//! Implements the pooling instance allocator.
//!
//! The pooling instance allocator maps memory in advance
//! and allocates instances, memories, tables, and stacks from
//! a pool of available resources.
//!
//! Using the pooling instance allocator can speed up module instantiation
//! when modules can be constrained based on configurable limits.

use super::borrow_limiter;
use super::{
    initialize_instance, initialize_vmcontext, InstanceAllocationRequest, InstanceAllocator,
    InstanceHandle, InstantiationError, ResourceLimiter,
};
use crate::{instance::Instance, Memory, Mmap, Table, VMContext};
use anyhow::{anyhow, bail, Context, Result};
use rand::Rng;
use std::convert::TryFrom;
use std::marker;
use std::mem;
use std::sync::{Arc, Mutex};
use wasmtime_environ::{
    entity::{EntitySet, PrimaryMap},
    HostPtr, MemoryStyle, Module, Tunables, VMOffsets, VMOffsetsFields, WASM_PAGE_SIZE,
};

cfg_if::cfg_if! {
    if #[cfg(windows)] {
        mod windows;
        use windows as imp;
    } else if #[cfg(all(feature = "uffd", target_os = "linux"))] {
        mod uffd;
        use uffd as imp;
        use imp::initialize_memory_pool;
    } else if #[cfg(target_os = "linux")] {
        mod linux;
        use linux as imp;
    } else {
        mod unix;
        use unix as imp;
    }
}

use imp::{commit_memory_pages, commit_table_pages, decommit_memory_pages, decommit_table_pages};

#[cfg(all(feature = "async", unix))]
use imp::{commit_stack_pages, decommit_stack_pages};

#[cfg(feature = "async")]
use super::FiberStackError;

fn round_up_to_pow2(n: usize, to: usize) -> usize {
    debug_assert!(to > 0);
    debug_assert!(to.is_power_of_two());
    (n + to - 1) & !(to - 1)
}

/// Represents the limits placed on a module for compiling with the pooling instance allocator.
#[derive(Debug, Copy, Clone)]
pub struct ModuleLimits {
    /// The maximum number of imported functions for a module.
    pub imported_functions: u32,

    /// The maximum number of imported tables for a module.
    pub imported_tables: u32,

    /// The maximum number of imported linear memories for a module.
    pub imported_memories: u32,

    /// The maximum number of imported globals for a module.
    pub imported_globals: u32,

    /// The maximum number of defined types for a module.
    pub types: u32,

    /// The maximum number of defined functions for a module.
    pub functions: u32,

    /// The maximum number of defined tables for a module.
    pub tables: u32,

    /// The maximum number of defined linear memories for a module.
    pub memories: u32,

    /// The maximum number of defined globals for a module.
    pub globals: u32,

    /// The maximum table elements for any table defined in a module.
    pub table_elements: u32,

    /// The maximum number of pages for any linear memory defined in a module.
    pub memory_pages: u32,
}

impl ModuleLimits {
    fn validate(&self, module: &Module) -> Result<()> {
        if module.num_imported_funcs > self.imported_functions as usize {
            bail!(
                "imported function count of {} exceeds the limit of {}",
                module.num_imported_funcs,
                self.imported_functions
            );
        }

        if module.num_imported_tables > self.imported_tables as usize {
            bail!(
                "imported tables count of {} exceeds the limit of {}",
                module.num_imported_tables,
                self.imported_tables
            );
        }

        if module.num_imported_memories > self.imported_memories as usize {
            bail!(
                "imported memories count of {} exceeds the limit of {}",
                module.num_imported_memories,
                self.imported_memories
            );
        }

        if module.num_imported_globals > self.imported_globals as usize {
            bail!(
                "imported globals count of {} exceeds the limit of {}",
                module.num_imported_globals,
                self.imported_globals
            );
        }

        if module.types.len() > self.types as usize {
            bail!(
                "defined types count of {} exceeds the limit of {}",
                module.types.len(),
                self.types
            );
        }

        let functions = module.functions.len() - module.num_imported_funcs;
        if functions > self.functions as usize {
            bail!(
                "defined functions count of {} exceeds the limit of {}",
                functions,
                self.functions
            );
        }

        let tables = module.table_plans.len() - module.num_imported_tables;
        if tables > self.tables as usize {
            bail!(
                "defined tables count of {} exceeds the limit of {}",
                tables,
                self.tables
            );
        }

        let memories = module.memory_plans.len() - module.num_imported_memories;
        if memories > self.memories as usize {
            bail!(
                "defined memories count of {} exceeds the limit of {}",
                memories,
                self.memories
            );
        }

        let globals = module.globals.len() - module.num_imported_globals;
        if globals > self.globals as usize {
            bail!(
                "defined globals count of {} exceeds the limit of {}",
                globals,
                self.globals
            );
        }

        for (i, plan) in module.table_plans.values().as_slice()[module.num_imported_tables..]
            .iter()
            .enumerate()
        {
            if plan.table.minimum > self.table_elements {
                bail!(
                    "table index {} has a minimum element size of {} which exceeds the limit of {}",
                    i,
                    plan.table.minimum,
                    self.table_elements
                );
            }
        }

        for (i, plan) in module.memory_plans.values().as_slice()[module.num_imported_memories..]
            .iter()
            .enumerate()
        {
            if plan.memory.minimum > self.memory_pages {
                bail!(
                    "memory index {} has a minimum page size of {} which exceeds the limit of {}",
                    i,
                    plan.memory.minimum,
                    self.memory_pages
                );
            }

            if let MemoryStyle::Dynamic = plan.style {
                bail!(
                    "memory index {} has an unsupported dynamic memory plan style",
                    i,
                );
            }
        }

        Ok(())
    }
}

impl Default for ModuleLimits {
    fn default() -> Self {
        // See doc comments for `wasmtime::ModuleLimits` for these default values
        Self {
            imported_functions: 1000,
            imported_tables: 0,
            imported_memories: 0,
            imported_globals: 0,
            types: 100,
            functions: 10000,
            tables: 1,
            memories: 1,
            globals: 10,
            table_elements: 10000,
            memory_pages: 160,
        }
    }
}

/// Represents the limits placed on instances by the pooling instance allocator.
#[derive(Debug, Copy, Clone)]
pub struct InstanceLimits {
    /// The maximum number of concurrent instances supported.
    pub count: u32,
}

impl Default for InstanceLimits {
    fn default() -> Self {
        // See doc comments for `wasmtime::InstanceLimits` for these default values
        Self { count: 1000 }
    }
}

/// The allocation strategy to use for the pooling instance allocator.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum PoolingAllocationStrategy {
    /// Allocate from the next available instance.
    NextAvailable,
    /// Allocate from a random available instance.
    Random,
}

impl PoolingAllocationStrategy {
    fn next(&self, free_count: usize) -> usize {
        debug_assert!(free_count > 0);

        match self {
            Self::NextAvailable => free_count - 1,
            Self::Random => rand::thread_rng().gen_range(0..free_count),
        }
    }
}

impl Default for PoolingAllocationStrategy {
    fn default() -> Self {
        Self::NextAvailable
    }
}

/// Represents a pool of maximal `Instance` structures.
///
/// Each index in the pool provides enough space for a maximal `Instance`
/// structure depending on the limits used to create the pool.
///
/// The pool maintains a free list for fast instance allocation.
///
/// The userfault handler relies on how instances are stored in the mapping,
/// so make sure the uffd implementation is kept up-to-date.
#[derive(Debug)]
struct InstancePool {
    mapping: Mmap,
    instance_size: usize,
    max_instances: usize,
    free_list: Mutex<Vec<usize>>,
    memories: MemoryPool,
    tables: TablePool,
    empty_module: Arc<Module>,
}

impl InstancePool {
    fn new(
        module_limits: &ModuleLimits,
        instance_limits: &InstanceLimits,
        tunables: &Tunables,
    ) -> Result<Self> {
        let page_size = region::page::size();

        // Calculate the maximum size of an Instance structure given the limits
        let offsets = VMOffsets::from(VMOffsetsFields {
            ptr: HostPtr,
            num_signature_ids: module_limits.types,
            num_imported_functions: module_limits.imported_functions,
            num_imported_tables: module_limits.imported_tables,
            num_imported_memories: module_limits.imported_memories,
            num_imported_globals: module_limits.imported_globals,
            num_defined_functions: module_limits.functions,
            num_defined_tables: module_limits.tables,
            num_defined_memories: module_limits.memories,
            num_defined_globals: module_limits.globals,
        });

        let instance_size = round_up_to_pow2(
            mem::size_of::<Instance>()
                .checked_add(offsets.size_of_vmctx() as usize)
                .ok_or_else(|| anyhow!("instance size exceeds addressable memory"))?,
            page_size,
        );

        let max_instances = instance_limits.count as usize;

        let allocation_size = instance_size
            .checked_mul(max_instances)
            .ok_or_else(|| anyhow!("total size of instance data exceeds addressable memory"))?;

        let mapping = Mmap::accessible_reserved(allocation_size, allocation_size)
            .context("failed to create instance pool mapping")?;

        let pool = Self {
            mapping,
            instance_size,
            max_instances,
            free_list: Mutex::new((0..max_instances).collect()),
            memories: MemoryPool::new(module_limits, instance_limits, tunables)?,
            tables: TablePool::new(module_limits, instance_limits)?,
            empty_module: Arc::new(Module::default()),
        };

        // Use a default module to initialize the instances to start
        for i in 0..instance_limits.count as usize {
            pool.initialize(module_limits, i);
        }

        Ok(pool)
    }

    unsafe fn instance(&self, index: usize) -> &mut Instance {
        debug_assert!(index < self.max_instances);
        &mut *(self.mapping.as_mut_ptr().add(index * self.instance_size) as *mut Instance)
    }

    fn initialize(&self, limits: &ModuleLimits, index: usize) {
        unsafe {
            let instance = self.instance(index);

            // Write a default instance with preallocated memory/table map storage to the ptr
            std::ptr::write(
                instance as _,
                Instance {
                    module: self.empty_module.clone(),
                    offsets: VMOffsets::new(HostPtr, &self.empty_module),
                    memories: PrimaryMap::with_capacity(limits.memories as usize),
                    tables: PrimaryMap::with_capacity(limits.tables as usize),
                    dropped_elements: EntitySet::new(),
                    dropped_data: EntitySet::new(),
                    host_state: Box::new(()),
                    vmctx: VMContext {
                        _marker: marker::PhantomPinned,
                    },
                },
            );
        }
    }

    unsafe fn setup_instance(
        &self,
        index: usize,
        mut req: InstanceAllocationRequest,
    ) -> Result<InstanceHandle, InstantiationError> {
        let instance = self.instance(index);

        instance.module = req.module.clone();
        instance.offsets = VMOffsets::new(HostPtr, instance.module.as_ref());
        instance.host_state = std::mem::replace(&mut req.host_state, Box::new(()));

        let mut limiter = req.store.and_then(|s| (*s).limiter());
        Self::set_instance_memories(
            instance,
            self.memories.get(index),
            self.memories.max_wasm_pages,
            borrow_limiter(&mut limiter),
        )?;

        Self::set_instance_tables(
            instance,
            self.tables.get(index).map(|x| x as *mut usize),
            self.tables.max_elements,
            borrow_limiter(&mut limiter),
        )?;

        initialize_vmcontext(instance, req);

        Ok(InstanceHandle {
            instance: instance as _,
        })
    }

    fn allocate(
        &self,
        strategy: PoolingAllocationStrategy,
        req: InstanceAllocationRequest,
    ) -> Result<InstanceHandle, InstantiationError> {
        let index = {
            let mut free_list = self.free_list.lock().unwrap();
            if free_list.is_empty() {
                return Err(InstantiationError::Limit(self.max_instances as u32));
            }
            let free_index = strategy.next(free_list.len());
            free_list.swap_remove(free_index)
        };

        unsafe {
            self.setup_instance(index, req).or_else(|e| {
                // Deallocate the allocated instance on error
                let instance = self.instance(index);
                self.deallocate(&InstanceHandle {
                    instance: instance as _,
                });
                Err(e)
            })
        }
    }

    fn deallocate(&self, handle: &InstanceHandle) {
        let addr = handle.instance as usize;
        let base = self.mapping.as_ptr() as usize;

        debug_assert!(addr >= base && addr < base + self.mapping.len());
        debug_assert!((addr - base) % self.instance_size == 0);

        let index = (addr - base) / self.instance_size;
        debug_assert!(index < self.max_instances);

        let instance = unsafe { &mut *handle.instance };

        // Decommit any linear memories that were used
        for (memory, base) in instance.memories.values_mut().zip(self.memories.get(index)) {
            let mut memory = mem::take(memory);
            debug_assert!(memory.is_static());

            // Reset any faulted guard pages as the physical memory may be reused for another instance in the future
            #[cfg(all(feature = "uffd", target_os = "linux"))]
            memory
                .reset_guard_pages()
                .expect("failed to reset guard pages");
            drop(&mut memory); // require mutable on all platforms, not just uffd

            let size = (memory.size() as usize) * (WASM_PAGE_SIZE as usize);
            drop(memory);
            decommit_memory_pages(base, size).expect("failed to decommit linear memory pages");
        }

        instance.memories.clear();
        instance.dropped_data.clear();

        // Decommit any tables that were used
        for (table, base) in instance.tables.values_mut().zip(self.tables.get(index)) {
            let table = mem::take(table);
            debug_assert!(table.is_static());

            let size = round_up_to_pow2(
                table.size() as usize * mem::size_of::<*mut u8>(),
                self.tables.page_size,
            );

            drop(table);
            decommit_table_pages(base, size).expect("failed to decommit table pages");
        }

        instance.tables.clear();
        instance.dropped_elements.clear();

        // Drop all `global` values which need a destructor, such as externref
        // values which now need their reference count dropped.
        instance.drop_globals();

        // Drop any host state
        instance.host_state = Box::new(());

        // And finally reset the module/offsets back to their original. This
        // should put everything back in a relatively pristine state for each
        // fresh allocation later on.
        instance.module = self.empty_module.clone();
        instance.offsets = VMOffsets::new(HostPtr, &self.empty_module);

        self.free_list.lock().unwrap().push(index);
    }

    fn set_instance_memories(
        instance: &mut Instance,
        mut memories: impl Iterator<Item = *mut u8>,
        max_pages: u32,
        mut limiter: Option<&mut dyn ResourceLimiter>,
    ) -> Result<(), InstantiationError> {
        let module = instance.module.as_ref();

        debug_assert!(instance.memories.is_empty());

        for plan in
            (&module.memory_plans.values().as_slice()[module.num_imported_memories..]).iter()
        {
            let memory = unsafe {
                std::slice::from_raw_parts_mut(
                    memories.next().unwrap(),
                    (max_pages as usize) * (WASM_PAGE_SIZE as usize),
                )
            };
            instance.memories.push(
                Memory::new_static(
                    plan,
                    memory,
                    commit_memory_pages,
                    borrow_limiter(&mut limiter),
                )
                .map_err(InstantiationError::Resource)?,
            );
        }

        debug_assert!(instance.dropped_data.is_empty());
        instance.dropped_data.resize(module.passive_data.len());

        Ok(())
    }

    fn set_instance_tables(
        instance: &mut Instance,
        mut tables: impl Iterator<Item = *mut usize>,
        max_elements: u32,
        mut limiter: Option<&mut dyn ResourceLimiter>,
    ) -> Result<(), InstantiationError> {
        let module = instance.module.as_ref();

        debug_assert!(instance.tables.is_empty());

        for plan in (&module.table_plans.values().as_slice()[module.num_imported_tables..]).iter() {
            let base = tables.next().unwrap();

            commit_table_pages(
                base as *mut u8,
                max_elements as usize * mem::size_of::<*mut u8>(),
            )
            .map_err(InstantiationError::Resource)?;

            let table = unsafe { std::slice::from_raw_parts_mut(base, max_elements as usize) };
            instance.tables.push(
                Table::new_static(plan, table, borrow_limiter(&mut limiter))
                    .map_err(InstantiationError::Resource)?,
            );
        }

        debug_assert!(instance.dropped_elements.is_empty());
        instance
            .dropped_elements
            .resize(module.passive_elements.len());

        Ok(())
    }
}

impl Drop for InstancePool {
    fn drop(&mut self) {
        unsafe {
            for i in 0..self.max_instances {
                let ptr = self.mapping.as_mut_ptr().add(i * self.instance_size) as *mut Instance;
                std::ptr::drop_in_place(ptr);
            }
        }
    }
}

/// Represents a pool of WebAssembly linear memories.
///
/// A linear memory is divided into accessible pages and guard pages.
///
/// Each instance index into the pool returns an iterator over the base addresses
/// of the instance's linear memories.
///
/// The userfault handler relies on how memories are stored in the mapping,
/// so make sure the uffd implementation is kept up-to-date.
#[derive(Debug)]
struct MemoryPool {
    mapping: Mmap,
    // The size, in bytes, of each linear memory's reservation plus the guard
    // region allocated for it.
    memory_size: usize,
    // The size, in bytes, of the offset to the first linear memory in this
    // pool. This is here to help account for the first region of guard pages,
    // if desired, before the first linear memory.
    initial_memory_offset: usize,
    max_memories: usize,
    max_instances: usize,
    max_wasm_pages: u32,
}

impl MemoryPool {
    fn new(
        module_limits: &ModuleLimits,
        instance_limits: &InstanceLimits,
        tunables: &Tunables,
    ) -> Result<Self> {
        // The maximum module memory page count cannot exceed 65536 pages
        if module_limits.memory_pages > 0x10000 {
            bail!(
                "module memory page limit of {} exceeds the maximum of 65536",
                module_limits.memory_pages
            );
        }

        // The maximum module memory page count cannot exceed the memory reservation size
        if module_limits.memory_pages > tunables.static_memory_bound {
            bail!(
                "module memory page limit of {} pages exceeds maximum static memory limit of {} pages",
                module_limits.memory_pages,
                tunables.static_memory_bound,
            );
        }

        let memory_size = if module_limits.memory_pages > 0 {
            usize::try_from(
                u64::from(tunables.static_memory_bound) * u64::from(WASM_PAGE_SIZE)
                    + tunables.static_memory_offset_guard_size,
            )
            .map_err(|_| anyhow!("memory reservation size exceeds addressable memory"))?
        } else {
            0
        };

        debug_assert!(
            memory_size % region::page::size() == 0,
            "memory size {} is not a multiple of system page size",
            memory_size
        );

        let max_instances = instance_limits.count as usize;
        let max_memories = module_limits.memories as usize;
        let initial_memory_offset = if tunables.guard_before_linear_memory {
            usize::try_from(tunables.static_memory_offset_guard_size).unwrap()
        } else {
            0
        };

        // The entire allocation here is the size of each memory times the
        // max memories per instance times the number of instances allowed in
        // this pool, plus guard regions.
        //
        // Note, though, that guard regions are required to be after each linear
        // memory. If the `guard_before_linear_memory` setting is specified,
        // then due to the contiguous layout of linear memories the guard pages
        // after one memory are also guard pages preceding the next linear
        // memory. This means that we only need to handle pre-guard-page sizes
        // specially for the first linear memory, hence the
        // `initial_memory_offset` variable here. If guards aren't specified
        // before linear memories this is set to `0`, otherwise it's set to
        // the same size as guard regions for other memories.
        let allocation_size = memory_size
            .checked_mul(max_memories)
            .and_then(|c| c.checked_mul(max_instances))
            .and_then(|c| c.checked_add(initial_memory_offset))
            .ok_or_else(|| {
                anyhow!("total size of memory reservation exceeds addressable memory")
            })?;

        // Create a completely inaccessible region to start
        let mapping = Mmap::accessible_reserved(0, allocation_size)
            .context("failed to create memory pool mapping")?;

        let pool = Self {
            mapping,
            memory_size,
            initial_memory_offset,
            max_memories,
            max_instances,
            max_wasm_pages: module_limits.memory_pages,
        };

        // uffd support requires some special setup for the memory pool
        #[cfg(all(feature = "uffd", target_os = "linux"))]
        initialize_memory_pool(&pool)?;

        Ok(pool)
    }

    fn get(&self, instance_index: usize) -> impl Iterator<Item = *mut u8> {
        debug_assert!(instance_index < self.max_instances);

        let base: *mut u8 = unsafe {
            self.mapping.as_mut_ptr().add(
                self.initial_memory_offset + instance_index * self.memory_size * self.max_memories,
            ) as _
        };

        let size = self.memory_size;
        (0..self.max_memories).map(move |i| unsafe { base.add(i * size) })
    }
}

/// Represents a pool of WebAssembly tables.
///
/// Each instance index into the pool returns an iterator over the base addresses
/// of the instance's tables.
#[derive(Debug)]
struct TablePool {
    mapping: Mmap,
    table_size: usize,
    max_tables: usize,
    max_instances: usize,
    page_size: usize,
    max_elements: u32,
}

impl TablePool {
    fn new(module_limits: &ModuleLimits, instance_limits: &InstanceLimits) -> Result<Self> {
        let page_size = region::page::size();

        let table_size = if module_limits.table_elements > 0 {
            round_up_to_pow2(
                mem::size_of::<*mut u8>()
                    .checked_mul(module_limits.table_elements as usize)
                    .ok_or_else(|| anyhow!("table size exceeds addressable memory"))?,
                page_size,
            )
        } else {
            0
        };

        let max_instances = instance_limits.count as usize;
        let max_tables = module_limits.tables as usize;

        let allocation_size = table_size
            .checked_mul(max_tables)
            .and_then(|c| c.checked_mul(max_instances))
            .ok_or_else(|| anyhow!("total size of instance tables exceeds addressable memory"))?;

        let mapping = Mmap::accessible_reserved(allocation_size, allocation_size)
            .context("failed to create table pool mapping")?;

        Ok(Self {
            mapping,
            table_size,
            max_tables,
            max_instances,
            page_size,
            max_elements: module_limits.table_elements,
        })
    }

    fn get(&self, instance_index: usize) -> impl Iterator<Item = *mut u8> {
        debug_assert!(instance_index < self.max_instances);

        let base: *mut u8 = unsafe {
            self.mapping
                .as_mut_ptr()
                .add(instance_index * self.table_size * self.max_tables) as _
        };

        let size = self.table_size;
        (0..self.max_tables).map(move |i| unsafe { base.add(i * size) })
    }
}

/// Represents a pool of execution stacks (used for the async fiber implementation).
///
/// Each index into the pool represents a single execution stack. The maximum number of
/// stacks is the same as the maximum number of instances.
///
/// As stacks grow downwards, each stack starts (lowest address) with a guard page
/// that can be used to detect stack overflow.
///
/// The top of the stack (starting stack pointer) is returned when a stack is allocated
/// from the pool.
#[cfg(all(feature = "async", unix))]
#[derive(Debug)]
struct StackPool {
    mapping: Mmap,
    stack_size: usize,
    max_instances: usize,
    page_size: usize,
    free_list: Mutex<Vec<usize>>,
}

#[cfg(all(feature = "async", unix))]
impl StackPool {
    fn new(instance_limits: &InstanceLimits, stack_size: usize) -> Result<Self> {
        let page_size = region::page::size();

        // Add a page to the stack size for the guard page when using fiber stacks
        let stack_size = if stack_size == 0 {
            0
        } else {
            round_up_to_pow2(stack_size, page_size)
                .checked_add(page_size)
                .ok_or_else(|| anyhow!("stack size exceeds addressable memory"))?
        };

        let max_instances = instance_limits.count as usize;

        let allocation_size = stack_size
            .checked_mul(max_instances)
            .ok_or_else(|| anyhow!("total size of execution stacks exceeds addressable memory"))?;

        let mapping = Mmap::accessible_reserved(allocation_size, allocation_size)
            .context("failed to create stack pool mapping")?;

        // Set up the stack guard pages
        if allocation_size > 0 {
            unsafe {
                for i in 0..max_instances {
                    // Make the stack guard page inaccessible
                    let bottom_of_stack = mapping.as_mut_ptr().add(i * stack_size);
                    region::protect(bottom_of_stack, page_size, region::Protection::NONE)
                        .context("failed to protect stack guard page")?;
                }
            }
        }

        Ok(Self {
            mapping,
            stack_size,
            max_instances,
            page_size,
            free_list: Mutex::new((0..max_instances).collect()),
        })
    }

    fn allocate(
        &self,
        strategy: PoolingAllocationStrategy,
    ) -> Result<wasmtime_fiber::FiberStack, FiberStackError> {
        if self.stack_size == 0 {
            return Err(FiberStackError::NotSupported);
        }

        let index = {
            let mut free_list = self.free_list.lock().unwrap();
            if free_list.is_empty() {
                return Err(FiberStackError::Limit(self.max_instances as u32));
            }
            let free_index = strategy.next(free_list.len());
            free_list.swap_remove(free_index)
        };

        debug_assert!(index < self.max_instances);

        unsafe {
            // Remove the guard page from the size
            let size_without_guard = self.stack_size - self.page_size;

            let bottom_of_stack = self
                .mapping
                .as_mut_ptr()
                .add((index * self.stack_size) + self.page_size);

            commit_stack_pages(bottom_of_stack, size_without_guard)
                .map_err(FiberStackError::Resource)?;

            wasmtime_fiber::FiberStack::from_top_ptr(bottom_of_stack.add(size_without_guard))
                .map_err(|e| FiberStackError::Resource(e.into()))
        }
    }

    fn deallocate(&self, stack: &wasmtime_fiber::FiberStack) {
        let top = stack
            .top()
            .expect("fiber stack not allocated from the pool") as usize;

        let base = self.mapping.as_ptr() as usize;
        let len = self.mapping.len();
        assert!(
            top > base && top <= (base + len),
            "fiber stack top pointer not in range"
        );

        // Remove the guard page from the size
        let stack_size = self.stack_size - self.page_size;
        let bottom_of_stack = top - stack_size;
        let start_of_stack = bottom_of_stack - self.page_size;
        debug_assert!(start_of_stack >= base && start_of_stack < (base + len));
        debug_assert!((start_of_stack - base) % self.stack_size == 0);

        let index = (start_of_stack - base) / self.stack_size;
        debug_assert!(index < self.max_instances);

        decommit_stack_pages(bottom_of_stack as _, stack_size).unwrap();

        self.free_list.lock().unwrap().push(index);
    }
}

/// Implements the pooling instance allocator.
///
/// This allocator internally maintains pools of instances, memories, tables, and stacks.
///
/// Note: the resource pools are manually dropped so that the fault handler terminates correctly.
#[derive(Debug)]
pub struct PoolingInstanceAllocator {
    strategy: PoolingAllocationStrategy,
    module_limits: ModuleLimits,
    instance_limits: InstanceLimits,
    // This is manually drop so that the pools unmap their memory before the page fault handler drops.
    instances: mem::ManuallyDrop<InstancePool>,
    #[cfg(all(feature = "async", unix))]
    stacks: StackPool,
    stack_size: usize,
    #[cfg(all(feature = "uffd", target_os = "linux"))]
    _fault_handler: imp::PageFaultHandler,
}

impl PoolingInstanceAllocator {
    /// Creates a new pooling instance allocator with the given strategy and limits.
    pub fn new(
        strategy: PoolingAllocationStrategy,
        module_limits: ModuleLimits,
        instance_limits: InstanceLimits,
        stack_size: usize,
        tunables: &Tunables,
    ) -> Result<Self> {
        if instance_limits.count == 0 {
            bail!("the instance count limit cannot be zero");
        }

        let instances = InstancePool::new(&module_limits, &instance_limits, tunables)?;

        #[cfg(all(feature = "uffd", target_os = "linux"))]
        let _fault_handler = imp::PageFaultHandler::new(&instances)?;

        Ok(Self {
            strategy,
            module_limits,
            instance_limits,
            instances: mem::ManuallyDrop::new(instances),
            #[cfg(all(feature = "async", unix))]
            stacks: StackPool::new(&instance_limits, stack_size)?,
            stack_size,
            #[cfg(all(feature = "uffd", target_os = "linux"))]
            _fault_handler,
        })
    }
}

impl Drop for PoolingInstanceAllocator {
    fn drop(&mut self) {
        // Manually drop the pools before the fault handler (if uffd is enabled)
        // This ensures that any fault handler thread monitoring the pool memory terminates
        unsafe {
            mem::ManuallyDrop::drop(&mut self.instances);
        }
    }
}

unsafe impl InstanceAllocator for PoolingInstanceAllocator {
    fn validate(&self, module: &Module) -> Result<()> {
        self.module_limits.validate(module)
    }

    fn adjust_tunables(&self, tunables: &mut Tunables) {
        // Treat the static memory bound as the maximum for unbounded Wasm memories
        // Because we guarantee a module cannot compile unless it fits in the limits of
        // the pool allocator, this ensures all memories are treated as static (i.e. immovable).
        tunables.static_memory_bound_is_maximum = true;
    }

    unsafe fn allocate(
        &self,
        req: InstanceAllocationRequest,
    ) -> Result<InstanceHandle, InstantiationError> {
        self.instances.allocate(self.strategy, req)
    }

    unsafe fn initialize(
        &self,
        handle: &mut InstanceHandle,
        module: &Module,
        is_bulk_memory: bool,
    ) -> Result<(), InstantiationError> {
        let instance = handle.instance_mut();

        cfg_if::cfg_if! {
            if #[cfg(all(feature = "uffd", target_os = "linux"))] {
                match &module.memory_initialization {
                    wasmtime_environ::MemoryInitialization::Paged{ out_of_bounds, .. } => {
                        if !is_bulk_memory {
                            super::check_init_bounds(instance, module)?;
                        }

                        // Initialize the tables
                        super::initialize_tables(instance, module)?;

                        // Don't initialize the memory; the fault handler will back the pages when accessed

                        // If there was an out of bounds access observed in initialization, return a trap
                        if *out_of_bounds {
                            return Err(InstantiationError::Trap(crate::traphandlers::Trap::wasm(
                                wasmtime_environ::ir::TrapCode::HeapOutOfBounds,
                            )));
                        }

                        Ok(())
                    },
                    _ => initialize_instance(instance, module, is_bulk_memory)
                }
            } else {
                initialize_instance(instance, module, is_bulk_memory)
            }
        }
    }

    unsafe fn deallocate(&self, handle: &InstanceHandle) {
        self.instances.deallocate(handle);
    }

    #[cfg(all(feature = "async", unix))]
    fn allocate_fiber_stack(&self) -> Result<wasmtime_fiber::FiberStack, FiberStackError> {
        self.stacks.allocate(self.strategy)
    }

    #[cfg(all(feature = "async", unix))]
    unsafe fn deallocate_fiber_stack(&self, stack: &wasmtime_fiber::FiberStack) {
        self.stacks.deallocate(stack);
    }

    #[cfg(all(feature = "async", windows))]
    fn allocate_fiber_stack(&self) -> Result<wasmtime_fiber::FiberStack, FiberStackError> {
        if self.stack_size == 0 {
            return Err(FiberStackError::NotSupported);
        }

        // On windows, we don't use a stack pool as we use the native fiber implementation
        wasmtime_fiber::FiberStack::new(self.stack_size)
            .map_err(|e| FiberStackError::Resource(e.into()))
    }

    #[cfg(all(feature = "async", windows))]
    unsafe fn deallocate_fiber_stack(&self, _stack: &wasmtime_fiber::FiberStack) {
        // A no-op as we don't own the fiber stack on Windows
    }
}

#[cfg(test)]
mod test {
    use super::*;
    use crate::{Imports, VMSharedSignatureIndex};
    use wasmtime_environ::{
        entity::EntityRef,
        ir::Type,
        wasm::{Global, GlobalInit, Memory, SignatureIndex, Table, TableElementType, WasmType},
        MemoryPlan, ModuleType, TablePlan, TableStyle,
    };

    #[test]
    fn test_module_imported_functions_limit() {
        let limits = ModuleLimits {
            imported_functions: 0,
            ..Default::default()
        };

        let mut module = Module::default();

        module.functions.push(SignatureIndex::new(0));
        assert!(limits.validate(&module).is_ok());

        module.num_imported_funcs = 1;
        assert_eq!(
            limits.validate(&module).map_err(|e| e.to_string()),
            Err("imported function count of 1 exceeds the limit of 0".into())
        );
    }

    #[test]
    fn test_module_imported_tables_limit() {
        let limits = ModuleLimits {
            imported_tables: 0,
            ..Default::default()
        };

        let mut module = Module::default();

        module.table_plans.push(TablePlan {
            style: TableStyle::CallerChecksSignature,
            table: Table {
                wasm_ty: WasmType::FuncRef,
                ty: TableElementType::Func,
                minimum: 0,
                maximum: None,
            },
        });

        assert!(limits.validate(&module).is_ok());

        module.num_imported_tables = 1;
        assert_eq!(
            limits.validate(&module).map_err(|e| e.to_string()),
            Err("imported tables count of 1 exceeds the limit of 0".into())
        );
    }

    #[test]
    fn test_module_imported_memories_limit() {
        let limits = ModuleLimits {
            imported_memories: 0,
            ..Default::default()
        };

        let mut module = Module::default();

        module.memory_plans.push(MemoryPlan {
            style: MemoryStyle::Static { bound: 0 },
            memory: Memory {
                minimum: 0,
                maximum: None,
                shared: false,
            },
            pre_guard_size: 0,
            offset_guard_size: 0,
        });

        assert!(limits.validate(&module).is_ok());

        module.num_imported_memories = 1;
        assert_eq!(
            limits.validate(&module).map_err(|e| e.to_string()),
            Err("imported memories count of 1 exceeds the limit of 0".into())
        );
    }

    #[test]
    fn test_module_imported_globals_limit() {
        let limits = ModuleLimits {
            imported_globals: 0,
            ..Default::default()
        };

        let mut module = Module::default();

        module.globals.push(Global {
            wasm_ty: WasmType::I32,
            ty: Type::int(32).unwrap(),
            mutability: false,
            initializer: GlobalInit::I32Const(0),
        });

        assert!(limits.validate(&module).is_ok());

        module.num_imported_globals = 1;
        assert_eq!(
            limits.validate(&module).map_err(|e| e.to_string()),
            Err("imported globals count of 1 exceeds the limit of 0".into())
        );
    }

    #[test]
    fn test_module_defined_types_limit() {
        let limits = ModuleLimits {
            types: 0,
            ..Default::default()
        };

        let mut module = Module::default();
        assert!(limits.validate(&module).is_ok());

        module
            .types
            .push(ModuleType::Function(SignatureIndex::new(0)));
        assert_eq!(
            limits.validate(&module).map_err(|e| e.to_string()),
            Err("defined types count of 1 exceeds the limit of 0".into())
        );
    }

    #[test]
    fn test_module_defined_functions_limit() {
        let limits = ModuleLimits {
            functions: 0,
            ..Default::default()
        };

        let mut module = Module::default();
        assert!(limits.validate(&module).is_ok());

        module.functions.push(SignatureIndex::new(0));
        assert_eq!(
            limits.validate(&module).map_err(|e| e.to_string()),
            Err("defined functions count of 1 exceeds the limit of 0".into())
        );
    }

    #[test]
    fn test_module_defined_tables_limit() {
        let limits = ModuleLimits {
            tables: 0,
            ..Default::default()
        };

        let mut module = Module::default();
        assert!(limits.validate(&module).is_ok());

        module.table_plans.push(TablePlan {
            style: TableStyle::CallerChecksSignature,
            table: Table {
                wasm_ty: WasmType::FuncRef,
                ty: TableElementType::Func,
                minimum: 0,
                maximum: None,
            },
        });
        assert_eq!(
            limits.validate(&module).map_err(|e| e.to_string()),
            Err("defined tables count of 1 exceeds the limit of 0".into())
        );
    }

    #[test]
    fn test_module_defined_memories_limit() {
        let limits = ModuleLimits {
            memories: 0,
            ..Default::default()
        };

        let mut module = Module::default();
        assert!(limits.validate(&module).is_ok());

        module.memory_plans.push(MemoryPlan {
            style: MemoryStyle::Static { bound: 0 },
            memory: Memory {
                minimum: 0,
                maximum: None,
                shared: false,
            },
            pre_guard_size: 0,
            offset_guard_size: 0,
        });
        assert_eq!(
            limits.validate(&module).map_err(|e| e.to_string()),
            Err("defined memories count of 1 exceeds the limit of 0".into())
        );
    }

    #[test]
    fn test_module_defined_globals_limit() {
        let limits = ModuleLimits {
            globals: 0,
            ..Default::default()
        };

        let mut module = Module::default();
        assert!(limits.validate(&module).is_ok());

        module.globals.push(Global {
            wasm_ty: WasmType::I32,
            ty: Type::int(32).unwrap(),
            mutability: false,
            initializer: GlobalInit::I32Const(0),
        });
        assert_eq!(
            limits.validate(&module).map_err(|e| e.to_string()),
            Err("defined globals count of 1 exceeds the limit of 0".into())
        );
    }

    #[test]
    fn test_module_table_minimum_elements_limit() {
        let limits = ModuleLimits {
            tables: 1,
            table_elements: 10,
            ..Default::default()
        };

        let mut module = Module::default();
        module.table_plans.push(TablePlan {
            style: TableStyle::CallerChecksSignature,
            table: Table {
                wasm_ty: WasmType::FuncRef,
                ty: TableElementType::Func,
                minimum: 11,
                maximum: None,
            },
        });
        assert_eq!(
            limits.validate(&module).map_err(|e| e.to_string()),
            Err(
                "table index 0 has a minimum element size of 11 which exceeds the limit of 10"
                    .into()
            )
        );
    }

    #[test]
    fn test_module_memory_minimum_size_limit() {
        let limits = ModuleLimits {
            memories: 1,
            memory_pages: 5,
            ..Default::default()
        };

        let mut module = Module::default();
        module.memory_plans.push(MemoryPlan {
            style: MemoryStyle::Static { bound: 0 },
            memory: Memory {
                minimum: 6,
                maximum: None,
                shared: false,
            },
            pre_guard_size: 0,
            offset_guard_size: 0,
        });
        assert_eq!(
            limits.validate(&module).map_err(|e| e.to_string()),
            Err("memory index 0 has a minimum page size of 6 which exceeds the limit of 5".into())
        );
    }

    #[test]
    fn test_module_with_dynamic_memory_style() {
        let limits = ModuleLimits {
            memories: 1,
            memory_pages: 5,
            ..Default::default()
        };

        let mut module = Module::default();
        module.memory_plans.push(MemoryPlan {
            style: MemoryStyle::Dynamic,
            memory: Memory {
                minimum: 1,
                maximum: None,
                shared: false,
            },
            offset_guard_size: 0,
            pre_guard_size: 0,
        });
        assert_eq!(
            limits.validate(&module).map_err(|e| e.to_string()),
            Err("memory index 0 has an unsupported dynamic memory plan style".into())
        );
    }

    #[test]
    fn test_next_available_allocation_strategy() {
        let strat = PoolingAllocationStrategy::NextAvailable;
        assert_eq!(strat.next(10), 9);
        assert_eq!(strat.next(5), 4);
        assert_eq!(strat.next(1), 0);
    }

    #[test]
    fn test_random_allocation_strategy() {
        let strat = PoolingAllocationStrategy::Random;
        assert!(strat.next(100) < 100);
        assert_eq!(strat.next(1), 0);
    }

    #[cfg(target_pointer_width = "64")]
    #[test]
    fn test_instance_pool() -> Result<()> {
        let module_limits = ModuleLimits {
            imported_functions: 0,
            imported_tables: 0,
            imported_memories: 0,
            imported_globals: 0,
            types: 0,
            functions: 0,
            tables: 1,
            memories: 1,
            globals: 0,
            table_elements: 10,
            memory_pages: 1,
        };
        let instance_limits = InstanceLimits { count: 3 };

        let instances = InstancePool::new(
            &module_limits,
            &instance_limits,
            &Tunables {
                static_memory_bound: 1,
                ..Tunables::default()
            },
        )?;

        // As of April 2021, the instance struct's size is largely below the size of a single page,
        // so it's safe to assume it's been rounded to the size of a single memory page here.
        assert_eq!(instances.instance_size, region::page::size());
        assert_eq!(instances.max_instances, 3);

        assert_eq!(&*instances.free_list.lock().unwrap(), &[0, 1, 2]);

        let mut handles = Vec::new();
        let module = Arc::new(Module::default());
        let finished_functions = &PrimaryMap::new();

        for _ in (0..3).rev() {
            handles.push(
                instances
                    .allocate(
                        PoolingAllocationStrategy::NextAvailable,
                        InstanceAllocationRequest {
                            module: module.clone(),
                            finished_functions,
                            imports: Imports {
                                functions: &[],
                                tables: &[],
                                memories: &[],
                                globals: &[],
                            },
                            shared_signatures: VMSharedSignatureIndex::default().into(),
                            host_state: Box::new(()),
                            store: None,
                        },
                    )
                    .expect("allocation should succeed"),
            );
        }

        assert_eq!(&*instances.free_list.lock().unwrap(), &[]);

        match instances.allocate(
            PoolingAllocationStrategy::NextAvailable,
            InstanceAllocationRequest {
                module: module.clone(),
                finished_functions,
                imports: Imports {
                    functions: &[],
                    tables: &[],
                    memories: &[],
                    globals: &[],
                },
                shared_signatures: VMSharedSignatureIndex::default().into(),
                host_state: Box::new(()),
                store: None,
            },
        ) {
            Err(InstantiationError::Limit(3)) => {}
            _ => panic!("unexpected error"),
        };

        for handle in handles.drain(..) {
            instances.deallocate(&handle);
        }

        assert_eq!(&*instances.free_list.lock().unwrap(), &[2, 1, 0]);

        Ok(())
    }

    #[cfg(target_pointer_width = "64")]
    #[test]
    fn test_memory_pool() -> Result<()> {
        let pool = MemoryPool::new(
            &ModuleLimits {
                imported_functions: 0,
                imported_tables: 0,
                imported_memories: 0,
                imported_globals: 0,
                types: 0,
                functions: 0,
                tables: 0,
                memories: 3,
                globals: 0,
                table_elements: 0,
                memory_pages: 1,
            },
            &InstanceLimits { count: 5 },
            &Tunables {
                static_memory_bound: 1,
                static_memory_offset_guard_size: 0,
                ..Tunables::default()
            },
        )?;

        assert_eq!(pool.memory_size, WASM_PAGE_SIZE as usize);
        assert_eq!(pool.max_memories, 3);
        assert_eq!(pool.max_instances, 5);
        assert_eq!(pool.max_wasm_pages, 1);

        let base = pool.mapping.as_ptr() as usize;

        for i in 0..5 {
            let mut iter = pool.get(i);

            for j in 0..3 {
                assert_eq!(
                    iter.next().unwrap() as usize - base,
                    ((i * 3) + j) * pool.memory_size
                );
            }

            assert_eq!(iter.next(), None);
        }

        Ok(())
    }

    #[cfg(target_pointer_width = "64")]
    #[test]
    fn test_table_pool() -> Result<()> {
        let pool = TablePool::new(
            &ModuleLimits {
                imported_functions: 0,
                imported_tables: 0,
                imported_memories: 0,
                imported_globals: 0,
                types: 0,
                functions: 0,
                tables: 4,
                memories: 0,
                globals: 0,
                table_elements: 100,
                memory_pages: 0,
            },
            &InstanceLimits { count: 7 },
        )?;

        let host_page_size = region::page::size();

        assert_eq!(pool.table_size, host_page_size);
        assert_eq!(pool.max_tables, 4);
        assert_eq!(pool.max_instances, 7);
        assert_eq!(pool.page_size, host_page_size);
        assert_eq!(pool.max_elements, 100);

        let base = pool.mapping.as_ptr() as usize;

        for i in 0..7 {
            let mut iter = pool.get(i);

            for j in 0..4 {
                assert_eq!(
                    iter.next().unwrap() as usize - base,
                    ((i * 4) + j) * pool.table_size
                );
            }

            assert_eq!(iter.next(), None);
        }

        Ok(())
    }

    #[cfg(all(unix, target_pointer_width = "64", feature = "async"))]
    #[test]
    fn test_stack_pool() -> Result<()> {
        let pool = StackPool::new(&InstanceLimits { count: 10 }, 1)?;

        let native_page_size = region::page::size();
        assert_eq!(pool.stack_size, 2 * native_page_size);
        assert_eq!(pool.max_instances, 10);
        assert_eq!(pool.page_size, native_page_size);

        assert_eq!(
            &*pool.free_list.lock().unwrap(),
            &[0, 1, 2, 3, 4, 5, 6, 7, 8, 9],
        );

        let base = pool.mapping.as_ptr() as usize;

        let mut stacks = Vec::new();
        for i in (0..10).rev() {
            let stack = pool
                .allocate(PoolingAllocationStrategy::NextAvailable)
                .expect("allocation should succeed");
            assert_eq!(
                ((stack.top().unwrap() as usize - base) / pool.stack_size) - 1,
                i
            );
            stacks.push(stack);
        }

        assert_eq!(&*pool.free_list.lock().unwrap(), &[]);

        match pool
            .allocate(PoolingAllocationStrategy::NextAvailable)
            .unwrap_err()
        {
            FiberStackError::Limit(10) => {}
            _ => panic!("unexpected error"),
        };

        for stack in stacks {
            pool.deallocate(&stack);
        }

        assert_eq!(
            &*pool.free_list.lock().unwrap(),
            &[9, 8, 7, 6, 5, 4, 3, 2, 1, 0],
        );

        Ok(())
    }

    #[test]
    fn test_pooling_allocator_with_zero_instance_count() {
        assert_eq!(
            PoolingInstanceAllocator::new(
                PoolingAllocationStrategy::Random,
                ModuleLimits::default(),
                InstanceLimits {
                    count: 0,
                    ..Default::default()
                },
                4096,
                &Tunables::default(),
            )
            .map_err(|e| e.to_string())
            .expect_err("expected a failure constructing instance allocator"),
            "the instance count limit cannot be zero"
        );
    }

    #[test]
    fn test_pooling_allocator_with_memory_pages_exceeded() {
        assert_eq!(
            PoolingInstanceAllocator::new(
                PoolingAllocationStrategy::Random,
                ModuleLimits {
                    memory_pages: 0x10001,
                    ..Default::default()
                },
                InstanceLimits { count: 1 },
                4096,
                &Tunables {
                    static_memory_bound: 1,
                    ..Tunables::default()
                },
            )
            .map_err(|e| e.to_string())
            .expect_err("expected a failure constructing instance allocator"),
            "module memory page limit of 65537 exceeds the maximum of 65536"
        );
    }

    #[test]
    fn test_pooling_allocator_with_reservation_size_exceeded() {
        assert_eq!(
            PoolingInstanceAllocator::new(
                PoolingAllocationStrategy::Random,
                ModuleLimits {
                    memory_pages: 2,
                    ..Default::default()
                },
                InstanceLimits { count: 1 },
                4096,
                &Tunables {
                    static_memory_bound: 1,
                    static_memory_offset_guard_size: 0,
                    ..Tunables::default()
                },
            )
            .map_err(|e| e.to_string())
            .expect_err("expected a failure constructing instance allocator"),
            "module memory page limit of 2 pages exceeds maximum static memory limit of 1 pages"
        );
    }

    #[cfg(all(unix, target_pointer_width = "64", feature = "async"))]
    #[test]
    fn test_stack_zeroed() -> Result<()> {
        // https://github.com/bytecodealliance/wasmtime/pull/2518#issuecomment-747280133
        if std::env::var("WASMTIME_TEST_NO_HOG_MEMORY").is_ok() {
            return Ok(());
        }

        let allocator = PoolingInstanceAllocator::new(
            PoolingAllocationStrategy::NextAvailable,
            ModuleLimits {
                imported_functions: 0,
                types: 0,
                functions: 0,
                tables: 0,
                memories: 0,
                globals: 0,
                table_elements: 0,
                memory_pages: 0,
                ..Default::default()
            },
            InstanceLimits { count: 1 },
            4096,
            &Tunables::default(),
        )?;

        unsafe {
            for _ in 0..10 {
                let stack = allocator.allocate_fiber_stack()?;

                // The stack pointer is at the top, so decrement it first
                let addr = stack.top().unwrap().sub(1);

                assert_eq!(*addr, 0);
                *addr = 1;

                allocator.deallocate_fiber_stack(&stack);
            }
        }

        Ok(())
    }
}