#![doc = include_str!("../Readme.md")]

use std::ffi::c_void;
use std::mem::size_of;
use std::os::raw::*;
use std::io;
use std::path::Path;
use std::ptr::*;

/// The error type of this library, [std::io::Error](https://doc.rust-lang.org/std/io/struct.Error.html)
pub type Error = std::io::Error;

/// The result type of this library, [std::io::Result](https://doc.rust-lang.org/std/io/struct.Result.html)
pub type Result<T> = std::io::Result<T>;

/// A loaded library handle.
#[derive(Clone, Copy, Debug, PartialEq, Eq, PartialOrd, Ord, Hash)]
#[repr(transparent)]
pub struct Library(NonNull<c_void>);
unsafe impl Send for Library {}
unsafe impl Sync for Library {}

/// *   Constructors
///     *   [`Library::load`]               &mdash; Load a library, forever, or return <code>[Err]\([io::Error])</code>.
/// *   Symbols (most of these functions implicitly transmute! Use extreme caution.)
///     *   [`Library::has_sym`]            &mdash; Check if a symbol, `"name\0"`, exists in the library.
///     *   [`Library::sym`]                &mdash; Load a symbol from the library by `"name\0"`, or return <code>[Err]\([io::Error])</code>.
///     *   [`Library::sym_opt`]            &mdash; Load a symbol from the library by `"name\0"`, or return [`None`].
///     *   [`Library::sym_by_ordinal`]     &mdash; Load a symbol from the library by windows ordinal, or return <code>[Err]\([io::Error])</code>.
///     *   [`Library::sym_opt_by_ordinal`] &mdash; Load a symbol from the library by windows ordinal, or return [`None`].
/// *   Interop
///     *   [`Library::from_ptr`]           &mdash; Wrap a forever-loaded library in [`Library`] for interop purpouses.
///     *   [`Library::from_non_null`]      &mdash; Wrap a forever-loaded library in [`Library`] for interop purpouses.
///     *   [`Library::as_ptr`]             &mdash; Return a raw handle pointer for interop purpouses.
///     *   [`Library::as_non_null`]        &mdash; Return a raw handle pointer for interop purpouses.
impl Library {
    /// Load a library, forever.
    ///
    /// | OS        | Behavior |
    /// | --------- | -------- |
    /// | Windows   | `LoadLibraryW(path)`
    /// | Unix      | `dlopen(path, ...)`
    pub fn load(path: impl AsRef<Path>) -> Result<Self> {
        let path = path.as_ref();

        #[cfg(windows)] let handle = {
            use std::os::windows::ffi::OsStrExt;
            let filename = path.as_os_str().encode_wide().chain([0].iter().copied()).collect::<Vec<u16>>();
            unsafe { LoadLibraryW(filename.as_ptr()) }
        };

        #[cfg(unix)] let handle = {
            use std::os::unix::ffi::OsStrExt;
            let filename = path.as_os_str().as_bytes().iter().copied().chain([0].iter().copied()).collect::<Vec<u8>>();
            let _ = unsafe { dlerror() }; // clear error code
            unsafe { dlopen(filename.as_ptr() as _, RTLD_LAZY) }
        };

        if let Some(handle) = NonNull::new(handle) {
            Ok(Self(handle))
        } else {
            #[cfg(windows)] {
                let err = Error::last_os_error();
                match err.raw_os_error() {
                    Some(ERROR_BAD_EXE_FORMAT) => {
                        Err(io::Error::new(io::ErrorKind::Other, format!(
                            "Unable to load {path}: ERROR_BAD_EXE_FORMAT (likely tried to load a {that}-bit DLL into this {this}-bit process)",
                            path = path.display(),
                            this = if cfg!(target_arch = "x86_64") { "64" } else { "32" },
                            that = if cfg!(target_arch = "x86_64") { "32" } else { "64" },
                        )))
                    },
                    Some(ERROR_MOD_NOT_FOUND) => {
                        Err(io::Error::new(io::ErrorKind::NotFound, format!(
                            "Unable to load {path}: NotFound",
                            path = path.display(),
                        )))
                    },
                    _ => Err(err)
                }
            }
            #[cfg(unix)] {
                // dlerror already contains path info
                Err(io::Error::new(io::ErrorKind::Other, dlerror_string_lossy()))
            }
        }
    }

    /// Wrap a forever-loaded library in [`Library`] for interop purpouses.
    ///
    /// Wrap a [`winapi::shared::minwindef::HMODULE`](https://docs.rs/winapi/0.3/winapi/shared/minwindef/type.HMODULE.html) with `Library::from_ptr(handle.cast())`.<br>
    /// Wrap a [`windows::Win32::Foundation::HMODULE`](https://microsoft.github.io/windows-docs-rs/doc/windows/Win32/Foundation/struct.HMODULE.html) with `Library::from_ptr(handle.0 as _)`.
    ///
    /// # Safety
    ///
    /// If `handle` is not null, it is expected to be a valid library handle for the duration of the program.
    ///
    /// # Platform
    ///
    /// | OS        | Expects   |
    /// | --------- | --------- |
    /// | Windows   | [`libloaderapi.h`](https://learn.microsoft.com/en-us/windows/win32/api/libloaderapi/)-compatible `HMODULE`
    /// | Unix      | [`dlfcn.h`](https://pubs.opengroup.org/onlinepubs/7908799/xsh/dlfcn.h.html)-compatible handle
    pub unsafe fn from_ptr(handle: *mut c_void) -> Option<Self> { Some(Self::from_non_null(NonNull::new(handle)?)) }

    /// Wrap a forever-loaded library in [`Library`] for interop purpouses.
    ///
    /// # Safety
    ///
    /// `handle` is expected to be a valid library handle for the duration of the program.
    ///
    /// # Platform
    ///
    /// | OS        | Expects   |
    /// | --------- | --------- |
    /// | Windows   | [`libloaderapi.h`](https://learn.microsoft.com/en-us/windows/win32/api/libloaderapi/)-compatible `HMODULE`
    /// | Unix      | [`dlfcn.h`](https://pubs.opengroup.org/onlinepubs/7908799/xsh/dlfcn.h.html)-compatible handle
    pub unsafe fn from_non_null(handle: NonNull<c_void>) -> Self { Self(handle) }

    /// Return a raw handle pointer for interop purpouses.
    ///
    /// Acquire a [`winapi::shared::minwindef::HMODULE`](https://docs.rs/winapi/0.3/winapi/shared/minwindef/type.HMODULE.html) with `handle.as_ptr() as HMODULE`.<br>
    /// Acquire a [`windows::Win32::Foundation::HMODULE`](https://microsoft.github.io/windows-docs-rs/doc/windows/Win32/Foundation/struct.HMODULE.html) with `HMODULE(handle.as_ptr() as _)`.
    ///
    /// # Safety
    ///
    /// Don't use this pointer to unload the library.
    pub fn as_ptr(&self) -> *mut c_void { self.0.as_ptr() }

    /// Return a raw handle pointer for interop purpouses.
    ///
    /// # Safety
    ///
    /// Don't use this pointer to unload the library.
    pub fn as_non_null(&self) -> NonNull<c_void> { self.0 }

    /// Load a symbol from the library.
    /// Note that the symbol name must end with '\0'.
    /// Limiting yourself to basic ASCII is also likely wise.
    ///
    /// # Safety
    ///
    /// This function implicitly transmutes!  Use extreme caution.
    ///
    /// # Platform
    ///
    /// | OS        | Behavior |
    /// | --------- | -------- |
    /// | Windows   | `GetProcAddress(..., name)`
    /// | Unix      | `dlsym(..., name)`
    pub unsafe fn sym<'a, T>(&self, name: impl AsRef<str>) -> io::Result<T> {
        let name = name.as_ref();
        self.sym_opt(name).ok_or_else(||{
            io::Error::new(io::ErrorKind::InvalidInput, format!("Symbol {:?} missing from library", &name[..name.len()-1]))
        })
    }

    /// Load a symbol from the library.
    /// Note that the symbol name must end with '\0'.
    /// Limiting yourself to basic ASCII is also likely wise.
    ///
    /// # Safety
    ///
    /// This function implicitly transmutes!  Use extreme caution.
    ///
    /// # Platform
    ///
    /// | OS        | Behavior |
    /// | --------- | -------- |
    /// | Windows   | `GetProcAddress(..., name)`
    /// | Unix      | `dlsym(..., name)`
    pub unsafe fn sym_opt<'a, T>(&self, name: impl AsRef<str>) -> Option<T> {
        let name = name.as_ref();
        let module = self.as_ptr();
        let n = name.len();
        assert_eq!(size_of::<T>(), size_of::<*mut c_void>(), "symbol result is not pointer sized!");
        assert!(name.ends_with('\0'),           "symbol name must end with '\0'");
        assert!(!name[..n-1].contains('\0'),    "symbol name mustn't contain '\0's, except to terminate the string");

        let cname = name.as_ptr() as _;
        #[cfg(windows)] let result = GetProcAddress(module, cname);
        #[cfg(unix)] let result = dlsym(module, cname);

        if result == null_mut() {
            None
        } else {
            Some(std::ptr::read(&result as *const *mut c_void as *const T))
        }
    }

    /// Load a symbol from the library by ordinal.
    ///
    /// # Safety
    ///
    /// This function implicitly transmutes!  Use extreme caution.
    /// Additionally, DLL ordinals are typically unstable and might change between minor versions of the same DLL, breaking your imports in nastily subtle ways.
    /// If a function name is available, use it instead!
    ///
    /// # Platform
    ///
    /// | OS        | Behavior |
    /// | --------- | -------- |
    /// | Windows   | `GetProcAddress(..., MAKEINTRESOURCE(ordinal))`
    /// | <strike>Unix</strike> | `Err(...)`
    pub unsafe fn sym_by_ordinal<T>(self, ordinal: u16) -> io::Result<T> {
        self.sym_opt_by_ordinal(ordinal).ok_or_else(||{
            io::Error::new(io::ErrorKind::InvalidInput, format!("Symbol @{} missing from library", ordinal))
        })
    }

    /// Load a symbol from the library by ordinal.
    ///
    /// # Safety
    ///
    /// This function implicitly transmutes!  Use extreme caution.
    /// Additionally, DLL ordinals are typically unstable and might change between minor versions of the same DLL, breaking your imports in nastily subtle ways.
    /// If a function name is available, use it instead!
    ///
    /// # Platform
    ///
    /// | OS        | Behavior |
    /// | --------- | -------- |
    /// | Windows   | `GetProcAddress(..., MAKEINTRESOURCE(ordinal))`
    /// | <strike>Unix</strike> | `None`
    pub unsafe fn sym_opt_by_ordinal<T>(self, ordinal: u16) -> Option<T> {
        assert_eq!(size_of::<T>(), size_of::<*mut c_void>(), "symbol result is not pointer sized!");

        // SAFETY: ✔️
        //  * `hModule`     ✔️ is a valid, non-dangling, loaded hmodule
        //  * `lpProcName`  ✔️ is a WORD/u16, meeting GetProcAddress's documented requirement:
        //                  "If this parameter is an ordinal value, it must be in the low-order word; the high-order word must be zero."
        #[cfg(windows)] let func = GetProcAddress(self.as_ptr(), ordinal as usize as *const _);
        #[cfg(unix)] let func = null_mut::<c_void>();
        #[cfg(unix)] let _ = ordinal;

        if func.is_null() {
            None
        } else {
            // SAFETY: ✔️
            //  * `T`   ✔️ is asserted to be the same size as `*mut c_void` via assert at start of function (can't enforce this at compile time)
            //  * `T`   ✔️ is assumed compatible with `*mut c_void` per the documented safety contract of this unsafe function
            Some(std::mem::transmute_copy::<*mut c_void, T>(&func))
        }
    }

    /// Check if a symbol existing in the library.
    /// Note that the symbol name must end with '\0'.
    /// Limiting yourself to basic ASCII is also likely wise.
    ///
    /// # Platform
    ///
    /// | OS        | Behavior |
    /// | --------- | -------- |
    /// | Windows   | `!!GetProcAddress(..., name)`
    /// | Unix      | `!!dlsym(..., name)`
    pub fn has_sym(self, name: impl AsRef<str>) -> bool {
        // SAFETY: ✔️ cast to `*mut c_void` should always be safe.
        let s : Option<*mut c_void> = unsafe { self.sym_opt(name) };
        s.is_some()
    }

    /// Attempt to unload the library.
    ///
    /// # Safety
    /// ❌ This is a **fundamentally unsound** operation that **may do nothing** and **invalidates everything** ❌
    ///
    /// You are practically guaranteed undefined behavior from some kind of dangling pointer or reference.
    /// Several platforms don't even bother implementing unloading.
    /// Those that do implement unloading often have ways of opting out of actually unloading.
    /// I'd argue the only valid use case for this fn is unit testing the unloading of your DLLs, when *someone else* was crazy enough to unload libraries in production.
    ///
    /// Based on that reasoning, this crate has been designed around the assumption that you will never do this.
    /// It lacks lifetimes and implements <code>[Library] : [Copy]</code>, which makes use-after-free bugs easy.
    /// Be especially careful - all of the following are invalidated when this function is called:
    /// *   All fns/pointers previously returned by `sym*` for this library, including through copies of `self`.
    /// *   The [`Library`] itself, and any [`Copy`] thereof.  This means even `self.has_sym(...)` is no longer sound to call, despite the fn being safe, even though it will compile without error (as self is `Copy`)!
    ///
    /// ## Safe Alternatives
    /// *   Restart the entire process (fine for production since process shutdown is actually tested)
    /// *   Use a sub-process and restart that (will also make your code more stable if a hot-reloading "plugin" crashes)
    /// *   Simply leak the library (fine for dev builds)
    ///     *   Export a function to free memory, join threads, close file handles, etc. if you want to reduce memory use / file locks
    ///     *   Load a temporary copy of the library instead of the original if you hate having a file lock on the original library
    ///
    /// ## Unsafe Alternatives
    /// A wrapper or crate with "better" support for this fundamentally flawed operation might:
    /// *   Limit support to plugin-shaped dynamic libraries that opt-in to claiming they're safe to unload (export a special fn/symbol?)
    /// *   Actively test unloading to catch the bugs in those libraries
    /// *   Introduce lifetimes (e.g. [`libloading::Symbol`](https://docs.rs/libloading/0.8.1/libloading/struct.Symbol.html)), or make fn pointers private, to help combat fn pointer invalidation bugs
    /// *   Not implement [`Copy`] for [`Library`] (or equivalent)
    /// *   Not default-implement [`Clone`] for [`Library`] (or equivalent - properly implemented refcounting might be OK)
    /// *   Implement [`Drop`] for [`Library`] (or equivalent) if you're arrogant enough to claim your unloading code is actually safe/sound.
    ///
    /// ## This might do nothing useful whatsoever
    /// *   ["musl’s dynamic loader loads libraries permanently for the lifetime of the process, until it exits or calls exec.<br>[...] only the musl behavior can satisfy the robustness conditions musl aims to provide"](https://wiki.musl-libc.org/functional-differences-from-glibc.html#Unloading-libraries)
    /// *   ["Prior to Mac OS X 10.5, only bundles could be unloaded.  Starting in Mac OS X 10.5, dynamic libraries may also be unloaded."](https://developer.apple.com/library/archive/documentation/System/Conceptual/ManPages_iPhoneOS/man3/dlclose.3.html)
    /// *   [`RTLD_NODELETE`](https://linux.die.net/man/3/dlopen) makes `dlclose` a noop
    /// *   [`GET_MODULE_HANDLE_EX_FLAG_PIN`](https://learn.microsoft.com/en-us/windows/win32/api/libloaderapi/nf-libloaderapi-getmodulehandleexa#parameters) makes `FreeLibrary*` a noop
    ///
    /// ## Threads are a problem
    /// Any thread (including those started automatically by the library itself on load) containing any library code anywhere in it's callstack will exhibit undefined behavior:
    /// *   Unwinding through library code via panic/exceptions/SEH will presumably use dangling unwinding information pointers
    /// *   Callstack snapshots (for allocation tracking, sentry.io event reporting, etc.) will contain dangling symbol pointers etc.
    /// *   Dangling instruction pointers (through execution or returning to library code) will, presumably:
    ///     *   Crash if unmapped, or mapped without execution permissions
    ///     *   Execute random, possibly "unreachable" code - or data misinterpreted as code - if mapped with execution permissions
    ///
    /// These are among the many reasons windows offers such a wide variety of unloading functions (lacking in POSIX systems):
    /// *   [`FreeLibrary`](https://learn.microsoft.com/en-us/windows/win32/api/libloaderapi/nf-libloaderapi-freelibrary)
    /// *   [`FreeLibraryAndExitThread`](https://learn.microsoft.com/en-us/windows/win32/api/libloaderapi/nf-libloaderapi-freelibraryandexitthread)
    /// *   [`FreeLibraryWhenCallbackReturns`](https://learn.microsoft.com/en-us/windows/win32/api/threadpoolapiset/nf-threadpoolapiset-freelibrarywhencallbackreturns)
    ///
    /// Some related reading:
    /// *   [What is the point of FreeLibraryAndExitThread?](https://devblogs.microsoft.com/oldnewthing/20131105-00/?p=2733) (The Old New Thing)
    /// *   [When is the correct time to call FreeLibraryWhenCallbackReturns?](https://devblogs.microsoft.com/oldnewthing/20151225-00/?p=92711) (The Old New Thing)
    ///
    /// Additionally, there are serious limitations on what `DllMain` / destructors can do without deadlocking or worse, limiting the DLL's ability to fix any of this:
    /// *   [Dynamic-Link Library Best Practices](https://learn.microsoft.com/en-us/windows/win32/dlls/dynamic-link-library-best-practices) (learn.microsoft.com)
    /// *   <https://github.com/mity/old-new-win32api#dllmain> (links to The Old New Thing)
    /// *   [Why are DLLs unloaded in the "wrong" order?](https://devblogs.microsoft.com/oldnewthing/20050523-05/?p=35573) (The Old New Thing)
    ///
    /// ## Callbacks are a problem
    /// The library has many ways of creating pointers into itself which will dangle if the library is unloaded:
    /// *   Error handling
    ///     *   [`std::panic::set_hook`](https://doc.rust-lang.org/std/panic/fn.set_hook.html) (100% safe - which other code can then [`take_hook`](https://doc.rust-lang.org/std/panic/fn.take_hook.html), preventing you from unregistering it!)
    ///     *   [Signal handlers](https://en.cppreference.com/w/c/program/signal)
    ///     *   [Unhandled exception handlers](https://learn.microsoft.com/en-us/windows/win32/debug/using-a-vectored-exception-handler)
    /// *   Windows callbacks
    ///     *   [Window Procedures](https://learn.microsoft.com/en-us/windows/win32/winmsg/window-procedures)
    ///     *   [COM Classes](https://learn.microsoft.com/en-us/windows/win32/api/combaseapi/nf-combaseapi-coregisterclassobject)
    ///     *   ...
    /// *   `Box<dyn Anything>`
    /// *   ...
    ///
    /// ## String literals and other `'static` references are a problem
    /// Allocator debugging code often likes to register references or pointers to `__FILE__` (common in C++ macros) or [`core::file!()`](https://doc.rust-lang.org/core/macro.file.html) (Rust).
    /// These are generally treated - by 100% safe code - as having `'static` lifetimes, and not deep copied.
    /// While this is sound within the context of a single module, those references and pointers will dangle - with all the undefined behavior that comes with that - if those references and pointers ever end up in another module that outlasts the library.
    ///
    /// This is only the most ubiquitous example of the larger problem:  Every reference or pointer to `const`, `static`, or literal variables of the library becomes a ticking timebomb and hazard in 100% "safe" code.
    ///
    /// ## Testing is a problem
    /// Nobody tests unloading dynamic libraries.  Nobody.
    /// I can't even unload debug CRTs without triggering assertions.
    /// Calling this function will simply invoke broken untested code.
    ///
    /// # Platform
    ///
    /// | OS        | Behavior |
    /// | --------- | -------- |
    /// | Windows   | `FreeLibrary(...)`
    /// | Unix      | `dlclose(...)`
    pub unsafe fn close_unsafe_unsound_possible_noop_do_not_use_in_production(self) -> io::Result<()> {
        #[cfg(windows)] match FreeLibrary(self.as_ptr()) {
            0 => Err(io::Error::last_os_error()),
            _ => Ok(()), // "If the function succeeds, the return value is nonzero." (https://learn.microsoft.com/en-us/windows/win32/api/libloaderapi/nf-libloaderapi-freelibrary)
        }
        #[cfg(unix)] match dlclose(self.as_ptr()) {
            0 => Ok(()), // "The function dlclose() returns 0 on success, and nonzero on error." (https://linux.die.net/man/3/dlclose)
            _ => Err(io::Error::new(io::ErrorKind::Other, dlerror_string_lossy()))
        }
    }
}

#[cfg(windows)] const ERROR_BAD_EXE_FORMAT : i32 = 0x00C1;
#[cfg(windows)] const ERROR_MOD_NOT_FOUND  : i32 = 0x007E;
#[cfg(windows)] extern "system" {
    fn GetProcAddress(hModule: *mut c_void, lpProcName: *const c_char) -> *mut c_void;
    fn LoadLibraryW(lpFileName: *const u16) -> *mut c_void;
    fn FreeLibrary(hModule: *mut c_void) -> u32;
}

#[cfg(unix)] fn dlerror_string_lossy() -> String {
    let e = unsafe { dlerror() };
    if e.is_null() { String::new() } else { unsafe { std::ffi::CStr::from_ptr(e) }.to_string_lossy().into() }
}

#[cfg(unix)] const RTLD_LAZY : c_int = 1;
#[cfg(unix)] extern "C" {
    fn dlopen(filename: *const c_char, flags: c_int) -> *mut c_void;
    fn dlsym(handle: *mut c_void, symbol: *const c_char) -> *mut c_void;
    fn dlerror() -> *const c_char;
    fn dlclose(handle: *mut c_void) -> c_int;
}