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//! The native stack contract for threads that drive the Harn VM.
//!
//! Harn has two independent stack hazards, each with its own owner:
//!
//! * Walking an arbitrarily deep *value* — `x = [x]` in a loop — is made
//! stack-size independent by [`crate::value::recursion`], which grows the
//! native stack on demand and tears values down iteratively.
//! * Walking an arbitrarily deep *program* — parse, type-check, compile, and
//! evaluate all recurse over nested syntax — is not. It relies on the thread
//! simply having enough stack, and that is what [`RUNTIME_STACK_SIZE`] is.
//!
//! The second contract lives entirely in the hosts: it holds only if every
//! thread that ends up running the VM asks for the size. Getting it wrong is
//! unusually expensive, because a stack overflow aborts the process instead of
//! failing one request, so this module also carries the structural check that
//! keeps new hosts honest.
/// Native stack size a thread needs in order to drive the Harn VM.
///
/// Compilation and execution walk nested program structure with recursive
/// frames, which can exceed Rust's 2 MiB default thread stack. A host that
/// runs the VM on a thread it spawns must request this size explicitly.
///
/// Relying on the ambient default is not safe, and neither is relying on
/// `RUST_MIN_STACK`: that variable is set by the CI test lanes but not by any
/// shipped binary, so a host that depends on it passes its own tests and then
/// aborts the whole process — a stack overflow is not a catchable panic — the
/// first time a customer runs a deep enough script.
///
/// The size is set by the deepest descent the runtime promises to *refuse*
/// rather than the deepest it expects to run. A nested agent descent costs
/// roughly 2 MiB of native stack per level, so the previous 16 MiB could carry
/// only seven levels while the nested-execution budget declares eight: the
/// refusal was undeliverable, and the process aborted on the level that should
/// have been denied. A bound the stack cannot reach is not a bound.
pub const RUNTIME_STACK_SIZE: usize = 32 * 1024 * 1024;
/// Run `body` on a thread that holds the [`RUNTIME_STACK_SIZE`] contract.
///
/// A caller that drives the VM from a thread it did not create borrows
/// whatever stack that thread was given. The test harness is where this keeps
/// happening: a case that builds a Tokio runtime on the libtest thread creates
/// no thread of its own, so it runs the VM on libtest's stack. That stack is
/// large enough only because every CI lane exports `RUST_MIN_STACK`, and a
/// developer machine without it aborts the whole test binary on one ordinary
/// agent loop (harn#7962). An abort is not a failed case: every later case in
/// the binary silently never runs.
///
/// Naming the thread the contract binds is the fix. A host that already spawns
/// its own VM thread with [`RUNTIME_STACK_SIZE`] does not need this; it exists
/// so a caller running on a borrowed stack can state the size once, at the
/// entry point, instead of depending on an environment variable no shipped
/// binary sets.
///
/// Panics propagate to the caller unchanged, so a failing assertion inside
/// `body` still fails its own test.
pub fn on_vm_stack<R: Send>(body: impl FnOnce() -> R + Send) -> R {
std::thread::scope(|scope| {
std::thread::Builder::new()
.name("harn-vm-contract-stack".to_owned())
.stack_size(RUNTIME_STACK_SIZE)
.spawn_scoped(scope, body)
.expect("spawn a thread holding the VM stack contract")
.join()
.unwrap_or_else(|payload| std::panic::resume_unwind(payload))
})
}
#[cfg(test)]
mod tests {
/// How much source to read past a spawn before deciding what it does.
const WINDOW: usize = 600;
/// Spawning forms that create a thread with the ambient default stack
/// unless the call site says otherwise.
const SPAWNS: [&str; 2] = ["std::thread::spawn(", "thread::Builder::new()"];
/// Building a current-thread Tokio runtime on a freshly spawned thread is
/// what "this thread is about to drive the VM" looks like across the
/// workspace: it is how every serve transport, the orchestrator's ACP
/// worker, and the CLI's scaffold and test workers are shaped.
const DRIVES_VM: &str = "tokio::runtime::Builder";
/// Either idiom for honoring the contract: the inline
/// `.stack_size(..._STACK_SIZE)` that `harn-cli` uses, or a helper such as
/// `harn-serve`'s `vm_thread` that applies it centrally (those call sites
/// match neither spawn form, so they never reach this check).
const HONORS_CONTRACT: &str = "stack_size(";
/// The source a spawn is judged on: everything from the spawn up to the
/// next attributed item, at any indentation.
///
/// Without the cut, a one-line spawn reads the runtime built by the
/// *following* function and reports a thread that does no VM work.
#[expect(
clippy::string_slice,
reason = "len is a sum of whole split_inclusive line lengths"
)]
fn spawn_body(window: &str) -> &str {
match window
.split_inclusive('\n')
.take_while(|line| !line.trim_start().starts_with("#["))
.map(str::len)
.sum::<usize>()
{
0 => "",
len => &window[..len],
}
}
/// Every thread in the workspace that builds a Tokio runtime to drive the
/// VM must ask for [`super::RUNTIME_STACK_SIZE`].
///
/// This is deliberately a *workspace* scan rather than a per-crate one.
/// The same defect shipped simultaneously in `harn-serve`, `harn-cli`, and
/// `harn-vm` (harn#6165) precisely because each crate's own tests could
/// only see that crate — and because every Rust test lane here exports
/// `RUST_MIN_STACK=16777216`, which makes an unsized spawn large enough in
/// CI and nowhere else.
///
/// Scope is `crates/`, which is every workspace member and so every shipped
/// host. `bench/` is out, and builds its runtimes on the current thread
/// rather than a spawned one, so there is nothing there to catch.
///
/// What this does *not* catch, so nobody over-trusts it: a thread that
/// drives the VM without building a Tokio runtime on itself. Of the ~76
/// non-test spawn sites in the workspace this judges only the ~10 shaped
/// like a transport or worker. `run_dap_adapter`, the counterfactual plan
/// runner, and the connector worker loop all drive the VM through a plain
/// function call and were found by reading, not by this scan. Deciding
/// those needs a call graph; recognizing the idiom that actually recurs
/// does not, and that idiom is where every instance so far has lived.
#[test]
fn vm_driving_threads_ask_for_the_runtime_stack() {
let crates_dir = std::path::Path::new(env!("CARGO_MANIFEST_DIR"))
.parent()
.expect("harn-vm lives below crates");
let mut offenders = Vec::new();
let mut scanned = 0usize;
for entry in walkdir::WalkDir::new(crates_dir)
.into_iter()
.filter_map(Result::ok)
.filter(|entry| {
entry.file_type().is_file()
&& entry.path().extension().and_then(std::ffi::OsStr::to_str) == Some("rs")
&& entry
.path()
.components()
.any(|component| component.as_os_str() == "src")
&& entry.file_name() != "runtime_stack.rs"
})
{
scanned += 1;
let source = std::fs::read_to_string(entry.path()).expect("read Rust source");
for pattern in SPAWNS {
for (offset, _) in source.match_indices(pattern) {
let end = (offset + WINDOW).min(source.len());
// A window can land mid-codepoint; the next match still
// covers this file and every marker here is ASCII.
let Some(window) = source.get(offset..end) else {
continue;
};
let window = spawn_body(window);
if window.contains(DRIVES_VM) && !window.contains(HONORS_CONTRACT) {
#[expect(
clippy::string_slice,
reason = "offset is a match_indices offset on source"
)]
let line = 1 + source[..offset].matches('\n').count();
offenders.push(format!("{}:{line}", entry.path().display()));
}
}
}
}
assert!(scanned > 100, "scan found only {scanned} sources to check");
assert!(
offenders.is_empty(),
"these threads build a Tokio runtime to drive the VM but take Rust's \
2 MiB default stack, so a deep script aborts the process instead of \
failing one request. Give them harn_vm::RUNTIME_STACK_SIZE — inline \
via `.stack_size(..)`, or through a helper like harn-serve's \
`vm_thread`:\n {}",
offenders.join("\n ")
);
}
/// A multi-thread Tokio runtime spawns its own worker threads, and those
/// workers run the VM.
///
/// [`vm_driving_threads_ask_for_the_runtime_stack`] cannot see this: it
/// judges *spawn sites*, and a multi-thread runtime built on the process's
/// main thread has none. Tokio creates the workers internally, at its own
/// 2 MiB default, and the CLI's runtime went that way for five releases
/// (harn#7961) — stopping a background sub-agent is polled on a worker, and
/// at the default size that overflowed and aborted the process. The
/// dedicated VM thread's `RUNTIME_STACK_SIZE` says nothing about a thread
/// Tokio made.
///
/// So every shipped multi-thread runtime must state
/// [`super::RUNTIME_STACK_SIZE`] for its workers. Unlike a behavioral test,
/// this fires under any ambient stack, which matters because the Rust test
/// lanes export `RUST_MIN_STACK=16777216` and would otherwise hide the
/// whole class.
///
/// Test code is out of scope: a path component named `tests` or ending in
/// `_tests`, and anything after a file's first `#[cfg(test)]`, is skipped.
/// Those runtimes never ship, and they run under the lanes' large stack.
#[test]
fn multi_thread_runtimes_size_their_worker_threads() {
/// Tokio builds these workers itself; nothing at the call site spawns.
const MULTI_THREAD: &str = "Builder::new_multi_thread()";
/// The builder method that states a worker stack size.
const SIZES_WORKERS: &str = "thread_stack_size(";
let crates_dir = std::path::Path::new(env!("CARGO_MANIFEST_DIR"))
.parent()
.expect("harn-vm lives below crates");
let mut offenders = Vec::new();
let mut scanned = 0usize;
for entry in walkdir::WalkDir::new(crates_dir)
.into_iter()
.filter_map(Result::ok)
.filter(|entry| {
entry.file_type().is_file()
&& entry.path().extension().and_then(std::ffi::OsStr::to_str) == Some("rs")
&& entry
.path()
.components()
.any(|component| component.as_os_str() == "src")
&& !entry.path().components().any(|component| {
let part = component.as_os_str().to_string_lossy();
let stem = part.strip_suffix(".rs").unwrap_or(&part);
stem == "tests" || stem.ends_with("_tests")
})
})
{
scanned += 1;
let source = std::fs::read_to_string(entry.path()).expect("read Rust source");
let test_mod = source.find("\n#[cfg(test)]").unwrap_or(source.len());
for (offset, _) in source.match_indices(MULTI_THREAD) {
if offset > test_mod {
continue;
}
let end = (offset + WINDOW).min(source.len());
let Some(window) = source.get(offset..end) else {
continue;
};
// The builder expression ends at its `build()`; past that is
// unrelated code that could carry the marker by accident.
#[expect(
clippy::string_slice,
reason = "find returns a char boundary in window"
)]
let window = match window.find(".build()") {
Some(cut) => &window[..cut],
None => window,
};
if !window.contains(SIZES_WORKERS) {
#[expect(
clippy::string_slice,
reason = "offset is a match_indices offset on source"
)]
let line = 1 + source[..offset].matches('\n').count();
offenders.push(format!("{}:{line}", entry.path().display()));
}
}
}
assert!(scanned > 100, "scan found only {scanned} sources to check");
assert!(
offenders.is_empty(),
"these multi-thread Tokio runtimes ship, so their worker threads run \
the VM, but they leave Tokio's 2 MiB default in place and a deep \
enough frame aborts the process. Add \
`.thread_stack_size(harn_vm::RUNTIME_STACK_SIZE)`:\n {}",
offenders.join("\n ")
);
}
/// Building a Tokio runtime is how a test says "this thread is about to
/// drive the VM"; executing a compiled chunk is it doing so.
const TEST_BUILDS_RUNTIME: &str = "tokio::runtime::Builder::new_";
/// The VM entry point these tests reach.
const TEST_DRIVES_VM: &str = "execute(&chunk";
/// Entering through the contract helper is what makes the case stack-size
/// independent, and it is the only accepted answer here: an inline
/// `stack_size` on a thread the test spawns itself is the shape the first
/// scan already judges.
const ENTERS_CONTRACT: &str = "on_vm_stack(";
/// Integration-test files that still drive the VM on whatever stack the
/// libtest harness handed them.
///
/// This is a shrinking ratchet, not a permission list. A row here is a
/// file whose cases pass today only because every CI lane exports
/// `RUST_MIN_STACK`; each one is a `super::on_vm_stack` wrap away from
/// leaving. Rows may be removed, never added, and a row that is no longer
/// an offender must be removed — a stale allowance is how a ratchet stops
/// measuring anything.
const HARNESS_STACK_BASELINE: [&str; 27] = [
"harn-vm/tests/agent_inbox_e2e.rs",
"harn-vm/tests/agent_terminal_ledger.rs",
"harn-vm/tests/call_frame_allocations.rs",
"harn-vm/tests/command_ledger_hold_paused_clock.rs",
"harn-vm/tests/harn_vm/agent_fanout.rs",
"harn-vm/tests/harn_vm/agent_loop_output_schema.rs",
"harn-vm/tests/harn_vm/agent_loop_steering_seams.rs",
"harn-vm/tests/harn_vm/agent_mcp_mid_conversation.rs",
"harn-vm/tests/harn_vm/agent_mcp_tool_ceiling.rs",
"harn-vm/tests/harn_vm/agent_prompt_prefix_stability.rs",
"harn-vm/tests/harn_vm/agent_sessions.rs",
"harn-vm/tests/harn_vm/builtin_call_dispatch.rs",
"harn-vm/tests/harn_vm/compaction_policy_primitive.rs",
"harn-vm/tests/harn_vm/external_agent_errors.rs",
"harn-vm/tests/harn_vm/github_stdlib_connectors.rs",
"harn-vm/tests/harn_vm/host_tool_batch_overlap.rs",
"harn-vm/tests/harn_vm/pool_multithread.rs",
"harn-vm/tests/harn_vm/runtime_introspection.rs",
"harn-vm/tests/harn_vm/skill_activation_evidence_conformance.rs",
"harn-vm/tests/harn_vm/stdlib_event_registration.rs",
"harn-vm/tests/harn_vm/tool_call_cancellation.rs",
"harn-vm/tests/harn_vm/tool_calling_bootcamp.rs",
"harn-vm/tests/harn_vm/tool_input_schema_spelling.rs",
"harn-vm/tests/harn_vm/tool_ref.rs",
"harn-vm/tests/harn_vm/worker_overlap.rs",
"harn-vm/tests/portable_kernel_parity.rs",
"harn-vm/tests/support/mod.rs",
];
/// A test that drives the VM on the harness thread escapes both scans
/// above, and the contract with it.
///
/// [`vm_driving_threads_ask_for_the_runtime_stack`] judges spawn sites and
/// [`multi_thread_runtimes_size_their_worker_threads`] judges the workers
/// Tokio makes. A case that builds a current-thread runtime on the libtest
/// thread has neither: it spawns nothing, and Tokio spawns nothing for it.
/// It runs the VM on libtest's stack, which is large enough only because
/// the lanes export `RUST_MIN_STACK=16777216`.
///
/// That is not a test-only concern. It makes the suite's green a statement
/// about the lane's environment rather than about the code, and when it
/// does break it breaks as `SIGABRT`, which kills the whole binary so every
/// later case silently never runs (harn#7962). Two cases in
/// `agent_loop_final_wrapup` and both cases in `workflow_replay_byte_compat`
/// abort this way once the ambient stack drops.
#[test]
fn vm_driving_tests_enter_through_the_contract_stack() {
let crates_dir = std::path::Path::new(env!("CARGO_MANIFEST_DIR"))
.parent()
.expect("harn-vm lives below crates");
let tests_dir = crates_dir.join("harn-vm").join("tests");
let mut offenders = Vec::new();
let mut scanned = 0usize;
for entry in walkdir::WalkDir::new(&tests_dir)
.into_iter()
.filter_map(Result::ok)
.filter(|entry| {
entry.file_type().is_file()
&& entry.path().extension().and_then(std::ffi::OsStr::to_str) == Some("rs")
})
{
scanned += 1;
let source = std::fs::read_to_string(entry.path()).expect("read Rust source");
if !source.contains(TEST_BUILDS_RUNTIME) || !source.contains(TEST_DRIVES_VM) {
continue;
}
if source.contains(ENTERS_CONTRACT) {
continue;
}
let relative = entry
.path()
.strip_prefix(crates_dir)
.expect("scan stays under crates")
.to_string_lossy()
.replace('\\', "/");
offenders.push(relative);
}
assert!(
scanned > 20,
"scan found only {scanned} test sources to check"
);
let unlisted: Vec<&String> = offenders
.iter()
.filter(|path| !HARNESS_STACK_BASELINE.contains(&path.as_str()))
.collect();
assert!(
unlisted.is_empty(),
"these tests build a Tokio runtime and execute a chunk on the libtest \
harness thread, so they drive the VM on a stack nobody sized and abort \
the whole test binary once the ambient stack drops. Wrap the entry \
point in `harn_vm::on_vm_stack(|| {{ .. }})`:\n {}",
unlisted
.iter()
.map(|path| path.as_str())
.collect::<Vec<_>>()
.join("\n ")
);
let stale: Vec<&str> = HARNESS_STACK_BASELINE
.into_iter()
.filter(|path| !offenders.iter().any(|found| found == path))
.collect();
assert!(
stale.is_empty(),
"these files are no longer offenders, so their rows in \
HARNESS_STACK_BASELINE allow something that no longer exists. Remove \
them; the ratchet only shrinks:\n {}",
stale.join("\n ")
);
}
}