Enable StartSuspended for all Unix platforms by Copilot · Pull Request #130347 · dotnet/runtime

Copilot · 2026-07-08T10:07:49Z

Extends ProcessStartInfo.StartSuspended and SafeProcessHandle.Resume() to work on all Unix platforms, not just macOS. Previously these APIs threw PlatformNotSupportedException on Linux and other Unix systems.

Approved API: #94127 (comment)

Implementation approach:

On non-macOS Unix, the child process calls raise(SIGSTOP) before execve, suspending itself until Resume() sends SIGCONT
The exec-waiting pipe is closed before stopping so the parent can return the child PID without blocking
fork() is used instead of vfork() when startSuspended is true (child must stop before exec, incompatible with vfork's shared-memory semantics)

Changes:

Ref assembly: Removed [SupportedOSPlatform("windows")] / [SupportedOSPlatform("macos")] from Resume() and StartSuspended
SafeProcessHandle.cs: Removed platform gate and PlatformNotSupportedException from Resume()
ProcessStartInfo.cs: Removed [SupportedOSPlatform] attributes and platform validation check
pal_process.c: Removed ENOTSUP early-return; added fork-over-vfork selection when suspended; added pipe-close + raise(SIGSTOP) before execve
Tests: Removed [PlatformSpecific] restriction and the StartSuspendedTests_NonWindowsNonMacOS class that tested the now-removed exception

- Remove [SupportedOSPlatform] attributes from StartSuspended and Resume() - Remove platform check in SafeProcessHandle.Resume() and ProcessStartInfo validation - In native pal_process.c: remove ENOTSUP check, use fork() instead of vfork() when startSuspended is true, and raise(SIGSTOP) before execve - Close the exec-waiting pipe before SIGSTOP so parent can proceed - Update tests: remove PlatformSpecific attribute and non-supported test class Co-authored-by: adamsitnik <6011991+adamsitnik@users.noreply.github.com>

dotnet-policy-service · 2026-07-08T10:29:22Z

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Copilot

Pull request overview

This PR expands ProcessStartInfo.StartSuspended and SafeProcessHandle.Resume() beyond macOS by implementing “start suspended” on non-macOS Unix using a pre-execve self-stop (SIGSTOP) and resuming via SIGCONT, and updates the public contract/tests accordingly.

Changes:

Updates native Unix process creation to support startSuspended in the fork/exec path (including fork-over-vfork selection).
Removes platform gating/attributes so StartSuspended / Resume() are no longer limited to Windows+macOS.
Broadens the existing StartSuspended test coverage beyond Windows/macOS and removes now-obsolete “throws PNSE” tests.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/native/libs/System.Native/pal_process.c	Implements non-macOS Unix StartSuspended via `SIGSTOP` before `execve` and switches to `fork()` when suspended.
src/libraries/System.Diagnostics.Process/tests/StartSuspendedTests.cs	Removes Windows/macOS-only restriction and deletes tests expecting PNSE on other Unix OSes.
src/libraries/System.Diagnostics.Process/src/System/Diagnostics/ProcessStartInfo.cs	Updates docs and removes runtime platform validation for `StartSuspended`.
src/libraries/System.Diagnostics.Process/src/Microsoft/Win32/SafeHandles/SafeProcessHandle.cs	Removes platform gate/PNSE from `Resume()` and updates remarks to “Unix”.
src/libraries/System.Diagnostics.Process/ref/System.Diagnostics.Process.cs	Updates ref contract by removing Windows/macOS-only platform annotations for `Resume()` and `StartSuspended`.

adamsitnik

@copilot please address the feedback

…, immediate-resume test - pal_process.c: add waitpid(WUNTRACED) in parent after pipe close for startSuspended to eliminate SIGCONT/SIGSTOP race condition - ProcessStartInfo.cs: clarify in docs that on non-macOS Unix suspension occurs before execve (process image not yet replaced) - StartSuspendedTests.cs: add StartSuspended_ResumeImmediately_Succeeds test that starts suspended and immediately calls Resume Co-authored-by: adamsitnik <6011991+adamsitnik@users.noreply.github.com>

- Remove redundant processId > 0 guard (success implies positive PID) - Separate WIFEXITED/WIFSIGNALED cases with clearer ECHILD error and comment Co-authored-by: adamsitnik <6011991+adamsitnik@users.noreply.github.com>

Copilot

Pull request overview

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

adamsitnik

LGTM, @copilot please apply my suggestion

…ucceeds test Co-authored-by: adamsitnik <6011991+adamsitnik@users.noreply.github.com>

adamsitnik

It LGTM!

@tmds could you please review it? So far we got the Windows and macOS (#129570) implementations merged, where this is a native concept.

tmds · 2026-07-13T09:03:04Z

@adamsitnik does this suspend implementation make sense? Are the semantics sufficiently similar to Windows? You are between fork and exec, so what is suspended is not the actual process.

adamsitnik · 2026-07-13T14:03:57Z

@adamsitnik does this suspend implementation make sense? Are the semantics sufficiently similar to Windows? You are between fork and exec, so what is suspended is not the actual process.

My goal is to make it as close as possible to POSIX_SPAWN_START_SUSPENDED (macOS). From https://keith.github.io/xcode-man-pages/posix_spawnattr_setflags.3.html:

POSIX_SPAWN_START_SUSPENDED: If this bit is set, then the child process will be created as if it immediately received a SIGSTOP signal, permitting debuggers, profilers, and other programs to manipulate the process before it begins execution in user space. This permits, for example, obtaining exact instruction counts, or debugging very early in dyld(1). To resume the child process, it must be sent a SIGCONT signal.

We suspend the process via SIGSTOP before any user code executes. The provided PID is also valid. SIGCONT is used to resume.

@tmds do you see any scenarios where it would fail?

tmds · 2026-07-13T14:19:04Z

The implementation looks mechanically correct, but I think what it ends up doing may not enable the same things as it does on Windows and macOS.

For example, if you'd attach a debugger on this suspended process, the target is not the application you want to debug. You can't set breakpoints, etc.

So my question is whether this enables what the users of the API are likely expecting.

tmds · 2026-07-15T05:51:56Z

My goal is to make it as close as possible to POSIX_SPAWN_START_SUSPENDED (macOS).

This one suspends after the exec, which is a major difference with what this implementation does.

To implement this on Linux, it seems we can use PTRACE_TRACEME (see https://man7.org/linux/man-pages/man2/ptrace.2.html).

adamsitnik · 2026-07-17T17:25:22Z

This one suspends after the exec, which is a major difference with what this implementation does.

I agree.

To implement this on Linux, it seems we can use PTRACE_TRACEME (see https://man7.org/linux/man-pages/man2/ptrace.2.html).

But this would have some side effects that could also be not expected.

I think I am going to close this PR as of now and simply wait for more user feedback. Perhaps people would be happy with what I am suggesting here or PTRACE_TRACEME would be better.

MattParkerDev · 2026-07-17T23:49:18Z

Why not use posix_spawn on Linux too?

MichalPetryka · 2026-07-18T06:38:02Z

Why not use posix_spawn on Linux too?

As per google AI: POSIX_SPAWN_START_SUSPENDED is an Apple-specific extension to the POSIX spawn API, supported on macOS, iOS, and other Darwin-based platforms.

Copilot AI assigned Copilot and adamsitnik Jul 8, 2026

Copilot AI requested review from adamsitnik and Copilot and removed request for Copilot July 8, 2026 10:07

Copilot finished work on behalf of adamsitnik July 8, 2026 10:08

Copilot created this pull request from a session on behalf of adamsitnik July 8, 2026 10:08 View session

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Comment thread src/native/libs/System.Native/pal_process.c

Comment thread src/libraries/System.Diagnostics.Process/src/System/Diagnostics/ProcessStartInfo.cs Outdated

Comment thread src/libraries/System.Diagnostics.Process/tests/StartSuspendedTests.cs

adamsitnik reviewed Jul 8, 2026

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Comment thread src/libraries/System.Diagnostics.Process/src/System/Diagnostics/ProcessStartInfo.cs Outdated

Comment thread src/native/libs/System.Native/pal_process.c

Comment thread src/libraries/System.Diagnostics.Process/tests/StartSuspendedTests.cs

Copilot started work on behalf of adamsitnik July 8, 2026 11:37 View session

Copilot AI requested review from Copilot and removed request for Copilot July 8, 2026 12:03

Refine waitpid error handling in startSuspended path

edf0452

- Remove redundant processId > 0 guard (success implies positive PID) - Separate WIFEXITED/WIFSIGNALED cases with clearer ECHILD error and comment Co-authored-by: adamsitnik <6011991+adamsitnik@users.noreply.github.com>

Copilot AI requested review from adamsitnik and Copilot and removed request for Copilot July 8, 2026 12:06

Copilot finished work on behalf of adamsitnik July 8, 2026 12:07

adamsitnik requested a review from Copilot July 8, 2026 13:41

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Comment thread src/native/libs/System.Native/pal_process.c

adamsitnik marked this pull request as ready for review July 8, 2026 13:59

adamsitnik approved these changes Jul 8, 2026

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Comment thread src/libraries/System.Diagnostics.Process/tests/StartSuspendedTests.cs Outdated

Copilot started work on behalf of adamsitnik July 8, 2026 14:03 View session

Apply using disposal to Process in StartSuspended_ResumeImmediately_S…

15443b5

…ucceeds test Co-authored-by: adamsitnik <6011991+adamsitnik@users.noreply.github.com>

Copilot AI requested review from Copilot and removed request for Copilot July 8, 2026 14:09

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adamsitnik requested a review from tmds July 8, 2026 16:44

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adamsitnik closed this Jul 17, 2026

adamsitnik mentioned this pull request Jul 17, 2026

[API Proposal]: ProcessStartInfo.CreateSuspended, Process.Resume #94127

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