Fix Unix SessionId in handshake to enable cross-terminal node reuse#13354
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rainersigwald merged 1 commit intodotnet:mainfrom Mar 10, 2026
Merged
Fix Unix SessionId in handshake to enable cross-terminal node reuse#13354rainersigwald merged 1 commit intodotnet:mainfrom
rainersigwald merged 1 commit intodotnet:mainfrom
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Pull request overview
This PR updates MSBuild’s node handshake behavior on Unix-like platforms to avoid terminal-specific session IDs, enabling out-of-proc nodes started in one terminal to be reused by builds started from another terminal.
Changes:
- Populate the handshake
SessionIdonly on Windows; use0on Unix-like platforms to avoid per-terminal session leader PID differences. - Add Unix-only unit tests intended to validate the updated handshake behavior.
Reviewed changes
Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.
| File | Description |
|---|---|
src/Shared/CommunicationsUtilities.cs |
Gates handshake SessionId retrieval to Windows to enable cross-terminal node reuse on Unix-like OSes. |
src/Build.UnitTests/BackEnd/UnixNodeReuseFixes_Tests.cs |
Adds Unix-only tests around handshake key/session-id behavior (needs adjustment to provide meaningful coverage and avoid brittle string parsing). |
This was referenced Mar 10, 2026
On Unix, Process.SessionId returns the session leader PID (from getsid()), which differs per terminal. This prevents nodes launched from one terminal from being reused by builds in another terminal. On Windows, SessionId correctly differentiates RDP sessions. Fix: Use SessionId=0 on non-Windows platforms, since Unix doesn't need RDP-style session isolation. This restores effective node reuse across terminals on macOS and Linux. Tests use [UnixOnlyFact] and directly assert SessionId=0 in the key. Note: The MSBuildTaskHost copy (src/MSBuildTaskHost/) has the same pattern but only runs on .NET Framework/Windows, so it's not affected.
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rainersigwald
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baronfel
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This was referenced Mar 10, 2026
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Problem
On Unix,
Process.SessionIdreturns the session leader PID (fromgetsid()), which differs per terminal. This prevents MSBuild nodes launched from one terminal from being reused by builds in another terminal — each terminal effectively gets its own set of idle nodes.Fix
Use
SessionId=0on non-Windows platforms. Unix doesn't need RDP-style session isolation, so 0 is safe and enables cross-terminal node reuse.Tests
Handshake_OnUnix_SessionIdIsZero— asserts the SessionId component is 0 in the keyHandshake_OnUnix_MatchesAcrossInstances— asserts matching keys across instancesBoth use
[UnixOnlyFact]per review feedback (no silent pass on Windows).Note: The MSBuildTaskHost copy (
src/MSBuildTaskHost/) usesProcess.SessionIddirectly but only runs on .NET Framework/Windows, so it's unaffected.Split from #13336 per reviewer request.