Warn on invalid jobserver file descriptors#16843
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weihanglo merged 2 commits intorust-lang:masterfrom Apr 7, 2026
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On Windows jobserver crate uses named semaphore so the error message is a bit different:
[99](https://github.com/rust-lang/cargo/actions/runs/24051846505/job/70149149235?pr=16843#step:15:5100)
---- expected: tests\testsuite\jobserver.rs:424:27
++++ actual: stderr
Warning: [0m - [WARNING] failed to connect to jobserver from environment variable `MAKEFLAGS="--jobserver-auth=200001,200002"`: cannot open file descriptor 200001 from the jobserver environment variable value: [..]
1 + [WARNING] failed to connect to jobserver from environment variable `MAKEFLAGS="--jobserver-auth=200001,200002"`: cannot open path or name 200001,200002 from the jobserver environment variable value: [NOT_FOUND]
2 2 | [COMPILING] foo v0.0.1 ([ROOT]/foo)
3 3 | [FINISHED] `dev` profile [unoptimized + debuginfo] target(s) in [ELAPSED]s
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Cargo submodule update 11 commits in a357df4c26fc14514e66aae2a269456b5545c7db..101549dddbd2b08e806f50154e3aa4cb3374cc21 2026-04-03 16:47:15 +0000 to 2026-04-08 12:51:20 +0000 - Never include use extra-filename in build scripts (rust-lang/cargo#16855) - fix(toml): Force script edition warnings on quiet (rust-lang/cargo#16848) - GitHub fast path uses `http_async` (rust-lang/cargo#16847) - feat(manifest): allow git dependency alongside alternate registry (rust-lang/cargo#16810) - fix(auth): add auth scheme hint to token rejected error for alt registries (rust-lang/cargo#16794) - Warn on invalid jobserver file descriptors (rust-lang/cargo#16843) - docs(unstable): List the minimum required MSRV for 'public' field (rust-lang/cargo#16841) - feat(lints): Emit unused_dependencies lint (rust-lang/cargo#16600) - fix(tree): clarify error message when `-i` is used without a package name (rust-lang/cargo#16818) - fix: Typo in target.<cfg>.linker (rust-lang/cargo#16839) - Send Content-Type header with cargo publish requests (rust-lang/cargo#16832) r? ghost
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What does this PR try to resolve?
Cargo will currently load a jobserver from file descriptors if
MAKEFLAGS,MFLAGS,RUST_MAKEFLAGSenvironment variables contain--jobserver-auth=N,N.The jobserver crate provides a method to attempt to validate whether these descriptors are actually pipes, which cargo was not using. This change enables the validation, and emits a warning if the file descriptors appear to be incorrect.
This helps avoid hard-to-debug problems where cargo interprets FDs as jobserver FDs, when they are actually not. This can occur when a process in between
makeandcargoforwards the environment variables, but not the FDs.How to test and review this PR?
This change is based on what rustc does.
First commit shows issue,
Second commit contains fix with warning showing.
r? @weihanglo