Fix lost-wakeup race on packageWaits in WaitForPackage - #3576
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peterebden merged 1 commit intoAug 17, 2026
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Concurrent callers waiting on the same unparsed package could both pass the Get(key) == nil check and both Set a fresh channel; the second Set overwrote the first, and LogParseResult closes only the channel currently in the map, so the first waiter slept forever and the build wedged with "N tasks left, 0 workers busy". Use AddOrGet, as pendingPackages and pendingTargets already do.
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Walks the workspace members and writes a BUILD file next to each: rust_library and rust_binary for the crate's products, rust_test for unit and integration tests, modules from the source tree, path dependencies mapped to member labels and registry dependencies to third_party aliases. Workspace-inherited edition and dependencies resolve through the root manifest. On a bare cargo repo it also scaffolds third_party/rust/BUILD, .plzconfig and plugins/BUILD, then chains the workspace's Cargo.lock into the existing lockfile import. Existing BUILD files are never overwritten; build scripts, optional features and renames are reported for follow-up. Proven end to end: a 40-crate cargo workspace imports, builds and tests green through the plugin. That repo also surfaced an upstream plz race (WaitForPackage lost-wakeup, wedged parse at scale) — root-caused, fix + regression test submitted as thought-machine/please#3576, noted in PARITY hardening. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
peterebden
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Ah yes, that sounds right; I think I found a similar thing in #3569 . Ended up changing Get completely, but this seems fine here.
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I've been building rust-rules, a Rust plugin for Please. While testing it on a repo with ~40 packages, each subincluding the plugin's build_defs, cold builds would intermittently hang at
N tasks left, 0 workers busy, parsing M BUILD files.The goroutine dump showed a single parser stuck in
subincludeTarget → WaitForPackage → waitOnChan. The cause is a race inWaitForPackage. Two parsers racing for the same unparsed package can both seepackageWaits.Get(key) == niland both callSet. The secondSetoverwrites the first thread's channel, andLogParseResultcloses only the channel currently in the map, so the first thread waits forever.The fix makes the insertion atomic with
AddOrGet, matchingpendingPackagesandpendingTargetsin the same file. One caller inserts the canonical channel and queues the parse; everyone else waits on that channel and is woken by the existing close.Added a regression test that races 32 callers against the package parse completing. It fails on master within a few iterations and passes with the fix, including under
-race.Reproduced on v17.27.0 and v17.31.2. Stock plz wedged 11 of 30 cold cycles on my machine; patched ran 30 clean.
plz test //src/core/...passes.