[astuyve]

What

MaxExecutionTime didn't bound Run while a builtin waited on an *os.File stdin pipe that never sends, which is the setup the docs recommend. 13 builtins hung forever: cat, head, tail, grep, wc, sort, uniq, tr, sed, cut, strings, xargs and jq. Only read, awk and sha256sum had their own way to wake up at the deadline.

Run now binds the runner's stdin to the run's context once (interp/stdin_deadline.go). When the context ends, a watchdog sets an already-expired read deadline on the file, which wakes any blocked Read with os.ErrDeadlineExceeded.

• The file is never closed and no bytes are read on the caller's behalf.
• The deadline is cleared when Run returns.
• Files without read deadlines (regular files, /dev/null) are left alone.

Limits, also noted in SHELL_FEATURES.md:

• Windows: anonymous pipes there don't support read deadlines, so a blocked stdin read can still delay return.
• Other stdin types: a stdin that isn't an *os.File isn't covered.
• Existing deadline: a read deadline the caller already had on stdin is cleared.

Credit

Found by Jacob Hayes with his deterministic-simulation tool patina (finding #5: a never-sending pipe leg plus a never-returns detector, about 140 runs). Findings thread: https://dd.slack.com/archives/D0AC8RLB21J/p1793475631273749

Tests

interp/stdin_pipe_deadline_test.go:

• TestMaxExecutionTimeBoundsBuiltinBlockedOnStdinPipe: each of the 16 stdin-reading builtins gets an idle os.Pipe stdin and a 200ms MaxExecutionTime, and must return context.DeadlineExceeded within 5s. On main exactly the 13 reported builtins fail.
• TestStdinPipeDeadlineDoesNotLeakOrStealBytes: after a normal run no deadline is left on the pipe and later bytes are still readable.

On Windows the test skips the 13 cases that rely on a read deadline (the limit above); read, awk and sha256sum still run there. Passes with -race. GOOS=windows/darwin go vet ./interp/ is clean. go test ./... passes except TestPSPentestLargePIDListDoesNotHang in builtins/ps, which fails the same way on clean main on this host.

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