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ExecuteAsync hangs forever (and leaves the child blocked) when the output handler throws or a strict encoding hits a decode error #89
Description
What's wrong
When a read faults, AsyncProcessStreamReader.Start breaks out of its read loop (RunCommand/AsyncProcessStreamReader.cs:74-77) and stops reading both pipes. A read faults when the handler callback throws, or when the handler's Encoding uses throwOnInvalidBytes: true, a case the code explicitly supports ("carries a decode error out to the caller").
RunCommand.cs:484-485 then awaits:
await Task.WhenAll(outputReader.Start(ct), process.WaitForExitAsync(ct));WhenAll doesn't complete until the process exits, and nothing kills the process. Once the child writes more than the pipe buffer holds (≈64 KB on Linux, less on Windows), it blocks on write() forever. The call never returns, and the exception is never surfaced.
Reproduction (Linux, net10.0)
new OutputHandler(s => throw new InvalidOperationException())withsh -c "echo first; sleep 0.5; head -c 1000000 /dev/zero"→ the task is still pending after 10 s, and theheadprocess is still alive.encoding: new UTF8Encoding(false, throwOnInvalidBytes: true)withsh -c "printf '\377\n'; sleep 0.5; head -c 1000000 /dev/zero"→ the task is still pending after 10 s.
Suggested fix
In RunAsync, observe the reader task on its own. If it faults, call the existing TryKill(process) (killing the whole process tree), then await the exit and rethrow the read exception. For example:
Task read = outputReader.Start(ct);
try { await read; } catch { TryKill(process); throw; }
await process.WaitForExitAsync(ct);Acceptance criteria
Both repros throw the handler or decode exception promptly (well under a second after the fault), and no child process is left running.
Activity
matt-edmondson commentedon Sep 26, 2026
Triage
ExecuteAsyncnever returns, the exception that caused it is never seen, and the child process is left blocked on a full pipe. Two triggers cause it: a handler that throws, and a strictEncoding, which the code documents as supported. There is no workaround short of making sure the handler never throws.RunAsynccompletion (RunCommand.cs:484-485) and the fault path inAsyncProcessStreamReader.Start. Maintainer: @matt-edmondson.OutputHandlercompletion contract.Startinto aReadToEndloop for each stream, and awaits both throughDrainOrAbandon. Once one stream's loop has faulted, nothing reads that pipe any more. The child can still block onwrite(), andWaitForExitAsynccan still wait forever. So Read each output stream to end of stream, not just until exit #87 is unlikely to fix this on its own, and it conflicts with any fix made here.Next steps: Land #87 first, then build this fix on its
ReadToEndstructure. When a read loop faults, callTryKill(process)so the whole process tree is killed, then await exit and rethrow the original exception. Add both repros as tests, with a timeout, and assert that no child process is still alive afterwards.Generated by Claude Code