Avoid race condition in hDuplicateTo
In our codebase we have some code along the lines of
newStdout <- hDuplicate stdout stderr `hDuplicateTo` stdout
to avoid stray
putStrLns from corrupting a protocol (LSP) that is
run over stdout.
On CI we have seen a bunch of issues where
this fails with
ResourceExhausted in Haskell.
I’ve spent some time looking at the docs for
dup2 and the code in
base and afaict the following race condition is being triggered
- The user calls
hDuplicateTo stderr stdout.
hClose_help stdout_, this closes the file handle for stdout.
- The file handle for stdout is now free, so another thread allocating a file might get stdout.
dup2is called while
stdout(now pointing to something else) is half-open, it returns EBUSY.
I think there might actually be an even worse case where
dup2 is run
after FD 1 is fully open again. In that case, you will end up not just
redirecting the original stdout to stderr but also the whatever
resulted in that file handle being allocated.
As far as I can tell,
dup2 takes care of closing the file handle
itself so there is no reason to do this in
hDuplicateTo. So this PR
replaces the call to
hClose_help by the only part of
that we actually care about, namely,
I tested this on our codebase fairly extensively and haven’t been able to reproduce the issue with this patch.
I’ve looked through the commit history but the commit that introduced it 7b067f2d is over 10 years old and doesn’t have any information on why the
hClose call was added originally.