[perl #78494] Pipes cause threads to hang on join()

or on close() in either thread.

close() in one thread blocks until close() is called in the other
thread, because both closes are waiting for the child process to end.

Since we have a reference-counting mechanism for the underlying
fileno, we can use that to determine whether close() should wait.

This does not solve the problem of close $OUT block when it has been
duplicated via open $OUT2, ">&" and $OUT2 is still in scope.

1 files changed, 4 insertions, 0 deletions

diff --git a/pod/perlfunc.pod b/pod/perlfunc.pod
index d85b3d7fd0..2047dd6047 100644
--- a/pod/perlfunc.pod
+++ b/pod/perlfunc.pod
@@ -910,6 +910,10 @@ on the pipe to exit--in case you wish to look at the output of the pipe
 afterwards--and implicitly puts the exit status value of that command into
 C<$?> and C<${^CHILD_ERROR_NATIVE}>.
 
+If there are multiple threads running, C<close> on a filehandle from a
+piped open returns true without waiting for the child process to terminate,
+if the filehandle is still open in another thread.
+
 Closing the read end of a pipe before the process writing to it at the
 other end is done writing results in the writer receiving a SIGPIPE.  If
 the other end can't handle that, be sure to read all the data before