internal/poll: readWriteLock should destroy the fd when there are no more references to it

The read lock in readWriteLock might be holding the last reference to
the fd. If the fd is closed while the read lock is still held, then the
fd will not be destroyed until the read lock is released and fd.destroy
is called.

The race windows is small and difficult to trigger on real workloads,
but it has been observed in CI.

Fixes #77609
Fixes #74754 (tentatively)

Change-Id: I17be2d23dced1258e1a986c7359e27a81fc51c53
Reviewed-on: https://go-review.googlesource.com/c/go/+/745601
Reviewed-by: Mark Freeman <markfreeman@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

1 files changed, 6 insertions, 2 deletions

diff --git a/src/internal/poll/fd_mutex.go b/src/internal/poll/fd_mutex.go
index aba22e9867..300628db79 100644
--- a/src/internal/poll/fd_mutex.go
+++ b/src/internal/poll/fd_mutex.go
@@ -269,7 +269,9 @@ func (fd *FD) readWriteLock() error {
 		return errClosing(fd.isFile)
 	}
 	if !fd.fdmu.rwlock(writeLock, waitLock) {
-		fd.fdmu.rwunlock(readlock) // unlock read lock acquired above
+		if fd.fdmu.rwunlock(readlock) {
+			fd.destroy()
+		}
 		return errClosing(fd.isFile)
 	}
 	return nil
@@ -286,7 +288,9 @@ func (fd *FD) tryReadWriteLock() (bool, error) {
 		return false, nil
 	}
 	if !fd.fdmu.rwlock(writeLock, tryLock) {
-		fd.fdmu.rwunlock(readlock) // unlock read lock acquired above
+		if fd.fdmu.rwunlock(readlock) {
+			fd.destroy()
+		}
 		if fd.closing() {
 			return false, errClosing(fd.isFile)
 		}