Fix asymetric mutex locking when joining thread.

If `join-thread' timeout, the thread mutex is not unlocked, resulting in
deadlock to the next call to it or deadlock of the thread itself when it
terminates.

Thus, always unlock the mutex.

Fixes <https://bugs.gnu.org/55356>.

* module/ice-9/threads.scm (join-thread): Always unlock thread mutex.
* test-suite/tests/threads.test (join-thread): New test to ensure the
mutex is released.
* NEWS: Update.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>

NEWS

@@ -37,6 +37,8 @@ the compiler reports it as "possibly unused".
    (<https://bugs.gnu.org/64666>)
 ** Avoid module resolution in 'call-with-new-thread', which could deadlock
    (<https://bugs.gnu.org/62691>)
+** Fix deadlock in 'join-thread' when timeout is hit
+   (<https://bugs.gnu.org/55356>)
 ** 'read-u8' in (scheme base) now defaults to (current-input-port)
    (<https://bugs.gnu.org/62690>)
 ** Hashing of UTF-8 symbols with non-ASCII characters avoids corruption

module/ice-9/threads.scm

@@ -204,7 +204,9 @@ terminates, unless the target @var{thread} has already terminated."
              (wait-condition-variable cv mutex timeout)
              (wait-condition-variable cv mutex))
          (lp))
-        (else timeoutval))))))
+        (else
+         (unlock-mutex mutex)
+         timeoutval))))))
 
 (define* (try-mutex mutex)
   "Try to lock @var{mutex}.  If the mutex is already locked, return

test-suite/tests/threads.test

@@ -332,7 +332,21 @@
 	      (sleep 2)
 	      (system-async-mark aproc)
 	      (join-thread other-thread)))
-	  #t))
+	  #t)
+
+        (pass-if "do not throw exception if trying to join after timeout"
+          (let ((other-thread (begin-thread (pause))))
+            (dynamic-wind
+              (const #f)
+              (lambda ()
+                (join-thread other-thread 1)
+                ;; Up to 3.0.9, this second call would throw: "mutex
+                ;; already locked by thread".
+                ;; See <https://bugs.gnu.org/55356>.
+                (join-thread other-thread 1)
+                #t)
+              (lambda ()
+                (cancel-thread other-thread))))))
 
       ;;
       ;; thread cancellation