bug#61058: [PATCH v2] Fix asymetric mutex locking when joining thread.

[Olivier Dion <olivier.dion@polymtl.ca>]

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.

Fix: #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
---
 module/ice-9/threads.scm      |  4 +++-
 test-suite/tests/threads.test | 13 ++++++++++++-
 2 files changed, 15 insertions(+), 2 deletions(-)

diff --git a/module/ice-9/threads.scm b/module/ice-9/threads.scm
index 048d8b085..a1e43b9fa 100644
--- a/module/ice-9/threads.scm
+++ b/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
diff --git a/test-suite/tests/threads.test b/test-suite/tests/threads.test
index efdf36db2..af8c8c75b 100644
--- a/test-suite/tests/threads.test
+++ b/test-suite/tests/threads.test
@@ -332,7 +332,18 @@
 	      (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)
+                (join-thread other-thread 1)
+                #t)
+              (lambda ()
+                (cancel-thread other-thread))))))
 
       ;;
       ;; thread cancellation
-- 
2.41.0