From 455ee49f5573baa1bc5237a8d49083ce588a13ee Mon Sep 17 00:00:00 2001
From: Olivier Dion <olivier.dion@polymtl.ca>
Date: Thu, 25 Jan 2024 16:45:47 -0500
Subject: [PATCH] Fix asymetric mutex locking when joining thread.
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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                          |  2 ++
 module/ice-9/threads.scm      |  4 +++-
 test-suite/tests/threads.test | 16 +++++++++++++++-
 3 files changed, 20 insertions(+), 2 deletions(-)

diff --git a/NEWS b/NEWS
index 57fc6f776..0bfbd1dd8 100644
--- a/NEWS
+++ b/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
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..fa89deeb2 100644
--- a/test-suite/tests/threads.test
+++ b/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