From e04ec66a2713e119e535e98f3f30a4e1f2bb6540 Mon Sep 17 00:00:00 2001
From: Victor Stinner <vstinner@python.org>
Date: Fri, 1 Oct 2021 10:29:13 +0200
Subject: [PATCH] bpo-41710: Fix PY_TIMEOUT_MAX value on Windows

Fix _thread.TIMEOUT_MAX value on Windows: the maximum timeout is
0x7FFFFFFF milliseconds (around 24.9 days), not 0xFFFFFFFF
milliseconds (around 49.7 days).

Set PY_TIMEOUT_MAX to 0x7FFFFFFF milliseconds, rather than 0xFFFFFFFF
milliseconds.
---
 Include/pythread.h                                   |  8 +++++---
 .../Library/2021-09-30-08-57-50.bpo-41710.JMsPAW.rst |  3 +++
 Python/thread_nt.h                                   | 12 +++++++++---
 3 files changed, 17 insertions(+), 6 deletions(-)
 create mode 100644 Misc/NEWS.d/next/Library/2021-09-30-08-57-50.bpo-41710.JMsPAW.rst

diff --git a/Include/pythread.h b/Include/pythread.h
index bb9d86412218ad..cf4cc9a7473f1a 100644
--- a/Include/pythread.h
+++ b/Include/pythread.h
@@ -61,9 +61,11 @@ PyAPI_FUNC(int) _PyThread_at_fork_reinit(PyThread_type_lock *lock);
       convert microseconds to nanoseconds. */
 #  define PY_TIMEOUT_MAX (LLONG_MAX / 1000)
 #elif defined (NT_THREADS)
-   /* In the NT API, the timeout is a DWORD and is expressed in milliseconds */
-#  if 0xFFFFFFFFLL * 1000 < LLONG_MAX
-#    define PY_TIMEOUT_MAX (0xFFFFFFFFLL * 1000)
+   /* In the NT API, the timeout is a DWORD and is expressed in milliseconds,
+    * a positive number between 0 and 0x7FFFFFFF (see WaitForSingleObject()
+    * documentation). */
+#  if 0x7FFFFFFFLL * 1000 < LLONG_MAX
+#    define PY_TIMEOUT_MAX (0x7FFFFFFFLL * 1000)
 #  else
 #    define PY_TIMEOUT_MAX LLONG_MAX
 #  endif
diff --git a/Misc/NEWS.d/next/Library/2021-09-30-08-57-50.bpo-41710.JMsPAW.rst b/Misc/NEWS.d/next/Library/2021-09-30-08-57-50.bpo-41710.JMsPAW.rst
new file mode 100644
index 00000000000000..516214a619e8eb
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2021-09-30-08-57-50.bpo-41710.JMsPAW.rst
@@ -0,0 +1,3 @@
+Fix :data:`_thread.TIMEOUT_MAX` value on Windows: the maximum timeout is
+0x7FFFFFFF milliseconds (around 24.9 days), not 0xFFFFFFFF milliseconds (around
+49.7 days).
diff --git a/Python/thread_nt.h b/Python/thread_nt.h
index 0ce5e94f89bf72..05ade1946aa373 100644
--- a/Python/thread_nt.h
+++ b/Python/thread_nt.h
@@ -295,6 +295,10 @@ PyThread_free_lock(PyThread_type_lock aLock)
     FreeNonRecursiveMutex(aLock) ;
 }
 
+// WaitForSingleObject() documentation: "The time-out value needs to be a
+// positive number between 0 and 0x7FFFFFFF." INFINITE is equal to 0xFFFFFFFF.
+const DWORD TIMEOUT_MS_MAX = 0x7FFFFFFF;
+
 /*
  * Return 1 on success if the lock was acquired
  *
@@ -312,9 +316,11 @@ PyThread_acquire_lock_timed(PyThread_type_lock aLock,
 
     if (microseconds >= 0) {
         milliseconds = microseconds / 1000;
-        if (microseconds % 1000 > 0)
-            ++milliseconds;
-        if (milliseconds > PY_DWORD_MAX) {
+        // Round milliseconds away from zero
+        if (microseconds % 1000 > 0) {
+            milliseconds++;
+        }
+        if (milliseconds > (PY_TIMEOUT_T)TIMEOUT_MS_MAX) {
             Py_FatalError("Timeout larger than PY_TIMEOUT_MAX");
         }
     }