This fix (across 4 files in 3 directories) solves a subtle problem with

gvanrossum · gvanrossum · commit 359bcaa539d9 · 1997-11-14T22:24:28.000Z
signal handlers in a fork()ed child process when Python is compiled with
thread support.  The bug was reported by Scott &lt;scott@chronis.icgroup.com&gt;.

What happens is that after a fork(), the variables used by the signal
module to determine whether this is the main thread or not are bogus,
and it decides that no thread is the main thread, so no signals will
be delivered.

The solution is the addition of PyOS_AfterFork(), which fixes the signal
module's variables.  A dummy version of the function is present in the
intrcheck.c source file which is linked when the signal module is not
used.
diff --git a/Include/intrcheck.h b/Include/intrcheck.h
@@ -37,6 +37,7 @@ PERFORMANCE OF THIS SOFTWARE.
 
 extern int PyOS_InterruptOccurred Py_PROTO((void));
 extern void PyOS_InitInterrupts Py_PROTO((void));
+void PyOS_AfterFork Py_PROTO((void));
 
 #ifdef __cplusplus
 }
diff --git a/Modules/posixmodule.c b/Modules/posixmodule.c
@@ -1082,6 +1082,7 @@ posix_fork(self, args)
 	pid = fork();
 	if (pid == -1)
 		return posix_error();
+	PyOS_AfterFork();
 	return PyInt_FromLong((long)pid);
 }
 #endif
diff --git a/Modules/signalmodule.c b/Modules/signalmodule.c
@@ -613,3 +613,12 @@ PyOS_InterruptOccurred()
 	}
 	return 0;
 }
+
+void
+PyOS_AfterFork()
+{
+#ifdef WITH_THREAD
+	main_thread = get_thread_ident();
+	main_pid = getpid();
+#endif
+}
diff --git a/Parser/intrcheck.c b/Parser/intrcheck.c
@@ -219,3 +219,8 @@ PyOS_InterruptOccurred()
 }
 
 #endif /* !OK */
+
+void
+PyOS_AfterFork()
+{
+}

Original file line number	Diff line number	Diff line change
`@@ -37,6 +37,7 @@ PERFORMANCE OF THIS SOFTWARE.`
`37`	`37`
`38`	`38`	`extern int PyOS_InterruptOccurred Py_PROTO((void));`
`39`	`39`	`extern void PyOS_InitInterrupts Py_PROTO((void));`
	`40`	`+void PyOS_AfterFork Py_PROTO((void));`
`40`	`41`
`41`	`42`	`#ifdef __cplusplus`
`42`	`43`	`}`
Original file line number	Diff line number	Diff line change
`@@ -1082,6 +1082,7 @@ posix_fork(self, args)`
`1082`	`1082`	`pid = fork();`
`1083`	`1083`	`if (pid == -1)`
`1084`	`1084`	`return posix_error();`
	`1085`	`+ PyOS_AfterFork();`
`1085`	`1086`	`return PyInt_FromLong((long)pid);`
`1086`	`1087`	`}`
`1087`	`1088`	`#endif`
Original file line number	Diff line number	Diff line change
`@@ -613,3 +613,12 @@ PyOS_InterruptOccurred()`
`613`	`613`	`}`
`614`	`614`	`return 0;`
`615`	`615`	`}`
	`616`	`+`
	`617`	`+void`
	`618`	`+PyOS_AfterFork()`
	`619`	`+{`
	`620`	`+#ifdef WITH_THREAD`
	`621`	`+ main_thread = get_thread_ident();`
	`622`	`+ main_pid = getpid();`
	`623`	`+#endif`
	`624`	`+}`
Original file line number	Diff line number	Diff line change
`@@ -219,3 +219,8 @@ PyOS_InterruptOccurred()`
`219`	`219`	`}`
`220`	`220`
`221`	`221`	`#endif /* !OK */`
	`222`	`+`
	`223`	`+void`
	`224`	`+PyOS_AfterFork()`
	`225`	`+{`
	`226`	`+}`