gh-123321: Fix Parser/myreadline.c to prevent a segfault during a multi-threaded race (#123323)

1 files changed, 10 insertions, 3 deletions

diff --git a/Parser/myreadline.c b/Parser/myreadline.c
index 1825665..6eab56a 100644
--- a/Parser/myreadline.c
+++ b/Parser/myreadline.c
@@ -392,9 +392,14 @@ PyOS_Readline(FILE *sys_stdin, FILE *sys_stdout, const char *prompt)
         }
     }
 
-    _PyOS_ReadlineTState = tstate;
     Py_BEGIN_ALLOW_THREADS
+
+    // GH-123321: We need to acquire the lock before setting
+    // _PyOS_ReadlineTState and after the release of the GIL, otherwise
+    // the variable may be nullified by a different thread or a deadlock
+    // may occur if the GIL is taken in any sub-function.
     PyThread_acquire_lock(_PyOS_ReadlineLock, 1);
+    _PyOS_ReadlineTState = tstate;
 
     /* This is needed to handle the unlikely case that the
      * interpreter is in interactive mode *and* stdin/out are not
@@ -418,11 +423,13 @@ PyOS_Readline(FILE *sys_stdin, FILE *sys_stdout, const char *prompt)
     else {
         rv = (*PyOS_ReadlineFunctionPointer)(sys_stdin, sys_stdout, prompt);
     }
-    Py_END_ALLOW_THREADS
 
+    // gh-123321: Must set the variable and then release the lock before
+    // taking the GIL. Otherwise a deadlock or segfault may occur.
+    _PyOS_ReadlineTState = NULL;
     PyThread_release_lock(_PyOS_ReadlineLock);
 
-    _PyOS_ReadlineTState = NULL;
+    Py_END_ALLOW_THREADS
 
     if (rv == NULL)
         return NULL;