gh-111174: Fix crash in getbuffer() called repeatedly for empty BytesIO (GH-111210)

1 files changed, 4 insertions, 3 deletions

diff --git a/Modules/_io/bytesio.c b/Modules/_io/bytesio.c
index f307420..16b8ac6 100644
--- a/Modules/_io/bytesio.c
+++ b/Modules/_io/bytesio.c
@@ -126,12 +126,13 @@ unshare_buffer(bytesio *self, size_t size)
 static int
 resize_buffer(bytesio *self, size_t size)
 {
+    assert(self->buf != NULL);
+    assert(self->exports == 0);
+
     /* Here, unsigned types are used to avoid dealing with signed integer
        overflow, which is undefined in C. */
     size_t alloc = PyBytes_GET_SIZE(self->buf);
 
-    assert(self->buf != NULL);
-
     /* For simplicity, stay in the range of the signed type. Anyway, Python
        doesn't allow strings to be longer than this. */
     if (size > PY_SSIZE_T_MAX)
@@ -1074,7 +1075,7 @@ bytesiobuf_getbuffer(bytesiobuf *obj, Py_buffer *view, int flags)
             "bytesiobuf_getbuffer: view==NULL argument is obsolete");
         return -1;
     }
-    if (SHARED_BUF(b)) {
+    if (b->exports == 0 && SHARED_BUF(b)) {
         if (unshare_buffer(b, b->string_size) < 0)
             return -1;
     }