gh-159191: Add a fast path to bytes.join() for exact bytes items

Lib/test/test_bytes.py

@@ -644,6 +644,7 @@ def test_join(self):
             self.assertEqual(self.type2test(b"").join(lst), b"abc")
             self.assertEqual(self.type2test(b"").join(tuple(lst)), b"abc")
             self.assertEqual(self.type2test(b"").join(iter(lst)), b"abc")
+        self.assertEqual(self.type2test(b".").join([b"ab", b"cd"]), b"ab.cd")
         dot_join = self.type2test(b".:").join
         self.assertEqual(dot_join([b"ab", b"cd"]), b"ab.:cd")
         self.assertEqual(dot_join([memoryview(b"ab"), b"cd"]), b"ab.:cd")
@@ -671,12 +672,13 @@ def test_join_concurrent_buffer_mutation(self):
         # mutate the joined sequence (simulated here by mutating in __buffer__).
         # See: https://github.com/python/cpython/issues/151295
         def make_seq(mutate):
-            # Item is only referenced from the list slot, so mutate() frees it.
+            # The first two items are only referenced from their list slots,
+            # so mutate() frees them.
             class Item:
                 def __buffer__(self, flags):
                     mutate(seq)
                     return memoryview(b'x')
-            seq = [b'a', Item(), b'c']
+            seq = [bytes(2), Item(), b'c']
             return seq
 
         for sep in (self.type2test(b''), self.type2test(b'::')):
@@ -686,11 +688,11 @@ def __buffer__(self, flags):
                 self.assertRaises(RuntimeError, sep.join, seq)
 
                 # The list length is unchanged, so the size-change recheck
-                # cannot fire: only keeping the item alive avoids the crash.
+                # cannot fire: only keeping the items alive avoids the crash.
                 def replace(seq):
-                    seq[1] = b'z'
+                    seq[0] = seq[1] = b'z'
                 seq = make_seq(replace)
-                self.assertEqual(sep.join(seq), sep.join([b'a', b'x', b'c']))
+                self.assertEqual(sep.join(seq), sep.join([bytes(2), b'x', b'c']))
 
     def test_count(self):
         b = self.type2test(b'mississippi')

Lib/test/test_free_threading/test_bytes_object.py

@@ -54,6 +54,27 @@ def reader():
 
         threading_helper.run_concurrently([writer] + [reader] * 4)
 
+    def test_racing_join_replace_large(self):
+        # join() suspends the critical section while it copies a result
+        # of 1 MiB or more, so the list can drop its items meanwhile.
+        size = 1 << 18
+        lst = [bytes(size) for _ in range(8)]
+        done = Event()
+
+        def writer():
+            try:
+                for _ in range(100):
+                    for i in range(len(lst)):
+                        lst[i] = bytes(size)
+            finally:
+                done.set()
+
+        def reader():
+            while not done.is_set():
+                self.assertEqual(b''.join(lst), bytes(size * len(lst)))
+
+        threading_helper.run_concurrently([writer] + [reader] * 4)
+
 
 if __name__ == "__main__":
     unittest.main()

Misc/NEWS.d/next/Core_and_Builtins/2026-10-08-17-36-53.gh-issue-159191.4YpkIS.rst

@@ -0,0 +1,3 @@
+Speed up :meth:`bytes.join` and :meth:`bytearray.join` when all items are
+:class:`bytes`. Patch by Pieter Eendebak and Christian Aurich Zanettini
+Martins.

Objects/stringlib/join.h

@@ -34,6 +34,44 @@ STRINGLIB(bytes_join_lock_held)(PyObject *sep, PyObject *seq)
         }
     }
 #endif
+
+    /* Fast path: all items are exact bytes and the result is too small to
+     * release the GIL for.  Nothing here runs Python code or suspends the
+     * critical section, so seq keeps the items alive and they can be copied
+     * without taking references. */
+    Py_ssize_t limit = GIL_THRESHOLD - seplen;
+    for (i = 0; i < seqlen; i++) {
+        item = PySequence_Fast_GET_ITEM(seq, i);
+        if (!PyBytes_CheckExact(item)
+            || PyBytes_GET_SIZE(item) >= limit - sz) {
+            break;
+        }
+        sz += PyBytes_GET_SIZE(item) + seplen;
+    }
+    if (i == seqlen) {
+        res = STRINGLIB_NEW(NULL, sz - seplen);
+        if (res == NULL) {
+            return NULL;
+        }
+        p = STRINGLIB_STR(res);
+        for (i = 0; i < seqlen; i++) {
+            if (i != 0 && seplen != 0) {
+                if (seplen == 1) {
+                    *p++ = *sepstr;
+                }
+                else {
+                    memcpy(p, sepstr, seplen);
+                    p += seplen;
+                }
+            }
+            item = PySequence_Fast_GET_ITEM(seq, i);
+            memcpy(p, PyBytes_AS_STRING(item), PyBytes_GET_SIZE(item));
+            p += PyBytes_GET_SIZE(item);
+        }
+        return res;
+    }
+    sz = 0;
+
     if (seqlen > NB_STATIC_BUFFERS) {
         buffers = PyMem_NEW(Py_buffer, seqlen);
         if (buffers == NULL) {
@@ -148,8 +186,16 @@ STRINGLIB(bytes_join_lock_held)(PyObject *sep, PyObject *seq)
 error:
     res = NULL;
 done:
-    for (i = 0; i < nbufs; i++)
-        PyBuffer_Release(&buffers[i]);
+    for (i = 0; i < nbufs; i++) {
+        PyObject *obj = buffers[i].obj;
+        if (obj != NULL && PyBytes_CheckExact(obj)) {
+            /* bytes has no bf_releasebuffer */
+            Py_DECREF(obj);
+        }
+        else {
+            PyBuffer_Release(&buffers[i]);
+        }
+    }
     if (buffers != static_buffers)
         PyMem_Free(buffers);
     return res;