bpo-30039: Don't run signal handlers while resuming a yield from stack (#1081)

If we have a chain of generators/coroutines that are 'yield from'ing
each other, then resuming the stack works like:

- call send() on the outermost generator
- this enters _PyEval_EvalFrameDefault, which re-executes the
  YIELD_FROM opcode
- which calls send() on the next generator
- which enters _PyEval_EvalFrameDefault, which re-executes the
  YIELD_FROM opcode
- ...etc.

However, every time we enter _PyEval_EvalFrameDefault, the first thing
we do is to check for pending signals, and if there are any then we
run the signal handler. And if it raises an exception, then we
immediately propagate that exception *instead* of starting to execute
bytecode. This means that e.g. a SIGINT at the wrong moment can "break
the chain" – it can be raised in the middle of our yield from chain,
with the bottom part of the stack abandoned for the garbage collector.

The fix is pretty simple: there's already a special case in
_PyEval_EvalFrameEx where it skips running signal handlers if the next
opcode is SETUP_FINALLY. (I don't see how this accomplishes anything
useful, but that's another story.) If we extend this check to also
skip running signal handlers when the next opcode is YIELD_FROM, then
that closes the hole – now the exception can only be raised at the
innermost stack frame.

This shouldn't have any performance implications, because the opcode
check happens inside the "slow path" after we've already determined
that there's a pending signal or something similar for us to process;
the vast majority of the time this isn't true and the new check
doesn't run at all.

1 files changed, 14 insertions, 3 deletions

diff --git a/Python/ceval.c b/Python/ceval.c
index 23fd088..302070b 100644
--- a/Python/ceval.c
+++ b/Python/ceval.c
@@ -1064,9 +1064,20 @@ _PyEval_EvalFrameDefault(PyFrameObject *f, int throwflag)
            Py_MakePendingCalls() above. */
 
         if (_Py_atomic_load_relaxed(&eval_breaker)) {
-            if (_Py_OPCODE(*next_instr) == SETUP_FINALLY) {
-                /* Make the last opcode before
-                   a try: finally: block uninterruptible. */
+            if (_Py_OPCODE(*next_instr) == SETUP_FINALLY ||
+                _Py_OPCODE(*next_instr) == YIELD_FROM) {
+                /* Two cases where we skip running signal handlers and other
+                   pending calls:
+                   - If we're about to enter the try: of a try/finally (not
+                     *very* useful, but might help in some cases and it's
+                     traditional)
+                   - If we're resuming a chain of nested 'yield from' or
+                     'await' calls, then each frame is parked with YIELD_FROM
+                     as its next opcode. If the user hit control-C we want to
+                     wait until we've reached the innermost frame before
+                     running the signal handler and raising KeyboardInterrupt
+                     (see bpo-30039).
+                */
                 goto fast_next_opcode;
             }
             if (_Py_atomic_load_relaxed(&pendingcalls_to_do)) {