Sync iterator wrapper has fewer possible states.

We use an enum to thoroughly document the possible states the yielding
function can be in, making the code a lot more readable IMHO!

once/_iterator_wrappers.py

@@ -1,5 +1,6 @@
 import asyncio
 import collections.abc
+import enum
 import threading
 
 # Before we begin, a note on the assert statements in this file:
@@ -91,6 +92,15 @@ async def yield_results(self) -> collections.abc.AsyncGenerator:
             i += 1
 
 
+class _IteratorAction(enum.Enum):
+    # Generating the next value from the underlying iterator
+    GENERATING = 1
+    # Yield an already computed value
+    YIELDING = 2
+    # Waiting for the underlying iterator, already triggered from another call.
+    WAITING = 3
+
+
 class GeneratorWrapper:
     """Wrapper around an sync generator which only runs once.
 
@@ -107,80 +117,56 @@ def __init__(self, func, *args, **kwargs) -> None:
         self.next_send = None
 
     def yield_results(self) -> collections.abc.Generator:
+        # Fast path for subsequent repeated call:
+        with self.lock:
+            finished = self.finished
+        if finished:
+            yield from self.results
+            return
         i = 0
+        yield_value = None
+        next_send = None
         # Fast path for subsequent calls will not require a lock
         while True:
-            # If we on before the penultimate entry, we can return now. When yielding the last
-            # element of results, we need to be recording next_send, so that needs the lock.
-            if i < len(self.results) - 1:
-                yield self.results[i]
-                i += 1
-                continue
-            # Because we don't hold a lock here, we can't make this assumption
-            # i == len(self.results) - 1 or i == len(self.results)
-            # because the iterator could have moved in the interim. However, it will no longer
-            # move once self.finished.
-            if self.finished:
-                if i < len(self.results):
-                    yield self.results[i]
-                    i += 1
-                    continue
-                if i == len(self.results):
-                    return
-
-            # Initial calls, and concurrent calls before completion will require the lock.
+            action: _IteratorAction | None = None
+            # With a lock, we figure out which action to take, and then we take it after release.
             with self.lock:
-                # Just in case a race condition prevented us from hitting these conditions before,
-                # check them again, so they can be handled by the code before the lock.
-                if i < len(self.results) - 1:
-                    continue
-                if self.finished:
-                    if i < len(self.results):
-                        continue
-                    if i == len(self.results):
+                if i == len(self.results):
+                    if self.finished:
                         return
-                assert i == len(self.results) - 1 or i == len(self.results)
-                # If we are at the end and waiting for the generator to complete, there is nothing
-                # to do!
-                if self.generating and i == len(self.results):
-                    continue
-
-                # At this point, there are 2 states to handle, which we will want to do outside the
-                # lock to avoid deadlocks.
-                # State #1: We are about to yield back the last entry in self.results and potentially
-                #           log next send. We can allow multiple calls to enter this state, as long
-                #           as we re-grab the lock before modifying self.next_send
-                # State #2: We are at the end of self.results, and need to call our underlying
-                #           iterator. Only one call may enter this state due to our check of
-                #           self.generating above.
-                if i == len(self.results) and not self.generating:
-                    self.generating = True
-                    next_send = self.next_send
-                    listening = False
+                    if self.generating:
+                        action = _IteratorAction.WAITING
+                    else:
+                        action = _IteratorAction.GENERATING
+                        next_send = self.next_send
+                        self.generating = True
                 else:
-                    assert i == len(self.results) - 1 or self.generating
-                    listening = True
-            # We break outside the lock to either listen or kick off a new generation.
-            if listening:
-                next_send = yield self.results[i]
+                    action = _IteratorAction.YIELDING
+                    yield_value = self.results[i]
+            if action == _IteratorAction.WAITING:
+                continue
+            if action == _IteratorAction.YIELDING:
+                next_send = yield yield_value
                 i += 1
+                # If we have just sent the last element and we have not yet kicked off the next
+                # iteration, we need to record the next send value.
                 with self.lock:
-                    if not self.finished and i == len(self.results):
+                    if i == len(self.results) and not self.generating:
                         self.next_send = next_send
                 continue
-            # We must be in generating state
-            assert self.generator is not None
-            try:
-                result = self.generator.send(next_send)
-            except StopIteration:
-                # This lock should be unnecessary, which by definition means there should be no
-                # contention on it, so we use it to preserve our assumptions about variables which
-                # are modified under lock.
-                with self.lock:
-                    self.finished = True
-                    self.generator = None  # Allow this to be GCed.
-                    self.generating = False
-                return
-            with self.lock:
-                self.results.append(result)
-                self.generating = False
+            if action == _IteratorAction.GENERATING:
+                assert self.generator is not None
+                try:
+                    result = self.generator.send(next_send)
+                except StopIteration:
+                    # This lock should be unnecessary, which by definition means there should be no
+                    # contention on it, so we use it to preserve our assumptions about variables which
+                    # are modified under lock.
+                    with self.lock:
+                        self.finished = True
+                        self.generator = None  # Allow this to be GCed.
+                        self.generating = False
+                else:
+                    with self.lock:
+                        self.generating = False
+                        self.results.append(result)