Fix GH-23301: nested "yield from" repeats a value after "yield from []"

NEWS

@@ -5,6 +5,8 @@ PHP                                                                        NEWS
 - Core:
   . Fixed bug GH-15375 (Nested "yield from" skips items after a valid() or
     next() call on the inner generator). (iliaal)
+  . Fixed bug GH-23301 (Nested "yield from" yields a value twice when the
+    middle generator delegates again). (Lazizbek Ergashev)
 
 - Opcache:
   . Fixed opcache.protect_memory race under ZTS. (realFlowControl)

Zend/tests/generators/backtrace_multi_yield_from.phpt

@@ -25,8 +25,8 @@ var_dump($gen2->current());
 ?>
 --EXPECTF--
 int(1)
-int(1)
 #0 %s(10): gen()
 #1 [internal function]: from(Object(Generator))
-#2 %s(19): Generator->next()
+#2 %s(17): Generator->next()
 int(2)
+NULL

Zend/tests/generators/gh15375.phpt

@@ -46,9 +46,9 @@ foreach (outer(withNext()) as $s) {
     echo $s, "\n";
 }
 
-// A shared, pre-primed generator consumed through two nested "yield from"
-// levels must still present its current value once to each consumer (the fix
-// must not over-clear the middle level's first-touch).
+// Reading a pre-primed generator through two nested "yield from" levels primes
+// the whole chain but advances nothing. A later next() on a middle level then
+// advances the shared generator, like any other next() would.
 echo "shared primed:\n";
 function counter() {
     yield 1;
@@ -58,11 +58,11 @@ $gen1 = counter();
 $gen1->valid();
 $gen2 = outer($gen1);
 $gen3 = outer($gen2);
-echo "gen3 current: ", $gen3->current(), "\n";
+var_dump($gen3->current());
 $gen2->next();
-echo "gen2 current: ", $gen2->current(), "\n";
+var_dump($gen2->current());
 $gen2->next();
-echo "gen2 current: ", $gen2->current(), "\n";
+var_dump($gen2->current());
 
 ?>
 --EXPECT--
@@ -84,6 +84,6 @@ six
 eight
 nine
 shared primed:
-gen3 current: 1
-gen2 current: 1
-gen2 current: 2
+int(1)
+int(2)
+NULL

Zend/tests/generators/gh23301.phpt

@@ -0,0 +1,44 @@
+--TEST--
+GH-23301 (Nested "yield from" yields a value twice when the middle generator delegates again)
+--FILE--
+<?php
+
+function inner() {
+    yield "B";
+}
+
+function middle() {
+    yield "A";
+    yield from inner();
+    yield "C";
+    yield from ["D"];
+}
+
+function delegate($gen) {
+    yield from $gen;
+}
+
+foreach (delegate(middle()) as $value) {
+    echo $value, "\n";
+}
+
+function tail($inner) {
+    yield from $inner;
+    yield from ["E"];
+}
+
+$middle = tail(inner());
+$outer = delegate($middle);
+
+var_dump($outer->current());
+$middle->next();
+var_dump($middle->current());
+
+?>
+--EXPECT--
+A
+B
+C
+D
+string(1) "B"
+string(1) "E"

Zend/zend_generators.c

@@ -776,10 +776,12 @@ ZEND_API void zend_generator_resume(zend_generator *orig_generator) /* {{{ */
 		return;
 	}
 
-	if (UNEXPECTED((delegator->flags & ZEND_GENERATOR_DO_INIT) != 0 && !Z_ISUNDEF(generator->value))) {
-		/* We must not advance Generator if we yield from a Generator being currently run */
-		orig_generator->flags &= ~ZEND_GENERATOR_DO_INIT;
-		return;
+	if (UNEXPECTED((delegator->flags & ZEND_GENERATOR_DO_INIT) != 0)) {
+		delegator->flags &= ~ZEND_GENERATOR_DO_INIT;
+		if (UNEXPECTED(!Z_ISUNDEF(generator->value))) {
+			/* We must not advance an already initialized delegate on first resumption */
+			return;
+		}
 	}
 
 	if (EG(active_fiber)) {
@@ -819,7 +821,7 @@ ZEND_API void zend_generator_resume(zend_generator *orig_generator) /* {{{ */
 			EG(current_execute_data) = original_execute_data;
 			EG(jit_trace_num) = original_jit_trace_num;
 
-			orig_generator->flags &= ~(ZEND_GENERATOR_DO_INIT | ZEND_GENERATOR_IN_FIBER);
+			orig_generator->flags &= ~ZEND_GENERATOR_IN_FIBER;
 			generator->flags &= ~(ZEND_GENERATOR_CURRENTLY_RUNNING | ZEND_GENERATOR_IN_FIBER);
 			return;
 		}
@@ -882,7 +884,6 @@ ZEND_API void zend_generator_resume(zend_generator *orig_generator) /* {{{ */
 		} else {
 			generator = zend_generator_get_current(orig_generator);
 			zend_generator_throw_exception(generator, NULL);
-			orig_generator->flags &= ~ZEND_GENERATOR_DO_INIT;
 			delegator = orig_generator;
 			goto try_again;
 		}
@@ -900,7 +901,7 @@ ZEND_API void zend_generator_resume(zend_generator *orig_generator) /* {{{ */
 		goto try_again;
 	}
 
-	orig_generator->flags &= ~(ZEND_GENERATOR_DO_INIT | ZEND_GENERATOR_IN_FIBER);
+	orig_generator->flags &= ~ZEND_GENERATOR_IN_FIBER;
 }
 /* }}} */