iter: propagate panics from the iterator passed to Pull

This change propagates panics from the iterator passed to Pull through
next and stop. Once the panic occurs, next and stop become no-ops (the
iterator is invalidated).

For #67712.

Change-Id: I05e45601d4d10acdf51b53e3164bd891c1b324ac
Reviewed-on: https://go-review.googlesource.com/c/go/+/589136
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Austin Clements <austin@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

Author: mknyszek

Diff for: src/iter/iter.go

@@ -50,11 +50,12 @@ func coroswitch(*coro)
 // simultaneously.
 func Pull[V any](seq Seq[V]) (next func() (V, bool), stop func()) {
 	var (
-		v         V
-		ok        bool
-		done      bool
-		yieldNext bool
-		racer     int
+		v          V
+		ok         bool
+		done       bool
+		yieldNext  bool
+		racer      int
+		panicValue any
 	)
 	c := newcoro(func(c *coro) {
 		race.Acquire(unsafe.Pointer(&racer))
@@ -72,14 +73,22 @@ func Pull[V any](seq Seq[V]) (next func() (V, bool), stop func()) {
 			race.Acquire(unsafe.Pointer(&racer))
 			return !done
 		}
+		// Recover and propagate panics from seq.
+		defer func() {
+			if p := recover(); p != nil {
+				done = true // Invalidate iterator.
+				panicValue = p
+			}
+			race.Release(unsafe.Pointer(&racer))
+		}()
 		seq(yield)
 		var v0 V
 		v, ok = v0, false
 		done = true
-		race.Release(unsafe.Pointer(&racer))
 	})
 	next = func() (v1 V, ok1 bool) {
 		race.Write(unsafe.Pointer(&racer)) // detect races
+
 		if done {
 			return
 		}
@@ -90,15 +99,26 @@ func Pull[V any](seq Seq[V]) (next func() (V, bool), stop func()) {
 		race.Release(unsafe.Pointer(&racer))
 		coroswitch(c)
 		race.Acquire(unsafe.Pointer(&racer))
+
+		// Propagate panics from seq.
+		if panicValue != nil {
+			panic(panicValue)
+		}
 		return v, ok
 	}
 	stop = func() {
 		race.Write(unsafe.Pointer(&racer)) // detect races
+
 		if !done {
 			done = true
 			race.Release(unsafe.Pointer(&racer))
 			coroswitch(c)
 			race.Acquire(unsafe.Pointer(&racer))
+
+			// Propagate panics from seq.
+			if panicValue != nil {
+				panic(panicValue)
+			}
 		}
 	}
 	return next, stop
@@ -125,12 +145,13 @@ func Pull[V any](seq Seq[V]) (next func() (V, bool), stop func()) {
 // simultaneously.
 func Pull2[K, V any](seq Seq2[K, V]) (next func() (K, V, bool), stop func()) {
 	var (
-		k         K
-		v         V
-		ok        bool
-		done      bool
-		yieldNext bool
-		racer     int
+		k          K
+		v          V
+		ok         bool
+		done       bool
+		yieldNext  bool
+		racer      int
+		panicValue any
 	)
 	c := newcoro(func(c *coro) {
 		race.Acquire(unsafe.Pointer(&racer))
@@ -148,15 +169,23 @@ func Pull2[K, V any](seq Seq2[K, V]) (next func() (K, V, bool), stop func()) {
 			race.Acquire(unsafe.Pointer(&racer))
 			return !done
 		}
+		// Recover and propagate panics from seq.
+		defer func() {
+			if p := recover(); p != nil {
+				done = true // Invalidate iterator.
+				panicValue = p
+			}
+			race.Release(unsafe.Pointer(&racer))
+		}()
 		seq(yield)
 		var k0 K
 		var v0 V
 		k, v, ok = k0, v0, false
 		done = true
-		race.Release(unsafe.Pointer(&racer))
 	})
 	next = func() (k1 K, v1 V, ok1 bool) {
 		race.Write(unsafe.Pointer(&racer)) // detect races
+
 		if done {
 			return
 		}
@@ -167,15 +196,26 @@ func Pull2[K, V any](seq Seq2[K, V]) (next func() (K, V, bool), stop func()) {
 		race.Release(unsafe.Pointer(&racer))
 		coroswitch(c)
 		race.Acquire(unsafe.Pointer(&racer))
+
+		// Propagate panics from seq.
+		if panicValue != nil {
+			panic(panicValue)
+		}
 		return k, v, ok
 	}
 	stop = func() {
 		race.Write(unsafe.Pointer(&racer)) // detect races
+
 		if !done {
 			done = true
 			race.Release(unsafe.Pointer(&racer))
 			coroswitch(c)
 			race.Acquire(unsafe.Pointer(&racer))
+
+			// Propagate panics from seq.
+			if panicValue != nil {
+				panic(panicValue)
+			}
 		}
 	}
 	return next, stop

Diff for: src/iter/pull_test.go

@@ -241,3 +241,82 @@ func storeYield2() Seq2[int, int] {
 }
 
 var yieldSlot2 func(int, int) bool
+
+func TestPullPanic(t *testing.T) {
+	t.Run("next", func(t *testing.T) {
+		next, stop := Pull(panicSeq())
+		if !panicsWith("boom", func() { next() }) {
+			t.Fatal("failed to propagate panic on first next")
+		}
+		// Make sure we don't panic again if we try to call next or stop.
+		if _, ok := next(); ok {
+			t.Fatal("next returned true after iterator panicked")
+		}
+		// Calling stop again should be a no-op.
+		stop()
+	})
+	t.Run("stop", func(t *testing.T) {
+		next, stop := Pull(panicSeq())
+		if !panicsWith("boom", func() { stop() }) {
+			t.Fatal("failed to propagate panic on first stop")
+		}
+		// Make sure we don't panic again if we try to call next or stop.
+		if _, ok := next(); ok {
+			t.Fatal("next returned true after iterator panicked")
+		}
+		// Calling stop again should be a no-op.
+		stop()
+	})
+}
+
+func panicSeq() Seq[int] {
+	return func(yield func(int) bool) {
+		panic("boom")
+	}
+}
+
+func TestPull2Panic(t *testing.T) {
+	t.Run("next", func(t *testing.T) {
+		next, stop := Pull2(panicSeq2())
+		if !panicsWith("boom", func() { next() }) {
+			t.Fatal("failed to propagate panic on first next")
+		}
+		// Make sure we don't panic again if we try to call next or stop.
+		if _, _, ok := next(); ok {
+			t.Fatal("next returned true after iterator panicked")
+		}
+		// Calling stop again should be a no-op.
+		stop()
+	})
+	t.Run("stop", func(t *testing.T) {
+		next, stop := Pull2(panicSeq2())
+		if !panicsWith("boom", func() { stop() }) {
+			t.Fatal("failed to propagate panic on first stop")
+		}
+		// Make sure we don't panic again if we try to call next or stop.
+		if _, _, ok := next(); ok {
+			t.Fatal("next returned true after iterator panicked")
+		}
+		// Calling stop again should be a no-op.
+		stop()
+	})
+}
+
+func panicSeq2() Seq2[int, int] {
+	return func(yield func(int, int) bool) {
+		panic("boom")
+	}
+}
+
+func panicsWith(v any, f func()) (panicked bool) {
+	defer func() {
+		if r := recover(); r != nil {
+			if r != v {
+				panic(r)
+			}
+			panicked = true
+		}
+	}()
+	f()
+	return
+}