okhttp: Fix bidirectional keep-alive causing spurious GO_AWAY

When bidirectional keep-alive is enabled (both client and server
sending keep-alive pings), the server incorrectly validates ping
acknowledgments (ACKs) sent in response to server-initiated pings.
This causes the KeepAliveEnforcer strike counter to increment for
legitimate protocol responses, eventually triggering a GO_AWAY with
ENHANCE_YOUR_CALM.

Move the keepAliveEnforcer.pingAcceptable() check inside the !ack
conditional block, so only client-initiated ping requests are
validated. Ping ACKs now bypass enforcement entirely, enabling
bidirectional keep-alive to work correctly.

Fixes #12417

Author: kssumin

okhttp/src/main/java/io/grpc/okhttp/OkHttpServerTransport.java

@@ -951,13 +951,13 @@ public void settings(boolean clearPrevious, Settings settings) {
 
     @Override
     public void ping(boolean ack, int payload1, int payload2) {
-      if (!keepAliveEnforcer.pingAcceptable()) {
-        abruptShutdown(ErrorCode.ENHANCE_YOUR_CALM, "too_many_pings",
-            Status.RESOURCE_EXHAUSTED.withDescription("Too many pings from client"), false);
-        return;
-      }
       long payload = (((long) payload1) << 32) | (payload2 & 0xffffffffL);
       if (!ack) {
+        if (!keepAliveEnforcer.pingAcceptable()) {
+          abruptShutdown(ErrorCode.ENHANCE_YOUR_CALM, "too_many_pings",
+              Status.RESOURCE_EXHAUSTED.withDescription("Too many pings from client"), false);
+          return;
+        }
         frameLogger.logPing(OkHttpFrameLogger.Direction.INBOUND, payload);
         synchronized (lock) {
           frameWriter.ping(true, payload1, payload2);

okhttp/src/test/java/io/grpc/okhttp/OkHttpServerTransportTest.java

@@ -1268,6 +1268,31 @@ public void keepAliveEnforcer_noticesActive() throws Exception {
         eq(ByteString.encodeString("too_many_pings", GrpcUtil.US_ASCII)));
   }
 
+  @Test
+  public void keepAliveEnforcer_doesNotEnforcePingAcks() throws Exception {
+    serverBuilder.permitKeepAliveTime(1, TimeUnit.HOURS)
+        .permitKeepAliveWithoutCalls(true);
+    initTransport();
+    handshake();
+
+    for (int i = 0; i < KeepAliveEnforcer.MAX_PING_STRIKES + 2; i++) {
+      int serverPingId = 0xDEAD + i;
+      clientFrameWriter.ping(true, serverPingId, 0);
+      clientFrameWriter.flush();
+    }
+
+    for (int i = 0; i < KeepAliveEnforcer.MAX_PING_STRIKES; i++) {
+      pingPong();
+    }
+
+    pingPongId++;
+    clientFrameWriter.ping(false, pingPongId, 0);
+    clientFrameWriter.flush();
+    assertThat(clientFrameReader.nextFrame(clientFramesRead)).isTrue();
+    verify(clientFramesRead).goAway(0, ErrorCode.ENHANCE_YOUR_CALM,
+        ByteString.encodeString("too_many_pings", GrpcUtil.US_ASCII));
+  }
+
   @Test
   public void maxConcurrentCallsPerConnection_failsWithRst() throws Exception {
     int maxConcurrentCallsPerConnection = 1;