gRPC client: release the call slot before the caller is notified

changes.md

@@ -218,6 +218,9 @@ Changes log
           their case (`Realm=`, `Charset=`), as required by RFC 9110. Issue #1455.
         - `ChallengeScheme.valueOf()` returned `HTTP_OAUTH` for the names of the Bearer and MAC schemes
           (`HTTP_Bearer`, `HTTP_MAC`) instead of `HTTP_OAUTH_BEARER` and `HTTP_OAUTH_MAC`. Issue #1618.
+        - The gRPC client could reject a call with `RESOURCE_EXHAUSTED` ("gRPC call limit exceeded")
+          right after the end of the previous one: the call slot was released after the caller was
+          notified. Issue #1622.
     - Misc
         - Fixed JDK HTTP client accessibility in core and WebSocket tests in Eclipse and VS Code,
           without adding a production module dependency.

org.restlet.ext.grpc/src/main/java/org/restlet/ext/grpc/internal/RestletClientStream.java

@@ -380,14 +380,13 @@ protected void http2ProcessingFailed(
 
         @Override
         public void deframerClosed(boolean partial) {
-            try {
-                super.deframerClosed(partial);
-            } finally {
-                if (!done) {
-                    done = true;
-                    finished.accept(RestletClientStream.this);
-                }
+            // Release the call slot first: the SDK notifies the caller from super.deframerClosed(),
+            // and a caller that starts its next call right away must not find the slot still taken.
+            if (!done) {
+                done = true;
+                finished.accept(RestletClientStream.this);
             }
+            super.deframerClosed(partial);
         }
     }
 

org.restlet.ext.grpc/src/test/java/org/restlet/ext/grpc/GrpcHttpValidationTestCase.java

@@ -9,6 +9,7 @@
 package org.restlet.ext.grpc;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNotEquals;
 import static org.junit.jupiter.api.Assertions.assertThrows;
 import static org.junit.jupiter.api.Assertions.assertTrue;
 
@@ -189,6 +190,41 @@ void malformedBinaryMetadataFailsExplicitly() throws Exception {
         assertRecovered();
     }
 
+    /**
+     * The call slot must be free as soon as the caller is told that a call failed, otherwise the
+     * next call of the same thread is rejected with RESOURCE_EXHAUSTED.
+     */
+    @Test
+    void failedCallReleasesItsSlotBeforeTheCallerIsNotified() throws Exception {
+        adapter.setMaxConcurrentCalls(1);
+        AtomicInteger requests = new AtomicInteger();
+        start(
+                new Restlet() {
+                    @Override
+                    public void handle(Request request, Response response) {
+                        if (requests.incrementAndGet() % 2 == 1) {
+                            response.setEntity("", MediaType.valueOf("application/grpc"));
+                        } else {
+                            response.setEntity(
+                                    new ByteArrayRepresentation(
+                                            frame("recovered"),
+                                            MediaType.valueOf("application/grpc")));
+                            response.getTrailers().add("grpc-status", "0");
+                        }
+                    }
+                });
+        for (int i = 0; i < 200; i++) {
+            int call = i;
+            Status status = failure();
+            // Only the slot matters here: depending on timing the failure is UNKNOWN or
+            // UNAVAILABLE.
+            assertNotEquals(Status.Code.OK, status.getCode(), () -> call + ": " + status);
+            assertNotEquals(
+                    Status.Code.RESOURCE_EXHAUSTED, status.getCode(), () -> call + ": " + status);
+            assertRecovered();
+        }
+    }
+
     @Test
     void wrongContentTypeFailsAndRecovers() throws Exception {
         startMalformedThenHealthy(response -> response.setEntity("not grpc", MediaType.TEXT_PLAIN));