servlet: add extra 5s to AsyncContext timeout

Author: panchenko

servlet/src/main/java/io/grpc/servlet/ServletAdapter.java

@@ -128,10 +128,9 @@ public void doPost(HttpServletRequest req, HttpServletResponse resp) throws IOEx
     }
 
     Long timeoutNanos = headers.get(TIMEOUT_KEY);
-    if (timeoutNanos == null) {
-      timeoutNanos = 0L;
-    }
-    asyncCtx.setTimeout(TimeUnit.NANOSECONDS.toMillis(timeoutNanos));
+    asyncCtx.setTimeout(timeoutNanos != null
+        ? TimeUnit.NANOSECONDS.toMillis(timeoutNanos) + ASYNC_TIMEOUT_SAFETY_MARGIN
+        : 0);
     StatsTraceContext statsTraceCtx =
         StatsTraceContext.newServerContext(streamTracerFactories, method, headers);
 
@@ -158,6 +157,11 @@ public void doPost(HttpServletRequest req, HttpServletResponse resp) throws IOEx
     asyncCtx.addListener(new GrpcAsyncListener(stream, logId));
   }
 
+  /**
+   * Deadlines are managed via Context, servlet async timeout is not supposed to happen.
+   */
+  private static final long ASYNC_TIMEOUT_SAFETY_MARGIN = 5_000;
+
   // This method must use Enumeration and its members, since that is the only way to read headers
   // from the servlet api.
   @SuppressWarnings("JdkObsolete")