Add APPENGINE_API_MAX_THREADS env variable for better api calls.

Author: ludoch

runtime/runtime_impl_jetty121/src/main/java/com/google/apphosting/runtime/http/HttpApiHostClient.java

@@ -129,6 +129,24 @@ Config config() {
     return config;
   }
 
+  static int getMaxThreads(Config config) {
+    int maxThreads = config.maxConnectionsPerDestination().orElse(100);
+    String maxThreadsEnv = System.getenv("APPENGINE_API_MAX_THREADS");
+    if (maxThreadsEnv != null) {
+      try {
+        int envMaxThreads = Integer.parseInt(maxThreadsEnv);
+        logger.atInfo().log(
+            "Overriding API max threads to %d from environment variable.", envMaxThreads);
+        return envMaxThreads;
+      } catch (NumberFormatException e) {
+        logger.atWarning().withCause(e).log(
+            "Invalid value for APPENGINE_API_MAX_THREADS: %s, using default %d",
+            maxThreadsEnv, maxThreads);
+      }
+    }
+    return maxThreads;
+  }
+
   static HttpApiHostClient create(String url, Config config) {
     if (System.getenv("APPENGINE_API_CALLS_USING_JDK_CLIENT") != null) {
       logger.atInfo().log("Using JDK HTTP client for API calls");

runtime/runtime_impl_jetty121/src/main/java/com/google/apphosting/runtime/http/JdkHttpApiHostClient.java

@@ -114,7 +114,7 @@ static JdkHttpApiHostClient create(String url, Config config) {
          * of threads under retry, which overwhelms the JVM and the internal Datastore
          * Appserver connection, forcing it to respond with masking INTERNAL_ERROR fallbacks.
          */
-        int maxThreads = config.maxConnectionsPerDestination().orElse(100);
+        int maxThreads = getMaxThreads(config);
         ThreadPoolExecutor tpe =
             new ThreadPoolExecutor(
                 maxThreads, maxThreads, 60L, SECONDS, new LinkedBlockingQueue<>(), factory);

runtime/runtime_impl_jetty121/src/main/java/com/google/apphosting/runtime/http/JettyHttpApiHostClient.java

@@ -120,7 +120,7 @@ static JettyHttpApiHostClient create(String url, Config config) {
      * If the system experiences a spike, Jetty will safely queue the outgoing RPCs, preventing the
      * JVM and the Appserver from being overwhelmed and eliminating the INTERNAL_ERROR fallback loop.
      */
-    int maxThreads = config.maxConnectionsPerDestination().orElse(100);
+    int maxThreads = getMaxThreads(config);
     QueuedThreadPool threadPool = new QueuedThreadPool(maxThreads, 10, 60000, null, myThreadGroup);
     threadPool.setName("JettyHttpApiHostClient");
     threadPool.setDaemon(true);