[SPARK-2313] Use socket to communicate GatewayServer port back to Python driver

This patch changes PySpark so that the GatewayServer's port is communicated back to the Python process that launches it over a local socket instead of a pipe.  The old pipe-based approach was brittle and could fail if `spark-submit` printed unexpected to stdout.

To accomplish this, I wrote a custom `PythonGatewayServer.main()` function to use in place of Py4J's `GatewayServer.main()`.

Closes #3424.

Author: Josh Rosen <joshrosen@databricks.com>

Closes #4603 from JoshRosen/SPARK-2313 and squashes the following commits:

6a7740b [Josh Rosen] Remove EchoOutputThread since it's no longer needed
0db501f [Josh Rosen] Use select() so that we don't block if GatewayServer dies.
9bdb4b6 [Josh Rosen] Handle case where getListeningPort returns -1
3fb7ed1 [Josh Rosen] Remove stdout=PIPE
2458934 [Josh Rosen] Use underscore to mark env var. as private
d12c95d [Josh Rosen] Use Logging and Utils.tryOrExit()
e5f9730 [Josh Rosen] Wrap everything in a giant try-block
2f70689 [Josh Rosen] Use stdin PIPE to share fate with driver
8bf956e [Josh Rosen] Initial cut at passing Py4J gateway port back to driver via socket

Author: JoshRosen

core/src/main/scala/org/apache/spark/api/python/PythonGatewayServer.scala

@@ -0,0 +1,64 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.spark.api.python
+
+import java.io.DataOutputStream
+import java.net.Socket
+
+import py4j.GatewayServer
+
+import org.apache.spark.Logging
+import org.apache.spark.util.Utils
+
+/**
+ * Process that starts a Py4J GatewayServer on an ephemeral port and communicates the bound port
+ * back to its caller via a callback port specified by the caller.
+ *
+ * This process is launched (via SparkSubmit) by the PySpark driver (see java_gateway.py).
+ */
+private[spark] object PythonGatewayServer extends Logging {
+  def main(args: Array[String]): Unit = Utils.tryOrExit {
+    // Start a GatewayServer on an ephemeral port
+    val gatewayServer: GatewayServer = new GatewayServer(null, 0)
+    gatewayServer.start()
+    val boundPort: Int = gatewayServer.getListeningPort
+    if (boundPort == -1) {
+      logError("GatewayServer failed to bind; exiting")
+      System.exit(1)
+    } else {
+      logDebug(s"Started PythonGatewayServer on port $boundPort")
+    }
+
+    // Communicate the bound port back to the caller via the caller-specified callback port
+    val callbackHost = sys.env("_PYSPARK_DRIVER_CALLBACK_HOST")
+    val callbackPort = sys.env("_PYSPARK_DRIVER_CALLBACK_PORT").toInt
+    logDebug(s"Communicating GatewayServer port to Python driver at $callbackHost:$callbackPort")
+    val callbackSocket = new Socket(callbackHost, callbackPort)
+    val dos = new DataOutputStream(callbackSocket.getOutputStream)
+    dos.writeInt(boundPort)
+    dos.close()
+    callbackSocket.close()
+
+    // Exit on EOF or broken pipe to ensure that this process dies when the Python driver dies:
+    while (System.in.read() != -1) {
+      // Do nothing
+    }
+    logDebug("Exiting due to broken pipe from Python driver")
+    System.exit(0)
+  }
+}

core/src/main/scala/org/apache/spark/deploy/SparkSubmit.scala

@@ -39,7 +39,6 @@ import org.apache.ivy.plugins.resolver.{ChainResolver, IBiblioResolver}
 
 import org.apache.spark.SPARK_VERSION
 import org.apache.spark.deploy.rest._
-import org.apache.spark.executor._
 import org.apache.spark.util.{ChildFirstURLClassLoader, MutableURLClassLoader, Utils}
 
 /**
@@ -284,8 +283,7 @@ object SparkSubmit {
     // If we're running a python app, set the main class to our specific python runner
     if (args.isPython && deployMode == CLIENT) {
       if (args.primaryResource == PYSPARK_SHELL) {
-        args.mainClass = "py4j.GatewayServer"
-        args.childArgs = ArrayBuffer("--die-on-broken-pipe", "0")
+        args.mainClass = "org.apache.spark.api.python.PythonGatewayServer"
       } else {
         // If a python file is provided, add it to the child arguments and list of files to deploy.
         // Usage: PythonAppRunner <main python file> <extra python files> [app arguments]

python/pyspark/java_gateway.py

@@ -17,19 +17,20 @@
 
 import atexit
 import os
-import sys
+import select
 import signal
 import shlex
+import socket
 import platform
 from subprocess import Popen, PIPE
-from threading import Thread
 from py4j.java_gateway import java_import, JavaGateway, GatewayClient
 
+from pyspark.serializers import read_int
+
 
 def launch_gateway():
     SPARK_HOME = os.environ["SPARK_HOME"]
 
-    gateway_port = -1
     if "PYSPARK_GATEWAY_PORT" in os.environ:
         gateway_port = int(os.environ["PYSPARK_GATEWAY_PORT"])
     else:
@@ -41,36 +42,42 @@ def launch_gateway():
         submit_args = submit_args if submit_args is not None else ""
         submit_args = shlex.split(submit_args)
         command = [os.path.join(SPARK_HOME, script)] + submit_args + ["pyspark-shell"]
+
+        # Start a socket that will be used by PythonGatewayServer to communicate its port to us
+        callback_socket = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
+        callback_socket.bind(('127.0.0.1', 0))
+        callback_socket.listen(1)
+        callback_host, callback_port = callback_socket.getsockname()
+        env = dict(os.environ)
+        env['_PYSPARK_DRIVER_CALLBACK_HOST'] = callback_host
+        env['_PYSPARK_DRIVER_CALLBACK_PORT'] = str(callback_port)
+
+        # Launch the Java gateway.
+        # We open a pipe to stdin so that the Java gateway can die when the pipe is broken
         if not on_windows:
             # Don't send ctrl-c / SIGINT to the Java gateway:
             def preexec_func():
                 signal.signal(signal.SIGINT, signal.SIG_IGN)
-            env = dict(os.environ)
             env["IS_SUBPROCESS"] = "1"  # tell JVM to exit after python exits
-            proc = Popen(command, stdout=PIPE, stdin=PIPE, preexec_fn=preexec_func, env=env)
+            proc = Popen(command, stdin=PIPE, preexec_fn=preexec_func, env=env)
         else:
             # preexec_fn not supported on Windows
-            proc = Popen(command, stdout=PIPE, stdin=PIPE)
+            proc = Popen(command, stdin=PIPE, env=env)
 
-        try:
-            # Determine which ephemeral port the server started on:
-            gateway_port = proc.stdout.readline()
-            gateway_port = int(gateway_port)
-        except ValueError:
-            # Grab the remaining lines of stdout
-            (stdout, _) = proc.communicate()
-            exit_code = proc.poll()
-            error_msg = "Launching GatewayServer failed"
-            error_msg += " with exit code %d!\n" % exit_code if exit_code else "!\n"
-            error_msg += "Warning: Expected GatewayServer to output a port, but found "
-            if gateway_port == "" and stdout == "":
-                error_msg += "no output.\n"
-            else:
-                error_msg += "the following:\n\n"
-                error_msg += "--------------------------------------------------------------\n"
-                error_msg += gateway_port + stdout
-                error_msg += "--------------------------------------------------------------\n"
-            raise Exception(error_msg)
+        gateway_port = None
+        # We use select() here in order to avoid blocking indefinitely if the subprocess dies
+        # before connecting
+        while gateway_port is None and proc.poll() is None:
+            timeout = 1  # (seconds)
+            readable, _, _ = select.select([callback_socket], [], [], timeout)
+            if callback_socket in readable:
+                gateway_connection = callback_socket.accept()[0]
+                # Determine which ephemeral port the server started on:
+                gateway_port = read_int(gateway_connection.makefile())
+                gateway_connection.close()
+                callback_socket.close()
+        if gateway_port is None:
+            raise Exception("Java gateway process exited before sending the driver its port number")
 
         # In Windows, ensure the Java child processes do not linger after Python has exited.
         # In UNIX-based systems, the child process can kill itself on broken pipe (i.e. when
@@ -88,21 +95,6 @@ def killChild():
                 Popen(["cmd", "/c", "taskkill", "/f", "/t", "/pid", str(proc.pid)])
             atexit.register(killChild)
 
-        # Create a thread to echo output from the GatewayServer, which is required
-        # for Java log output to show up:
-        class EchoOutputThread(Thread):
-
-            def __init__(self, stream):
-                Thread.__init__(self)
-                self.daemon = True
-                self.stream = stream
-
-            def run(self):
-                while True:
-                    line = self.stream.readline()
-                    sys.stderr.write(line)
-        EchoOutputThread(proc.stdout).start()
-
     # Connect to the gateway
     gateway = JavaGateway(GatewayClient(port=gateway_port), auto_convert=False)