Catch exceptions in project list generator

If the generator that produces per-project worker arguments raises an
exception it triggers python bug http://bugs.python.org/issue8296.
Rewrite the generator expression as a generator function, and catch
Exceptions and KeyboardInterrupts to end the iteration.

Also add a pool worker initializer to disable SIGINT to prevent
KeyboardInterrupts inside multiprocessing.Pool in the worker threads
causing the same problem.

Fixes easy-to-reproduce hangs when hitting ctrl-c during
repo forall -c echo

Change-Id: Ie4a65b3e1e07a64ed6bb6ff20f3912c4326718ca

subcmds/forall.py

@@ -20,6 +20,7 @@ import multiprocessing
 import re
 import os
 import select
+import signal
 import sys
 import subprocess
 
@@ -207,14 +208,12 @@ without iterating through the remaining projects.
 
     os.environ['REPO_COUNT'] = str(len(projects))
 
-    pool = multiprocessing.Pool(opt.jobs)
+    pool = multiprocessing.Pool(opt.jobs, InitWorker)
     try:
       config = self.manifest.manifestProject.config
       results_it = pool.imap(
          DoWorkWrapper,
-         ([mirror, opt, cmd, shell, cnt, config, self._SerializeProject(p)]
-          for cnt, p in enumerate(projects))
-      )
+         self.ProjectArgs(projects, mirror, opt, cmd, shell, config))
       pool.close()
       for r in results_it:
         rc = rc or r
@@ -236,12 +235,28 @@ without iterating through the remaining projects.
     if rc != 0:
       sys.exit(rc)
 
+  def ProjectArgs(self, projects, mirror, opt, cmd, shell, config):
+    for cnt, p in enumerate(projects):
+      try:
+        project = self._SerializeProject(p)
+      except Exception as e:
+        print('Project list error: %r' % e,
+              file=sys.stderr)
+        return
+      except KeyboardInterrupt:
+        print('Project list interrupted',
+              file=sys.stderr)
+        return
+      yield [mirror, opt, cmd, shell, cnt, config, project]
 
 class WorkerKeyboardInterrupt(Exception):
   """ Keyboard interrupt exception for worker processes. """
   pass
 
 
+def InitWorker():
+  signal.signal(signal.SIGINT, signal.SIG_IGN)
+
 def DoWorkWrapper(args):
   """ A wrapper around the DoWork() method.