sync: Limit -j to file descriptors

Each worker thread requires at least 3 file descriptors to run the
forked 'git fetch' child to operate against the local repository.
Mac OS X has the RLIMIT_NOFILE set to 256 by default, which means
a sync -j128 often fails when the workers run out of pipes within
the Python parent process.

Change-Id: I2cdb14621b899424b079daf7969bc8c16b85b903
Signed-off-by: Shawn O. Pearce <sop@google.com>

subcmds/sync.py

@@ -28,6 +28,14 @@ try:
 except ImportError:
   import dummy_threading as _threading
 
+try:
+  import resource
+  def _rlimit_nofile():
+    return resource.getrlimit(resource.RLIMIT_NOFILE)
+except ImportError:
+  def _rlimit_nofile():
+    return (256, 256)
+
 from git_command import GIT
 from git_refs import R_HEADS
 from project import HEAD
@@ -312,6 +320,10 @@ uncommitted changes are present' % project.relpath
   def Execute(self, opt, args):
     if opt.jobs:
       self.jobs = opt.jobs
+    if self.jobs > 1:
+      soft_limit, _ = _rlimit_nofile()
+      self.jobs = min(self.jobs, (soft_limit - 5) / 3)
+
     if opt.network_only and opt.detach_head:
       print >>sys.stderr, 'error: cannot combine -n and -d'
       sys.exit(1)