Flush more often in gclient's SubprocessCallAndFilter.

Speed up flushing of output on platforms that may not
flush as often as we expect (like Windows).  Wait at
least 10s between flushes to avoid overloading busy
readers.

BUG=none
TEST=gclient flushes on Windows more often, regular
gclient operations continue to work as expected
Review URL: http://codereview.chromium.org/3140013

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@56217 0039d316-1c4b-4281-b951-d872f2087c98

gclient_utils.py

@@ -280,6 +280,11 @@ def SubprocessCallAndFilter(command,
       shell=(sys.platform == 'win32'), stdout=subprocess.PIPE,
       stderr=subprocess.STDOUT)
 
+  # Do a flush of sys.stdout before we begin reading from the subprocess's
+  # stdout.
+  last_flushed_at = time.time()
+  sys.stdout.flush()
+
   # Also, we need to forward stdout to prevent weird re-ordering of output.
   # This has to be done on a per byte basis to make sure it is not buffered:
   # normally buffering is done for each line, but if svn requests input, no
@@ -296,9 +301,16 @@ def SubprocessCallAndFilter(command,
         sys.stdout.write(in_byte)
       if in_byte != '\n':
         in_line += in_byte
-    if in_byte == '\n' and filter_fn:
-      filter_fn(in_line)
+    if in_byte == '\n':
+      if filter_fn:
+        filter_fn(in_line)
       in_line = ''
+      # Flush at least 10 seconds between line writes.  We wait at least 10
+      # seconds to avoid overloading the reader that called us with output,
+      # which can slow busy readers down.
+      if (time.time() - last_flushed_at) > 10:
+        last_flushed_at = time.time()
+        sys.stdout.flush()
     in_byte = kid.stdout.read(1)
   rv = kid.wait()