fixing drt-trampoline

tools/testing/drt-trampoline.py

Index: tools/testing/drt-trampoline.py
diff --git a/tools/testing/drt-trampoline.py b/tools/testing/drt-trampoline.py
index 2c7b033e009e2aa0cc64013d483043f6fabd0b90..89363a565373d05ad09c06b5f74a3e82b21cf3cc 100644
--- a/tools/testing/drt-trampoline.py
+++ b/tools/testing/drt-trampoline.py
@@ -49,19 +49,19 @@ def main(argv):
 
 
   p = subprocess.Popen(cmd, env=env, stdout=subprocess.PIPE)
-  p.wait()

[Siggi Cherem (dart-lang), 2012/06/28 21:23:57]

I believe the issue is related to pipeing output and using wait.

The failing test had a very long output. I believe that piping output uses a
limited buffer that gets filled up. If we don't read from it, 'wait' will block
forever.  'communicate' seems to be better suited for reading data out of the
subprocess.

[Emily Fortuna, 2012/06/28 21:44:39]

I think you're right. I seem to remember reading in Python's documentation some
warning about p.wait if the buffer gets filled.

+  output, error = p.communicate()
   if p.returncode != 0:
     raise Exception('Failed to run command. return code=%s' % p.returncode)
 
   if out_expected_file:
     # Compare output to the given expectation file.
-    output = None
     expectation = None
-    with p.stdout as res:
-      if is_png:
-        # DRT prints the image to STDOUT, but includes 5 header lines.
-        for i in range(4): res.readline()
-      output = res.read()
+    if is_png:
+      # DRT prints the image to STDOUT, but includes extra text that we trim:
+      # - 5 header lines
+      # - a '#EOF\n' at the end
+      output = output.replace('\n', '_', 3)
+      output = output[output.find('\n') + 1: -5]

[Emily Fortuna, 2012/06/28 21:44:39]

Perhaps I'm misunderstanding how this code works, but why do we replace the
first three newlines and then start with the 4th? How about:
output = output.replace('\n', '_', 4)
 output = output[output.find('\n') + 1: -len('#EOF\n')] (this makes the -5 more
clear, too)

[Siggi Cherem (dart-lang), 2012/06/28 21:55:06]

Done. Turns out that it was 4 header lines, not 5... 

     if os.path.exists(out_expected_file):
       with open(out_expected_file, 'r') as f:
         expectation = f.read()
@@ -87,7 +87,7 @@ def main(argv):
     print '#EOF'
   else:
     # Pipe through the output for non-layout tests.
-    sys.stdout.write(p.stdout.read())
+    print output
 
 if __name__ == '__main__':
   try: