[Telemetry] Make unhandled exception output more useful.

tools/telemetry/telemetry/test_runner.py

Index: tools/telemetry/telemetry/test_runner.py
diff --git a/tools/telemetry/telemetry/test_runner.py b/tools/telemetry/telemetry/test_runner.py
index adb95a83618e1b1d2ff968df9211b297fda26d68..ec6213258ca9d32bf598e14ad97d2932788b94bb 100644
--- a/tools/telemetry/telemetry/test_runner.py
+++ b/tools/telemetry/telemetry/test_runner.py
@@ -10,9 +10,11 @@ actually running the test is in Test and PageRunner."""
 import copy
 import inspect
 import json
+import math
 import optparse
 import os
 import sys
+import traceback
 
 from telemetry import test
 from telemetry.core import browser_options
@@ -20,6 +22,71 @@ from telemetry.core import discover
 from telemetry.core import util
 
 
+def PrintFormattedException(exception_class, exception, tb):
+  """Prints an Exception in a more useful format than the default.
+
+  TODO(tonyg): Consider further enhancements. For instance:
+    - Report stacks to maintainers like depot_tools does.
+    - Add a debug flag to automatically start pdb upon exception.
+  """
+  def _GetFinalFrame(frame):
+    final_frame = None
+    while frame is not None:
+      final_frame = frame
+      frame = frame.tb_next
+    return final_frame
+
+  def _AbbreviateMiddle(target, middle, length):
+    assert length >= 0, 'Must provide positive length'
+    assert len(middle) <= length, 'middle must not be greater than length'
+    if len(target) <= length:
+      return target
+    half_length = (length - len(middle)) / 2.
+    return '%s%s%s' % (target[:int(math.floor(half_length))],
+                       middle,
+                       target[-int(math.ceil(half_length)):])
+
+  base_dir = os.path.abspath(util.GetChromiumSrcDir())
+  formatted_exception = traceback.format_exception(
+      exception_class, exception, tb)
+  extracted_tb = traceback.extract_tb(tb)
+  traceback_header = formatted_exception[0].strip()
+  exception = formatted_exception[-1].strip()
+  local_variables = _GetFinalFrame(tb).tb_frame.f_locals
+
+  # Format the traceback.
+  print >> sys.stderr
+  print >> sys.stderr, traceback_header
+  for filename, line, function, text in extracted_tb:
+    filename = os.path.abspath(filename).lstrip(base_dir)
+    print >> sys.stderr, '  %s at %s:%d' % (function, filename, line)
+    print >> sys.stderr, '    %s' % text
+
+  # Format the locals.
+  if local_variables:
+    print >> sys.stderr
+    print >> sys.stderr, 'Locals:'
+    longest_variable = max([len(v) for v in local_variables.keys()])
+    for variable, value in sorted(local_variables.iteritems()):
+      if variable == 'self':
+        continue
+      value = repr(value)
+      possibly_truncated_value = _AbbreviateMiddle(value, ' ... ', 128)
+      truncation_indication = ''
+      if len(possibly_truncated_value) != len(value):
+        truncation_indication = ' (truncated)'
+      print >> sys.stderr, '  %s: %s%s' % (variable.ljust(longest_variable + 1),
+                                           possibly_truncated_value,
+                                           truncation_indication)
+
+  # Format the exception.
+  print >> sys.stderr
+  print >> sys.stderr, exception
+
+
+sys.excepthook = PrintFormattedException
+
+
 class Command(object):
   usage = ''