Add a small tool to answer questions like "Why does target A depend on target B".

tools/gyp-explain.py

Index: tools/gyp-explain.py
diff --git a/tools/gyp-explain.py b/tools/gyp-explain.py
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+++ b/tools/gyp-explain.py
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+#!/usr/bin/env python
+# Copyright (c) 2011 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+"""Prints paths between gyp targets.
+"""
+
+import json
+import os
+import sys
+import time
+
+from collections import deque
+
+def usage():
+  print """\
+Usage:
+  tools/gyp-explain.py chrome_dll gtest#
+"""
+
+
+def GetPath(graph, fro, to):
+  """Given a graph in (node -> list of successor nodes) dictionary format,
+  yields all paths from |fro| to |to|, starting with the shortest."""
+  # Storing full paths in the queue is a bit wasteful, but good enough for this.
+  q = deque([(fro, [])])
+  while q:
+    t, path = q.popleft()
+    if t == to:
+      yield path + [t]
+    for d in graph[t]:
+      q.append((d, path + [t]))
+
+
+def MatchNode(graph, substring):
+  """Given a dictionary, returns the key that matches |substring| best. Exits
+  if there's not one single best match."""
+  candidates = []
+  for target in graph:
+    if substring in target:
+      candidates.append(target)
+
+  if not candidates:
+    print 'No targets match "%s"' % substring
+    sys.exit(1)
+  if len(candidates) > 1:
+    print 'More than one target matches "%s": %s' % (
+        substring, ' '.join(candidates))
+    sys.exit(1)
+  return candidates[0]
+
+
+def Main(argv):
+  # Check that dump.json exists and that it's not too old.
+  dump_json_dirty = False
+  try:
+    st = os.stat('dump.json')
+    file_age_s = time.time() - st.st_mtime
+    if file_age_s > 2 * 60 * 60:
+      print 'dump.json is more than 2 hours old.'
+      dump_json_dirty = True
+  except IOError:
+    print 'dump.json not found.'
+    dump_json_dirty = True
+
+  if dump_json_dirty:
+    print 'Run'
+    print '    GYP_GENERATORS=dump_dependency_json build/gyp_chromium'
+    print 'first, then try again.'
+    sys.exit(1)
+
+  g = json.load(open('dump.json'))
+
+  if len(argv) != 3:
+    usage()
+    sys.exit(1)
+
+  fro = MatchNode(g, argv[1])
+  to = MatchNode(g, argv[2])
+
+  paths = list(GetPath(g, fro, to))
+  if len(paths) > 0:
+    print 'These paths lead from %s to %s:' % (fro, to)
+    for path in paths:
+      print ' -> '.join(path)
+  else:
+    print 'No paths found from %s to %s.' % (fro, to)
+
+
+if __name__ == '__main__':
+  Main(sys.argv)