Factor clobber out of build/landmines.py

build/clobber.py

Index: build/clobber.py
diff --git a/build/clobber.py b/build/clobber.py
new file mode 100755
index 0000000000000000000000000000000000000000..785011a171d836f9cbf8dc0ddd101a015b3335de
--- /dev/null
+++ b/build/clobber.py
@@ -0,0 +1,110 @@
+#!/usr/bin/env python
+# Copyright 2015 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.
+
+"""This script provides methods for clobbering build directories."""
+
+import argparse
+import os
+import shutil
+import sys
+
+
+def extract_gn_build_commands(build_ninja_file):
+  """Extracts from a build.ninja the commands to run GN.
+
+  The commands to run GN are the gn rule and build.ninja build step at the
+  top of the build.ninja file. We want to keep these when deleting GN builds
+  since we want to preserve the command-line flags to GN.
+
+  On error, returns the empty string."""
+  result = ""
+  with open(build_ninja_file, 'r') as f:
+    # Read until the second blank line. The first thing GN writes to the file
+    # is the "rule gn" and the second is the section for "build build.ninja",
+    # separated by blank lines.
+    num_blank_lines = 0
+    while num_blank_lines < 2:
+      line = f.readline()
+      if len(line) == 0:
+        return ''  # Unexpected EOF.
+      result += line
+      if line[0] == '\n':
+        num_blank_lines = num_blank_lines + 1
+  return result
+
+
+def delete_build_dir(build_dir):
+  # GN writes a build.ninja.d file. Note that not all GN builds have args.gn.
+  build_ninja_d_file = os.path.join(build_dir, 'build.ninja.d')
+  if not os.path.exists(build_ninja_d_file):
+    shutil.rmtree(build_dir)
+    return
+
+  # GN builds aren't automatically regenerated when you sync. To avoid
+  # messing with the GN workflow, erase everything but the args file, and
+  # write a dummy build.ninja file that will automatically rerun GN the next
+  # time Ninja is run.
+  build_ninja_file = os.path.join(build_dir, 'build.ninja')
+  build_commands = extract_gn_build_commands(build_ninja_file)
+
+  try:
+    gn_args_file = os.path.join(build_dir, 'args.gn')
+    with open(gn_args_file, 'r') as f:
+      args_contents = f.read()
+  except IOError:
+    args_contents = ''
+
+  shutil.rmtree(build_dir)
+
+  # Put back the args file (if any).
+  os.mkdir(build_dir)
+  if args_contents != '':
+    with open(gn_args_file, 'w') as f:
+      f.write(args_contents)
+
+  # Write the build.ninja file sufficiently to regenerate itself.
+  with open(os.path.join(build_dir, 'build.ninja'), 'w') as f:
+    if build_commands != '':
+      f.write(build_commands)
+    else:
+      # Couldn't parse the build.ninja file, write a default thing.
+      f.write('''rule gn
+command = gn -q gen //out/%s/
+description = Regenerating ninja files
+
+build build.ninja: gn
+generator = 1
+depfile = build.ninja.d
+''' % (os.path.split(build_dir)[1]))
+
+  # Write a .d file for the build which references a nonexistant file. This
+  # will make Ninja always mark the build as dirty.
+  with open(build_ninja_d_file, 'w') as f:
+    f.write('build.ninja: nonexistant_file.gn\n')
+
+
+def clobber(out_dir):
+  """Clobber contents of build directory.
+
+  Don't delete the directory itself: some checkouts have the build directory
+  mounted."""
+  for f in os.listdir(out_dir):
+    path = os.path.join(out_dir, f)
+    if os.path.isfile(path):
+      os.unlink(path)
+    elif os.path.isdir(path):
+      delete_build_dir(path)
+
+
+def main():
+  parser = argparse.ArgumentParser()
+  parser.add_argument('out_dir', help='The output directory to clobber')
+  args = parser.parse_args()
+  clobber(args.out_dir)
+  return 0
+
+
+if __name__ == '__main__':
+  sys.exit(main())