Make landmines handle GN build directories better.

GN has user-managed build output directories, and the build arguments are saved in those directories. This means that when you hit a landmine, the user will be forced to regenerate that build directory, and to respecify their build arguments.

This change makes the landmine script look for GN build directories. If one is found, the build and command-line arguments to GN are preserved, and enough of a bootstrap build is left in that directory to regenerate itself the next time the build is run. This should make landmines transparent to GN users (except of course that everything will be rebuilt).

Review URL: https://codereview.chromium.org/709813003

Cr-Commit-Position: refs/heads/master@{#303528}

build/landmines.py

@@ -55,6 +55,79 @@ def get_build_dir(build_tool, is_iphone=False):
   return os.path.abspath(ret)
 
 
+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_if_necessary(new_landmines):
   """Does the work of setting, planting, and triggering landmines."""
   out_dir = get_build_dir(landmine_utils.builder())
@@ -83,7 +156,7 @@ def clobber_if_necessary(new_landmines):
         if os.path.isfile(path):
           os.unlink(path)
         elif os.path.isdir(path):
-          shutil.rmtree(path)
+          delete_build_dir(path)
 
   # Save current set of landmines for next time.
   with open(landmines_path, 'w') as f: