[Vim/YCM] Fix hang/crash when no Clang command line is available.

Previously, we tried to determine the command line for building a source
file by looking at the first output of said source file. This doesn't
work if the first output doesn't yield a clang command line. This patch
attempts to resolve this issue by going through all the build outputs of
a source file until one is found that yields a clang command line.

It is still possible to not find a Clang command line. In this case,
patch causes a graceful failure, rather than a crash.

R=jbroman@chromium.org,johnme@chromium.org
BUG=497787

Review URL: https://codereview.chromium.org/1156223007

Cr-Commit-Position: refs/heads/master@{#336467}

1 files changed, 92 insertions, 27 deletions

diff --git a/tools/vim/chromium.ycm_extra_conf.py b/tools/vim/chromium.ycm_extra_conf.py
index 5715e29..2145000 100644
--- a/tools/vim/chromium.ycm_extra_conf.py
+++ b/tools/vim/chromium.ycm_extra_conf.py
@@ -169,19 +169,18 @@ def FindChromeSrcFromFilename(filename):
   return curdir
 
 
-def GetDefaultCppFile(chrome_root, filename):
-  """Returns the default target file to use for |filename|.
+def GetDefaultSourceFile(chrome_root, filename):
+  """Returns the default source file to use as an alternative to |filename|.
 
-  The default target is some source file that is known to exist and loosely
-  related to |filename|. Compile flags used to build the default target is
-  assumed to be a close-enough approximation for building |filename|.
+  Compile flags used to build the default source file is assumed to be a
+  close-enough approximation for building |filename|.
 
   Args:
     chrome_root: (String) Absolute path to the root of Chromium checkout.
-    filename: (String) Absolute path to the target source file.
+    filename: (String) Absolute path to the source file.
 
   Returns:
-    (String) Absolute path to substitute target file.
+    (String) Absolute path to substitute source file.
   """
   blink_root = os.path.join(chrome_root, 'third_party', 'WebKit')
   if filename.startswith(blink_root):
@@ -190,15 +189,20 @@ def GetDefaultCppFile(chrome_root, filename):
     return os.path.join(chrome_root, 'base', 'logging.cc')
 
 
-def GetBuildTargetForSourceFile(chrome_root, filename):
-  """Returns a build target corresponding to |filename|.
+def GetBuildableSourceFile(chrome_root, filename):
+  """Returns a buildable source file corresponding to |filename|.
+
+  A buildable source file is one which is likely to be passed into clang as a
+  source file during the build. For .h files, returns the closest matching .cc,
+  .cpp or .c file. If no such file is found, returns the same as
+  GetDefaultSourceFile().
 
   Args:
     chrome_root: (String) Absolute path to the root of Chromium checkout.
     filename: (String) Absolute path to the target source file.
 
   Returns:
-    (String) Absolute path to build target.
+    (String) Absolute path to source file.
   """
   if filename.endswith('.h'):
     # Header files can't be built. Instead, try to match a header file to its
@@ -209,46 +213,104 @@ def GetBuildTargetForSourceFile(chrome_root, filename):
       if os.path.exists(alt_name):
         return alt_name
 
-    # Failing that, build a default file instead and assume that the resulting
-    # commandline options are valid for the .h file.
-    return GetDefaultCppFile(chrome_root, filename)
+    return GetDefaultSourceFile(chrome_root, filename)
 
   return filename
 
 
-def GetClangCommandLineFromNinjaForFilename(out_dir, filename):
-  """Returns the Clang command line for building |filename|
+def GetNinjaBuildOutputsForSourceFile(out_dir, filename):
+  """Returns a list of build outputs for filename.
 
-  Asks ninja for the list of commands used to build |filename| and returns the
-  final Clang invocation.
+  The list is generated by invoking 'ninja -t query' tool to retrieve a list of
+  inputs and outputs of |filename|. This list is then filtered to only include
+  .o and .obj outputs.
 
   Args:
     out_dir: (String) Absolute path to ninja build output directory.
     filename: (String) Absolute path to source file.
 
   Returns:
-    (String) Clang command line or None if command line couldn't be determined.
+    (List of Strings) List of target names. Will return [] if |filename| doesn't
+        yield any .o or .obj outputs.
   """
   # Ninja needs the path to the source file relative to the output build
   # directory.
   rel_filename = os.path.relpath(os.path.realpath(filename), out_dir)
 
-  # Ask ninja how it would build our source file.
-  p = subprocess.Popen(['ninja', '-v', '-C', out_dir, '-t',
-                        'commands', rel_filename + '^'],
+  p = subprocess.Popen(['ninja', '-C', out_dir, '-t', 'query', rel_filename],
+                       stdout=subprocess.PIPE, stderr=subprocess.STDOUT)
+  stdout, _ = p.communicate()
+  if p.returncode:
+    return []
+
+  # The output looks like:
+  #   ../../relative/path/to/source.cc:
+  #     outputs:
+  #       obj/reative/path/to/target.source.o
+  #       obj/some/other/target2.source.o
+  #       another/target.txt
+  #
+  outputs_text = stdout.partition('\n  outputs:\n')[2]
+  output_lines = [line.strip() for line in outputs_text.split('\n')]
+  return [target for target in output_lines
+          if target and (target.endswith('.o') or target.endswith('.obj'))]
+
+
+def GetClangCommandLineForNinjaOutput(out_dir, build_target):
+  """Returns the Clang command line for building |build_target|
+
+  Asks ninja for the list of commands used to build |filename| and returns the
+  final Clang invocation.
+
+  Args:
+    out_dir: (String) Absolute path to ninja build output directory.
+    build_target: (String) A build target understood by ninja
+
+  Returns:
+    (String or None) Clang command line or None if a Clang command line couldn't
+        be determined.
+  """
+  p = subprocess.Popen(['ninja', '-v', '-C', out_dir,
+                        '-t', 'commands', build_target],
                        stdout=subprocess.PIPE)
   stdout, stderr = p.communicate()
   if p.returncode:
     return None
 
-  # Ninja might execute several commands to build something. We want the last
-  # clang command.
+  # Ninja will return multiple build steps for all dependencies up to
+  # |build_target|. The build step we want is the last Clang invocation, which
+  # is expected to be the one that outputs |build_target|.
   for line in reversed(stdout.split('\n')):
     if 'clang' in line:
       return line
   return None
 
 
+def GetClangCommandLineFromNinjaForSource(out_dir, filename):
+  """Returns a Clang command line used to build |filename|.
+
+  The same source file could be built multiple times using different tool
+  chains. In such cases, this command returns the first Clang invocation. We
+  currently don't prefer one toolchain over another. Hopefully the tool chain
+  corresponding to the Clang command line is compatible with the Clang build
+  used by YCM.
+
+  Args:
+    out_dir: (String) Absolute path to Chromium checkout.
+    filename: (String) Absolute path to source file.
+
+  Returns:
+    (String or None): Command line for Clang invocation using |filename| as a
+        source. Returns None if no such command line could be found.
+  """
+  build_targets = GetNinjaBuildOutputsForSourceFile(out_dir, filename)
+  for build_target in build_targets:
+    command_line = GetClangCommandLineForNinjaOutput(out_dir, build_target)
+    if command_line:
+      return command_line
+  return None
+
+
 def GetNormalizedClangCommand(command, out_dir):
   """Gets the normalized Clang binary path if |command| is a Clang command.
 
@@ -363,14 +425,17 @@ def GetClangOptionsFromNinjaForFilename(chrome_root, filename):
   from ninja_output import GetNinjaOutputDirectory
   out_dir = os.path.realpath(GetNinjaOutputDirectory(chrome_root))
 
-  clang_line = GetClangCommandLineFromNinjaForFilename(
-      out_dir, GetBuildTargetForSourceFile(chrome_root, filename))
+  clang_line = GetClangCommandLineFromNinjaForSource(
+      out_dir, GetBuildableSourceFile(chrome_root, filename))
   if not clang_line:
     # If ninja didn't know about filename or it's companion files, then try a
     # default build target. It is possible that the file is new, or build.ninja
     # is stale.
-    clang_line = GetClangCommandLineFromNinjaForFilename(
-        out_dir, GetDefaultCppFile(chrome_root, filename))
+    clang_line = GetClangCommandLineFromNinjaForSource(
+        out_dir, GetDefaultSourceFile(chrome_root, filename))
+
+  if not clang_line:
+    return (additional_flags, [])
 
   return GetClangOptionsFromCommandLine(clang_line, out_dir, additional_flags)