bisect_driver.py

# Copyright 2016 Google Inc. All Rights Reserved.
#
# This script is used to help the compiler wrapper in the Android build system
# bisect for bad object files.
"""Utilities for bisection of Android object files.

This module contains a set of utilities to allow bisection between
two sets (good and bad) of object files. Mostly used to find compiler
bugs.

Reference page:
https://sites.google.com/a/google.com/chromeos-toolchain-team-home2/home/team-tools-and-scripts/bisecting-chromeos-compiler-problems/bisection-compiler-wrapper

Design doc:
https://docs.google.com/document/d/1yDgaUIa2O5w6dc3sSTe1ry-1ehKajTGJGQCbyn0fcEM
"""

from __future__ import print_function

import contextlib
import fcntl
import os
import shutil
import subprocess
import sys

VALID_MODES = ['POPULATE_GOOD', 'POPULATE_BAD', 'TRIAGE']
GOOD_CACHE = 'good'
BAD_CACHE = 'bad'
LIST_FILE = os.path.join(GOOD_CACHE, '_LIST')

CONTINUE_ON_MISSING = os.environ.get('BISECT_CONTINUE_ON_MISSING', None) == '1'
WRAPPER_SAFE_MODE = os.environ.get('BISECT_WRAPPER_SAFE_MODE', None) == '1'


class Error(Exception):
  """The general compiler wrapper error class."""
  pass


@contextlib.contextmanager
def lock_file(path, mode):
  """Lock file and block if other process has lock on file.

  Acquire exclusive lock for file. Only blocks other processes if they attempt
  to also acquire lock through this method. If only reading (modes 'r' and 'rb')
  then the lock is shared (i.e. many reads can happen concurrently, but only one
  process may write at a time).

  This function is a contextmanager, meaning it's meant to be used with the
  "with" statement in Python. This is so cleanup and setup happens automatically
  and cleanly. Execution of the outer "with" statement happens at the "yield"
  statement. Execution resumes after the yield when the outer "with" statement
  ends.

  Args:
    path: path to file being locked
    mode: mode to open file with ('w', 'r', etc.)
  """
  with open(path, mode) as f:
    # Share the lock if just reading, make lock exclusive if writing
    if f.mode == 'r' or f.mode == 'rb':
      lock_type = fcntl.LOCK_SH
    else:
      lock_type = fcntl.LOCK_EX

    try:
      fcntl.lockf(f, lock_type)
      yield f
      f.flush()
    except:
      raise
    finally:
      fcntl.lockf(f, fcntl.LOCK_UN)


def log_to_file(path, execargs, link_from=None, link_to=None):
  """Common logging function.

  Log current working directory, current execargs, and a from-to relationship
  between files.
  """
  with lock_file(path, 'a') as log:
    log.write('cd: %s; %s\n' % (os.getcwd(), ' '.join(execargs)))
    if link_from and link_to:
      log.write('%s -> %s\n' % (link_from, link_to))


def exec_and_return(execargs):
  """Execute process and return.

  Execute according to execargs and return immediately. Don't inspect
  stderr or stdout.
  """
  return subprocess.call(execargs)


def which_cache(obj_file):
  """Determine which cache an object belongs to.

  The binary search tool creates two files for each search iteration listing
  the full set of bad objects and full set of good objects. We use this to
  determine where an object file should be linked from (good or bad).
  """
  bad_set_file = os.environ.get('BISECT_BAD_SET')
  ret = subprocess.call(['grep', '-x', '-q', obj_file, bad_set_file])
  if ret == 0:
    return BAD_CACHE
  else:
    return GOOD_CACHE


def makedirs(path):
  """Try to create directories in path."""
  try:
    os.makedirs(path)
  except os.error:
    if not os.path.isdir(path):
      raise


def get_obj_path(execargs):
  """Get the object path for the object file in the list of arguments.

  Returns:
    Absolute object path from execution args (-o argument). If no object being
    outputted or output doesn't end in ".o" then return empty string.
  """
  try:
    i = execargs.index('-o')
  except ValueError:
    return ''

  obj_path = execargs[i + 1]
  if not obj_path.endswith(('.o',)):
    # TODO: what suffixes do we need to contemplate
    # TODO: add this as a warning
    # TODO: need to handle -r compilations
    return ''

  return os.path.abspath(obj_path)


def get_dep_path(execargs):
  """Get the dep file path for the dep file in the list of arguments.

  Returns:
    Absolute path of dependency file path from execution args (-o argument). If
    no dependency being outputted then return empty string.
  """
  if '-MD' not in execargs and '-MMD' not in execargs:
    return ''

  # If -MF given this is the path of the dependency file. Otherwise the
  # dependency file is the value of -o but with a .d extension
  if '-MF' in execargs:
    i = execargs.index('-MF')
    dep_path = execargs[i + 1]
    return os.path.abspath(dep_path)

  full_obj_path = get_obj_path(execargs)
  if not full_obj_path:
    return ''

  return full_obj_path[:-2] + '.d'


def get_dwo_path(execargs):
  """Get the dwo file path for the dwo file in the list of arguments.

  Returns:
    Absolute dwo file path from execution args (-gsplit-dwarf argument) If no
    dwo file being outputted then return empty string.
  """
  if '-gsplit-dwarf' not in execargs:
    return ''

  full_obj_path = get_obj_path(execargs)
  if not full_obj_path:
    return ''

  return full_obj_path[:-2] + '.dwo'


def in_object_list(obj_name, list_filename):
  """Check if object file name exist in file with object list."""
  if not obj_name:
    return False

  with lock_file(list_filename, 'r') as list_file:
    for line in list_file:
      if line.strip() == obj_name:
        return True

    return False


def get_side_effects(execargs):
  """Determine side effects generated by compiler

  Returns:
    List of paths of objects that the compiler generates as side effects.
  """
  side_effects = []

  # Cache dependency files
  full_dep_path = get_dep_path(execargs)
  if full_dep_path:
    side_effects.append(full_dep_path)

  # Cache dwo files
  full_dwo_path = get_dwo_path(execargs)
  if full_dwo_path:
    side_effects.append(full_dwo_path)

  return side_effects


def cache_file(execargs, bisect_dir, cache, abs_file_path):
  """Cache compiler output file (.o/.d/.dwo)."""
  # os.path.join fails with absolute paths, use + instead
  bisect_path = os.path.join(bisect_dir, cache) + abs_file_path
  bisect_path_dir = os.path.dirname(bisect_path)
  makedirs(bisect_path_dir)
  pop_log = os.path.join(bisect_dir, cache, '_POPULATE_LOG')
  log_to_file(pop_log, execargs, abs_file_path, bisect_path)

  try:
    if os.path.exists(abs_file_path):
      shutil.copy2(abs_file_path, bisect_path)
  except Exception:
    print('Could not cache file %s' % abs_file_path, file=sys.stderr)
    raise


def restore_file(bisect_dir, cache, abs_file_path):
  """Restore file from cache (.o/.d/.dwo)."""
  # os.path.join fails with absolute paths, use + instead
  cached_path = os.path.join(bisect_dir, cache) + abs_file_path
  if os.path.exists(cached_path):
    if os.path.exists(abs_file_path):
      os.remove(abs_file_path)
    try:
      os.link(cached_path, abs_file_path)
    except OSError:
      shutil.copyfile(cached_path, abs_file_path)
  else:
    raise Error(('%s is missing from %s cache! Unsure how to proceed. Make '
                 'will now crash.' % (cache, cached_path)))


def bisect_populate(execargs, bisect_dir, population_name):
  """Add necessary information to the bisect cache for the given execution.

  Extract the necessary information for bisection from the compiler
  execution arguments and put it into the bisection cache. This
  includes copying the created object file, adding the object
  file path to the cache list and keeping a log of the execution.

  Args:
    execargs: compiler execution arguments.
    bisect_dir: bisection directory.
    population_name: name of the cache being populated (good/bad).
  """
  retval = exec_and_return(execargs)
  if retval:
    return retval

  full_obj_path = get_obj_path(execargs)
  # If not a normal compiler call then just exit
  if not full_obj_path:
    return

  cache_file(execargs, bisect_dir, population_name, full_obj_path)

  population_dir = os.path.join(bisect_dir, population_name)
  with lock_file(os.path.join(population_dir, '_LIST'), 'a') as object_list:
    object_list.write('%s\n' % full_obj_path)

  for side_effect in get_side_effects(execargs):
    cache_file(execargs, bisect_dir, population_name, side_effect)


def bisect_triage(execargs, bisect_dir):
  full_obj_path = get_obj_path(execargs)
  obj_list = os.path.join(bisect_dir, LIST_FILE)

  # If the output isn't an object file just call compiler
  if not full_obj_path:
    return exec_and_return(execargs)

  # If this isn't a bisected object just call compiler
  # This shouldn't happen!
  if not in_object_list(full_obj_path, obj_list):
    if CONTINUE_ON_MISSING:
      log_file = os.path.join(bisect_dir, '_MISSING_CACHED_OBJ_LOG')
      log_to_file(log_file, execargs, '? compiler', full_obj_path)
      return exec_and_return(execargs)
    else:
      raise Error(('%s is missing from cache! To ignore export '
                   'BISECT_CONTINUE_ON_MISSING=1. See documentation for more '
                   'details on this option.' % full_obj_path))

  cache = which_cache(full_obj_path)

  # If using safe WRAPPER_SAFE_MODE option call compiler and overwrite the
  # result from the good/bad cache. This option is safe and covers all compiler
  # side effects, but is very slow!
  if WRAPPER_SAFE_MODE:
    retval = exec_and_return(execargs)
    if retval:
      return retval
    os.remove(full_obj_path)
    restore_file(bisect_dir, cache, full_obj_path)
    return

  # Generate compiler side effects. Trick Make into thinking compiler was
  # actually executed.
  for side_effect in get_side_effects(execargs):
    restore_file(bisect_dir, cache, side_effect)

  # If generated object file happened to be pruned/cleaned by Make then link it
  # over from cache again.
  if not os.path.exists(full_obj_path):
    restore_file(bisect_dir, cache, full_obj_path)


def bisect_driver(bisect_stage, bisect_dir, execargs):
  """Call appropriate bisection stage according to value in bisect_stage."""
  if bisect_stage == 'POPULATE_GOOD':
    bisect_populate(execargs, bisect_dir, GOOD_CACHE)
  elif bisect_stage == 'POPULATE_BAD':
    bisect_populate(execargs, bisect_dir, BAD_CACHE)
  elif bisect_stage == 'TRIAGE':
    bisect_triage(execargs, bisect_dir)
  else:
    raise ValueError('wrong value for BISECT_STAGE: %s' % bisect_stage)