Add python coverage 3.7 to depot tools.

third_party/coverage/control.py

Index: third_party/coverage/control.py
diff --git a/third_party/coverage/control.py b/third_party/coverage/control.py
new file mode 100644
index 0000000000000000000000000000000000000000..4b76121cefebaf35d1a5ed4cf286d13bd9b628fd
--- /dev/null
+++ b/third_party/coverage/control.py
@@ -0,0 +1,776 @@
+"""Core control stuff for Coverage."""
+
+import atexit, os, random, socket, sys
+
+from coverage.annotate import AnnotateReporter
+from coverage.backward import string_class, iitems, sorted  # pylint: disable=W0622
+from coverage.codeunit import code_unit_factory, CodeUnit
+from coverage.collector import Collector
+from coverage.config import CoverageConfig
+from coverage.data import CoverageData
+from coverage.debug import DebugControl
+from coverage.files import FileLocator, TreeMatcher, FnmatchMatcher
+from coverage.files import PathAliases, find_python_files, prep_patterns
+from coverage.html import HtmlReporter
+from coverage.misc import CoverageException, bool_or_none, join_regex
+from coverage.misc import file_be_gone
+from coverage.results import Analysis, Numbers
+from coverage.summary import SummaryReporter
+from coverage.xmlreport import XmlReporter
+
+# Pypy has some unusual stuff in the "stdlib".  Consider those locations
+# when deciding where the stdlib is.
+try:
+    import _structseq       # pylint: disable=F0401
+except ImportError:
+    _structseq = None
+
+
+class coverage(object):
+    """Programmatic access to coverage.py.
+
+    To use::
+
+        from coverage import coverage
+
+        cov = coverage()
+        cov.start()
+        #.. call your code ..
+        cov.stop()
+        cov.html_report(directory='covhtml')
+
+    """
+    def __init__(self, data_file=None, data_suffix=None, cover_pylib=None,
+                auto_data=False, timid=None, branch=None, config_file=True,
+                source=None, omit=None, include=None, debug=None,
+                debug_file=None):
+        """
+        `data_file` is the base name of the data file to use, defaulting to
+        ".coverage".  `data_suffix` is appended (with a dot) to `data_file` to
+        create the final file name.  If `data_suffix` is simply True, then a
+        suffix is created with the machine and process identity included.
+
+        `cover_pylib` is a boolean determining whether Python code installed
+        with the Python interpreter is measured.  This includes the Python
+        standard library and any packages installed with the interpreter.
+
+        If `auto_data` is true, then any existing data file will be read when
+        coverage measurement starts, and data will be saved automatically when
+        measurement stops.
+
+        If `timid` is true, then a slower and simpler trace function will be
+        used.  This is important for some environments where manipulation of
+        tracing functions breaks the faster trace function.
+
+        If `branch` is true, then branch coverage will be measured in addition
+        to the usual statement coverage.
+
+        `config_file` determines what config file to read.  If it is a string,
+        it is the name of the config file to read.  If it is True, then a
+        standard file is read (".coveragerc").  If it is False, then no file is
+        read.
+
+        `source` is a list of file paths or package names.  Only code located
+        in the trees indicated by the file paths or package names will be
+        measured.
+
+        `include` and `omit` are lists of filename patterns. Files that match
+        `include` will be measured, files that match `omit` will not.  Each
+        will also accept a single string argument.
+
+        `debug` is a list of strings indicating what debugging information is
+        desired. `debug_file` is the file to write debug messages to,
+        defaulting to stderr.
+
+        """
+        from coverage import __version__
+
+        # A record of all the warnings that have been issued.
+        self._warnings = []
+
+        # Build our configuration from a number of sources:
+        # 1: defaults:
+        self.config = CoverageConfig()
+
+        # 2: from the coveragerc file:
+        if config_file:
+            if config_file is True:
+                config_file = ".coveragerc"
+            try:
+                self.config.from_file(config_file)
+            except ValueError:
+                _, err, _ = sys.exc_info()
+                raise CoverageException(
+                    "Couldn't read config file %s: %s" % (config_file, err)
+                    )
+
+        # 3: from environment variables:
+        self.config.from_environment('COVERAGE_OPTIONS')
+        env_data_file = os.environ.get('COVERAGE_FILE')
+        if env_data_file:
+            self.config.data_file = env_data_file
+
+        # 4: from constructor arguments:
+        self.config.from_args(
+            data_file=data_file, cover_pylib=cover_pylib, timid=timid,
+            branch=branch, parallel=bool_or_none(data_suffix),
+            source=source, omit=omit, include=include, debug=debug,
+            )
+
+        # Create and configure the debugging controller.
+        self.debug = DebugControl(self.config.debug, debug_file or sys.stderr)
+
+        self.auto_data = auto_data
+
+        # _exclude_re is a dict mapping exclusion list names to compiled
+        # regexes.
+        self._exclude_re = {}
+        self._exclude_regex_stale()
+
+        self.file_locator = FileLocator()
+
+        # The source argument can be directories or package names.
+        self.source = []
+        self.source_pkgs = []
+        for src in self.config.source or []:
+            if os.path.exists(src):
+                self.source.append(self.file_locator.canonical_filename(src))
+            else:
+                self.source_pkgs.append(src)
+
+        self.omit = prep_patterns(self.config.omit)
+        self.include = prep_patterns(self.config.include)
+
+        self.collector = Collector(
+            self._should_trace, timid=self.config.timid,
+            branch=self.config.branch, warn=self._warn
+            )
+
+        # Suffixes are a bit tricky.  We want to use the data suffix only when
+        # collecting data, not when combining data.  So we save it as
+        # `self.run_suffix` now, and promote it to `self.data_suffix` if we
+        # find that we are collecting data later.
+        if data_suffix or self.config.parallel:
+            if not isinstance(data_suffix, string_class):
+                # if data_suffix=True, use .machinename.pid.random
+                data_suffix = True
+        else:
+            data_suffix = None
+        self.data_suffix = None
+        self.run_suffix = data_suffix
+
+        # Create the data file.  We do this at construction time so that the
+        # data file will be written into the directory where the process
+        # started rather than wherever the process eventually chdir'd to.
+        self.data = CoverageData(
+            basename=self.config.data_file,
+            collector="coverage v%s" % __version__,
+            debug=self.debug,
+            )
+
+        # The dirs for files considered "installed with the interpreter".
+        self.pylib_dirs = []
+        if not self.config.cover_pylib:
+            # Look at where some standard modules are located. That's the
+            # indication for "installed with the interpreter". In some
+            # environments (virtualenv, for example), these modules may be
+            # spread across a few locations. Look at all the candidate modules
+            # we've imported, and take all the different ones.
+            for m in (atexit, os, random, socket, _structseq):
+                if m is not None and hasattr(m, "__file__"):
+                    m_dir = self._canonical_dir(m)
+                    if m_dir not in self.pylib_dirs:
+                        self.pylib_dirs.append(m_dir)
+
+        # To avoid tracing the coverage code itself, we skip anything located
+        # where we are.
+        self.cover_dir = self._canonical_dir(__file__)
+
+        # The matchers for _should_trace.
+        self.source_match = None
+        self.pylib_match = self.cover_match = None
+        self.include_match = self.omit_match = None
+
+        # Set the reporting precision.
+        Numbers.set_precision(self.config.precision)
+
+        # Is it ok for no data to be collected?
+        self._warn_no_data = True
+        self._warn_unimported_source = True
+
+        # State machine variables:
+        # Have we started collecting and not stopped it?
+        self._started = False
+        # Have we measured some data and not harvested it?
+        self._measured = False
+
+        atexit.register(self._atexit)
+
+    def _canonical_dir(self, morf):
+        """Return the canonical directory of the module or file `morf`."""
+        return os.path.split(CodeUnit(morf, self.file_locator).filename)[0]
+
+    def _source_for_file(self, filename):
+        """Return the source file for `filename`."""
+        if not filename.endswith(".py"):
+            if filename[-4:-1] == ".py":
+                filename = filename[:-1]
+            elif filename.endswith("$py.class"): # jython
+                filename = filename[:-9] + ".py"
+        return filename
+
+    def _should_trace_with_reason(self, filename, frame):
+        """Decide whether to trace execution in `filename`, with a reason.
+
+        This function is called from the trace function.  As each new file name
+        is encountered, this function determines whether it is traced or not.
+
+        Returns a pair of values:  the first indicates whether the file should
+        be traced: it's a canonicalized filename if it should be traced, None
+        if it should not.  The second value is a string, the resason for the
+        decision.
+
+        """
+        if not filename:
+            # Empty string is pretty useless
+            return None, "empty string isn't a filename"
+
+        if filename.startswith('<'):
+            # Lots of non-file execution is represented with artificial
+            # filenames like "<string>", "<doctest readme.txt[0]>", or
+            # "<exec_function>".  Don't ever trace these executions, since we
+            # can't do anything with the data later anyway.
+            return None, "not a real filename"
+
+        self._check_for_packages()
+
+        # Compiled Python files have two filenames: frame.f_code.co_filename is
+        # the filename at the time the .pyc was compiled.  The second name is
+        # __file__, which is where the .pyc was actually loaded from.  Since
+        # .pyc files can be moved after compilation (for example, by being
+        # installed), we look for __file__ in the frame and prefer it to the
+        # co_filename value.
+        dunder_file = frame.f_globals.get('__file__')
+        if dunder_file:
+            filename = self._source_for_file(dunder_file)
+
+        # Jython reports the .class file to the tracer, use the source file.
+        if filename.endswith("$py.class"):
+            filename = filename[:-9] + ".py"
+
+        canonical = self.file_locator.canonical_filename(filename)
+
+        # If the user specified source or include, then that's authoritative
+        # about the outer bound of what to measure and we don't have to apply
+        # any canned exclusions. If they didn't, then we have to exclude the
+        # stdlib and coverage.py directories.
+        if self.source_match:
+            if not self.source_match.match(canonical):
+                return None, "falls outside the --source trees"
+        elif self.include_match:
+            if not self.include_match.match(canonical):
+                return None, "falls outside the --include trees"
+        else:
+            # If we aren't supposed to trace installed code, then check if this
+            # is near the Python standard library and skip it if so.
+            if self.pylib_match and self.pylib_match.match(canonical):
+                return None, "is in the stdlib"
+
+            # We exclude the coverage code itself, since a little of it will be
+            # measured otherwise.
+            if self.cover_match and self.cover_match.match(canonical):
+                return None, "is part of coverage.py"
+
+        # Check the file against the omit pattern.
+        if self.omit_match and self.omit_match.match(canonical):
+            return None, "is inside an --omit pattern"
+
+        return canonical, "because we love you"
+
+    def _should_trace(self, filename, frame):
+        """Decide whether to trace execution in `filename`.
+
+        Calls `_should_trace_with_reason`, and returns just the decision.
+
+        """
+        canonical, reason = self._should_trace_with_reason(filename, frame)
+        if self.debug.should('trace'):
+            if not canonical:
+                msg = "Not tracing %r: %s" % (filename, reason)
+            else:
+                msg = "Tracing %r" % (filename,)
+            self.debug.write(msg)
+        return canonical
+
+    def _warn(self, msg):
+        """Use `msg` as a warning."""
+        self._warnings.append(msg)
+        sys.stderr.write("Coverage.py warning: %s\n" % msg)
+
+    def _check_for_packages(self):
+        """Update the source_match matcher with latest imported packages."""
+        # Our self.source_pkgs attribute is a list of package names we want to
+        # measure.  Each time through here, we see if we've imported any of
+        # them yet.  If so, we add its file to source_match, and we don't have
+        # to look for that package any more.
+        if self.source_pkgs:
+            found = []
+            for pkg in self.source_pkgs:
+                try:
+                    mod = sys.modules[pkg]
+                except KeyError:
+                    continue
+
+                found.append(pkg)
+
+                try:
+                    pkg_file = mod.__file__
+                except AttributeError:
+                    pkg_file = None
+                else:
+                    d, f = os.path.split(pkg_file)
+                    if f.startswith('__init__'):
+                        # This is actually a package, return the directory.
+                        pkg_file = d
+                    else:
+                        pkg_file = self._source_for_file(pkg_file)
+                    pkg_file = self.file_locator.canonical_filename(pkg_file)
+                    if not os.path.exists(pkg_file):
+                        pkg_file = None
+
+                if pkg_file:
+                    self.source.append(pkg_file)
+                    self.source_match.add(pkg_file)
+                else:
+                    self._warn("Module %s has no Python source." % pkg)
+
+            for pkg in found:
+                self.source_pkgs.remove(pkg)
+
+    def use_cache(self, usecache):
+        """Control the use of a data file (incorrectly called a cache).
+
+        `usecache` is true or false, whether to read and write data on disk.
+
+        """
+        self.data.usefile(usecache)
+
+    def load(self):
+        """Load previously-collected coverage data from the data file."""
+        self.collector.reset()
+        self.data.read()
+
+    def start(self):
+        """Start measuring code coverage.
+
+        Coverage measurement actually occurs in functions called after `start`
+        is invoked.  Statements in the same scope as `start` won't be measured.
+
+        Once you invoke `start`, you must also call `stop` eventually, or your
+        process might not shut down cleanly.
+
+        """
+        if self.run_suffix:
+            # Calling start() means we're running code, so use the run_suffix
+            # as the data_suffix when we eventually save the data.
+            self.data_suffix = self.run_suffix
+        if self.auto_data:
+            self.load()
+
+        # Create the matchers we need for _should_trace
+        if self.source or self.source_pkgs:
+            self.source_match = TreeMatcher(self.source)
+        else:
+            if self.cover_dir:
+                self.cover_match = TreeMatcher([self.cover_dir])
+            if self.pylib_dirs:
+                self.pylib_match = TreeMatcher(self.pylib_dirs)
+        if self.include:
+            self.include_match = FnmatchMatcher(self.include)
+        if self.omit:
+            self.omit_match = FnmatchMatcher(self.omit)
+
+        # The user may want to debug things, show info if desired.
+        if self.debug.should('config'):
+            self.debug.write("Configuration values:")
+            config_info = sorted(self.config.__dict__.items())
+            self.debug.write_formatted_info(config_info)
+
+        if self.debug.should('sys'):
+            self.debug.write("Debugging info:")
+            self.debug.write_formatted_info(self.sysinfo())
+
+        self.collector.start()
+        self._started = True
+        self._measured = True
+
+    def stop(self):
+        """Stop measuring code coverage."""
+        self._started = False
+        self.collector.stop()
+
+    def _atexit(self):
+        """Clean up on process shutdown."""
+        if self._started:
+            self.stop()
+        if self.auto_data:
+            self.save()
+
+    def erase(self):
+        """Erase previously-collected coverage data.
+
+        This removes the in-memory data collected in this session as well as
+        discarding the data file.
+
+        """
+        self.collector.reset()
+        self.data.erase()
+
+    def clear_exclude(self, which='exclude'):
+        """Clear the exclude list."""
+        setattr(self.config, which + "_list", [])
+        self._exclude_regex_stale()
+
+    def exclude(self, regex, which='exclude'):
+        """Exclude source lines from execution consideration.
+
+        A number of lists of regular expressions are maintained.  Each list
+        selects lines that are treated differently during reporting.
+
+        `which` determines which list is modified.  The "exclude" list selects
+        lines that are not considered executable at all.  The "partial" list
+        indicates lines with branches that are not taken.
+
+        `regex` is a regular expression.  The regex is added to the specified
+        list.  If any of the regexes in the list is found in a line, the line
+        is marked for special treatment during reporting.
+
+        """
+        excl_list = getattr(self.config, which + "_list")
+        excl_list.append(regex)
+        self._exclude_regex_stale()
+
+    def _exclude_regex_stale(self):
+        """Drop all the compiled exclusion regexes, a list was modified."""
+        self._exclude_re.clear()
+
+    def _exclude_regex(self, which):
+        """Return a compiled regex for the given exclusion list."""
+        if which not in self._exclude_re:
+            excl_list = getattr(self.config, which + "_list")
+            self._exclude_re[which] = join_regex(excl_list)
+        return self._exclude_re[which]
+
+    def get_exclude_list(self, which='exclude'):
+        """Return a list of excluded regex patterns.
+
+        `which` indicates which list is desired.  See `exclude` for the lists
+        that are available, and their meaning.
+
+        """
+        return getattr(self.config, which + "_list")
+
+    def save(self):
+        """Save the collected coverage data to the data file."""
+        data_suffix = self.data_suffix
+        if data_suffix is True:
+            # If data_suffix was a simple true value, then make a suffix with
+            # plenty of distinguishing information.  We do this here in
+            # `save()` at the last minute so that the pid will be correct even
+            # if the process forks.
+            extra = ""
+            if _TEST_NAME_FILE:
+                f = open(_TEST_NAME_FILE)
+                test_name = f.read()
+                f.close()
+                extra = "." + test_name
+            data_suffix = "%s%s.%s.%06d" % (
+                socket.gethostname(), extra, os.getpid(),
+                random.randint(0, 999999)
+                )
+
+        self._harvest_data()
+        self.data.write(suffix=data_suffix)
+
+    def combine(self):
+        """Combine together a number of similarly-named coverage data files.
+
+        All coverage data files whose name starts with `data_file` (from the
+        coverage() constructor) will be read, and combined together into the
+        current measurements.
+
+        """
+        aliases = None
+        if self.config.paths:
+            aliases = PathAliases(self.file_locator)
+            for paths in self.config.paths.values():
+                result = paths[0]
+                for pattern in paths[1:]:
+                    aliases.add(pattern, result)
+        self.data.combine_parallel_data(aliases=aliases)
+
+    def _harvest_data(self):
+        """Get the collected data and reset the collector.
+
+        Also warn about various problems collecting data.
+
+        """
+        if not self._measured:
+            return
+
+        self.data.add_line_data(self.collector.get_line_data())
+        self.data.add_arc_data(self.collector.get_arc_data())
+        self.collector.reset()
+
+        # If there are still entries in the source_pkgs list, then we never
+        # encountered those packages.
+        if self._warn_unimported_source:
+            for pkg in self.source_pkgs:
+                self._warn("Module %s was never imported." % pkg)
+
+        # Find out if we got any data.
+        summary = self.data.summary()
+        if not summary and self._warn_no_data:
+            self._warn("No data was collected.")
+
+        # Find files that were never executed at all.
+        for src in self.source:
+            for py_file in find_python_files(src):
+                py_file = self.file_locator.canonical_filename(py_file)
+
+                if self.omit_match and self.omit_match.match(py_file):
+                    # Turns out this file was omitted, so don't pull it back
+                    # in as unexecuted.
+                    continue
+
+                self.data.touch_file(py_file)
+
+        self._measured = False
+
+    # Backward compatibility with version 1.
+    def analysis(self, morf):
+        """Like `analysis2` but doesn't return excluded line numbers."""
+        f, s, _, m, mf = self.analysis2(morf)
+        return f, s, m, mf
+
+    def analysis2(self, morf):
+        """Analyze a module.
+
+        `morf` is a module or a filename.  It will be analyzed to determine
+        its coverage statistics.  The return value is a 5-tuple:
+
+        * The filename for the module.
+        * A list of line numbers of executable statements.
+        * A list of line numbers of excluded statements.
+        * A list of line numbers of statements not run (missing from
+          execution).
+        * A readable formatted string of the missing line numbers.
+
+        The analysis uses the source file itself and the current measured
+        coverage data.
+
+        """
+        analysis = self._analyze(morf)
+        return (
+            analysis.filename, analysis.statements, analysis.excluded,
+            analysis.missing, analysis.missing_formatted()
+            )
+
+    def _analyze(self, it):
+        """Analyze a single morf or code unit.
+
+        Returns an `Analysis` object.
+
+        """
+        self._harvest_data()
+        if not isinstance(it, CodeUnit):
+            it = code_unit_factory(it, self.file_locator)[0]
+
+        return Analysis(self, it)
+
+    def report(self, morfs=None, show_missing=True, ignore_errors=None,
+                file=None,                          # pylint: disable=W0622
+                omit=None, include=None
+                ):
+        """Write a summary report to `file`.
+
+        Each module in `morfs` is listed, with counts of statements, executed
+        statements, missing statements, and a list of lines missed.
+
+        `include` is a list of filename patterns.  Modules whose filenames
+        match those patterns will be included in the report. Modules matching
+        `omit` will not be included in the report.
+
+        Returns a float, the total percentage covered.
+
+        """
+        self._harvest_data()
+        self.config.from_args(
+            ignore_errors=ignore_errors, omit=omit, include=include,
+            show_missing=show_missing,
+            )
+        reporter = SummaryReporter(self, self.config)
+        return reporter.report(morfs, outfile=file)
+
+    def annotate(self, morfs=None, directory=None, ignore_errors=None,
+                    omit=None, include=None):
+        """Annotate a list of modules.
+
+        Each module in `morfs` is annotated.  The source is written to a new
+        file, named with a ",cover" suffix, with each line prefixed with a
+        marker to indicate the coverage of the line.  Covered lines have ">",
+        excluded lines have "-", and missing lines have "!".
+
+        See `coverage.report()` for other arguments.
+
+        """
+        self._harvest_data()
+        self.config.from_args(
+            ignore_errors=ignore_errors, omit=omit, include=include
+            )
+        reporter = AnnotateReporter(self, self.config)
+        reporter.report(morfs, directory=directory)
+
+    def html_report(self, morfs=None, directory=None, ignore_errors=None,
+                    omit=None, include=None, extra_css=None, title=None):
+        """Generate an HTML report.
+
+        The HTML is written to `directory`.  The file "index.html" is the
+        overview starting point, with links to more detailed pages for
+        individual modules.
+
+        `extra_css` is a path to a file of other CSS to apply on the page.
+        It will be copied into the HTML directory.
+
+        `title` is a text string (not HTML) to use as the title of the HTML
+        report.
+
+        See `coverage.report()` for other arguments.
+
+        Returns a float, the total percentage covered.
+
+        """
+        self._harvest_data()
+        self.config.from_args(
+            ignore_errors=ignore_errors, omit=omit, include=include,
+            html_dir=directory, extra_css=extra_css, html_title=title,
+            )
+        reporter = HtmlReporter(self, self.config)
+        return reporter.report(morfs)
+
+    def xml_report(self, morfs=None, outfile=None, ignore_errors=None,
+                    omit=None, include=None):
+        """Generate an XML report of coverage results.
+
+        The report is compatible with Cobertura reports.
+
+        Each module in `morfs` is included in the report.  `outfile` is the
+        path to write the file to, "-" will write to stdout.
+
+        See `coverage.report()` for other arguments.
+
+        Returns a float, the total percentage covered.
+
+        """
+        self._harvest_data()
+        self.config.from_args(
+            ignore_errors=ignore_errors, omit=omit, include=include,
+            xml_output=outfile,
+            )
+        file_to_close = None
+        delete_file = False
+        if self.config.xml_output:
+            if self.config.xml_output == '-':
+                outfile = sys.stdout
+            else:
+                outfile = open(self.config.xml_output, "w")
+                file_to_close = outfile
+        try:
+            try:
+                reporter = XmlReporter(self, self.config)
+                return reporter.report(morfs, outfile=outfile)
+            except CoverageException:
+                delete_file = True
+                raise
+        finally:
+            if file_to_close:
+                file_to_close.close()
+                if delete_file:
+                    file_be_gone(self.config.xml_output)
+
+    def sysinfo(self):
+        """Return a list of (key, value) pairs showing internal information."""
+
+        import coverage as covmod
+        import platform, re
+
+        try:
+            implementation = platform.python_implementation()
+        except AttributeError:
+            implementation = "unknown"
+
+        info = [
+            ('version', covmod.__version__),
+            ('coverage', covmod.__file__),
+            ('cover_dir', self.cover_dir),
+            ('pylib_dirs', self.pylib_dirs),
+            ('tracer', self.collector.tracer_name()),
+            ('config_files', self.config.attempted_config_files),
+            ('configs_read', self.config.config_files),
+            ('data_path', self.data.filename),
+            ('python', sys.version.replace('\n', '')),
+            ('platform', platform.platform()),
+            ('implementation', implementation),
+            ('executable', sys.executable),
+            ('cwd', os.getcwd()),
+            ('path', sys.path),
+            ('environment', sorted([
+                ("%s = %s" % (k, v)) for k, v in iitems(os.environ)
+                    if re.search(r"^COV|^PY", k)
+                ])),
+            ('command_line', " ".join(getattr(sys, 'argv', ['???']))),
+            ]
+        if self.source_match:
+            info.append(('source_match', self.source_match.info()))
+        if self.include_match:
+            info.append(('include_match', self.include_match.info()))
+        if self.omit_match:
+            info.append(('omit_match', self.omit_match.info()))
+        if self.cover_match:
+            info.append(('cover_match', self.cover_match.info()))
+        if self.pylib_match:
+            info.append(('pylib_match', self.pylib_match.info()))
+
+        return info
+
+
+def process_startup():
+    """Call this at Python startup to perhaps measure coverage.
+
+    If the environment variable COVERAGE_PROCESS_START is defined, coverage
+    measurement is started.  The value of the variable is the config file
+    to use.
+
+    There are two ways to configure your Python installation to invoke this
+    function when Python starts:
+
+    #. Create or append to sitecustomize.py to add these lines::
+
+        import coverage
+        coverage.process_startup()
+
+    #. Create a .pth file in your Python installation containing::
+
+        import coverage; coverage.process_startup()
+
+    """
+    cps = os.environ.get("COVERAGE_PROCESS_START")
+    if cps:
+        cov = coverage(config_file=cps, auto_data=True)
+        cov.start()
+        cov._warn_no_data = False
+        cov._warn_unimported_source = False
+
+
+# A hack for debugging testing in subprocesses.
+_TEST_NAME_FILE = "" #"/tmp/covtest.txt"