Improve py.test covered paths reporting.

Use a coverage plugin instead of `coverage combine` to open the door to
using a single PEX test source chroot even when testing against multiple
repo source roots at once.

Groundwork for pantsbuild#5426.

3rdparty/python/requirements.txt

@@ -2,7 +2,7 @@ ansicolors==1.0.2
 beautifulsoup4>=4.3.2,<4.4
 cffi==1.11.1
 contextlib2==0.5.5
-coverage>=4.3.4,<4.4
+coverage>=4.5,<4.6
 docutils>=0.12,<0.13
 fasteners==0.14.1
 faulthandler==2.6
@@ -20,8 +20,8 @@ pyflakes==1.1.0
 Pygments==1.4
 pyopenssl==17.3.0
 pystache==0.5.3
-pytest-cov>=2.4,<2.5
-pytest>=3.0.7,<4.0
+pytest-cov>=2.5,<2.6
+pytest>=3.4,<4.0
 pywatchman==1.4.1
 requests[security]>=2.5.0,<2.19
 scandir==1.2

src/python/pants/backend/python/tasks/BUILD

@@ -11,6 +11,7 @@ python_library(
     'src/python/pants/backend/python:interpreter_cache',
     'src/python/pants/backend/python/subsystems',
     'src/python/pants/backend/python/targets',
+    'src/python/pants/backend/python/tasks/coverage:plugin',
     'src/python/pants/base:build_environment',
     'src/python/pants/base:exceptions',
     'src/python/pants/base:fingerprint_strategy',

src/python/pants/backend/python/tasks/coverage/BUILD

@@ -0,0 +1,7 @@
+# Copyright 2018 Pants project contributors (see CONTRIBUTORS.md).
+# Licensed under the Apache License, Version 2.0 (see LICENSE).
+
+resources(
+  name='plugin',
+  sources=['plugin.py']
+)

src/python/pants/backend/python/tasks/coverage/plugin.py

@@ -0,0 +1,94 @@
+# coding=utf-8
+# Copyright 2018 Pants project contributors (see CONTRIBUTORS.md).
+# Licensed under the Apache License, Version 2.0 (see LICENSE).
+
+from __future__ import (absolute_import, division, generators, nested_scopes, print_function,
+                        unicode_literals, with_statement)
+
+import json
+import os
+
+from coverage import CoveragePlugin, FileTracer
+from coverage.config import DEFAULT_PARTIAL, DEFAULT_PARTIAL_ALWAYS
+from coverage.misc import join_regex
+from coverage.parser import PythonParser
+from coverage.python import PythonFileReporter
+
+
+class MyFileTracer(FileTracer):
+  def __init__(self, filename):
+    super(MyFileTracer, self).__init__()
+    self._filename = filename
+
+  def source_filename(self):
+    return self._filename
+
+
+class MyFileReporter(PythonFileReporter):
+  """A python file reporter that knows how to map Pants PEX chroots back to repo source code."""
+
+  def __init__(self, morf, relpath):
+    super(MyFileReporter, self).__init__(morf, coverage=None)
+    self._relpath = relpath
+
+  def relative_filename(self):
+    return self._relpath
+
+  # TODO(John Sirois): Kill the workaround overrides below if there is a useable upstream
+  # resolution to:
+  #   https://bitbucket.org/ned/coveragepy/issues/646/modifying-coverage-reporting-for-python
+
+  @property
+  def parser(self):
+    if self._parser is None:
+      self._parser = PythonParser(filename=self.filename)
+      self._parser.parse_source()
+    return self._parser
+
+  def no_branch_lines(self):
+    return self.parser.lines_matching(join_regex(DEFAULT_PARTIAL[:]),
+                                      join_regex(DEFAULT_PARTIAL_ALWAYS[:]))
+
+
+class MyPlugin(CoveragePlugin):
+  """A plugin that knows how to map Pants PEX chroots back to repo source code when reporting."""
+
+  def __init__(self, buildroot, src_to_chroot):
+    super(MyPlugin, self).__init__()
+    self._buildroot = buildroot
+    self._src_to_chroot = src_to_chroot
+
+  def find_executable_files(self, top):
+    for chroot_path in self._src_to_chroot.values():
+      if top.startswith(chroot_path):
+        for root, _, files in os.walk(top):
+          for f in files:
+            if f.endswith('.py'):
+              yield os.path.join(root, f)
+        break
+
+  def file_tracer(self, filename):
+    for chroot_path in self._src_to_chroot.values():
+      if filename.startswith(chroot_path):
+        return MyFileTracer(filename)
+
+  def file_reporter(self, filename):
+    src_file = self._map_to_src(filename)
+    mapped_relpath = os.path.relpath(src_file, self._buildroot)
+    return MyFileReporter(filename, mapped_relpath)
+
+  def _map_to_src(self, chroot):
+    for src_dir, chroot_dir in self._src_to_chroot.items():
+      if chroot.startswith(chroot_dir):
+        return src_dir + chroot[len(chroot_dir):]
+    raise AssertionError('Failed to map traced file {} to any source root via '
+                         'source root -> chroot mappings:\n\t{}'
+                         .format(chroot, '\n\t'.join(sorted('{} -> {}'.format(src_dir, chroot_dir)
+                                                            for src_dir, chroot_dir
+                                                            in self._src_to_chroot.items()))))
+
+
+def coverage_init(reg, options):
+  buildroot = options['buildroot']
+  src_to_chroot = json.loads(options['src_to_chroot'])
+  reg.add_file_tracer(MyPlugin(buildroot, src_to_chroot))

src/python/pants/backend/python/tasks/pytest_prep.py

@@ -7,6 +7,7 @@
 
 import os
 
+import pkg_resources
 from pex.pex_info import PexInfo
 
 from pants.backend.python.subsystems.pytest import PyTest
@@ -38,6 +39,10 @@ def config_path(self):
       """
       return os.path.join(self._pex.path(), 'pytest.ini')
 
+  @classmethod
+  def implementation_version(cls):
+    return super(PytestPrep, cls).implementation_version() + [('PytestPrep', 1)]
+
   @classmethod
   def product_types(cls):
     return [cls.PytestBinary]
@@ -52,6 +57,11 @@ def extra_requirements(self):
   def extra_files(self):
     yield self.ExtraFile.empty('pytest.ini')
 
+    enclosing_dir = os.path.dirname(__name__.replace('.', os.sep))
+    plugin_path = os.path.join(enclosing_dir, 'coverage/plugin.py')
+    yield self.ExtraFile(path=plugin_path,
+                         content=pkg_resources.resource_string(__name__, 'coverage/plugin.py'))
+
   def execute(self):
     pex_info = PexInfo.default()
     pex_info.entry_point = 'pytest'