Ensure source dirs are absolute

This `COV_CORE_SOURCE` environment variable is key for making sure that
child processes continue computing code coverage. However, there's no
guarantee that child processes start in the same directory as their parent
process, which screws up coverage reporting if you're using relative
paths for coverage sources. The fix is to make sure we're dealing with
absolute paths.

This is a little tricky to get right, because sources can include both
dirs and packages.

This fixes #465

Author: jfly

Diff for: src/pytest_cov/engine.py

@@ -108,7 +108,17 @@ def set_env(self):
         if self.cov_source is None:
             os.environ['COV_CORE_SOURCE'] = os.pathsep
         else:
-            os.environ['COV_CORE_SOURCE'] = os.pathsep.join(self.cov_source)
+            os.environ['COV_CORE_SOURCE'] = os.pathsep.join(
+                # We must make the paths absolute to ensure that subprocesses started in different
+                # directories still find the same sources: https://github.com/nedbat/coveragepy/issues/1942.
+                # Unfortunately, coverage.py doesn't provide a mechanism
+                # for unambiguously specifying source_dirs, so we have to detect if a source is
+                # a directory or a package by looking in the filesystem: https://github.com/nedbat/coveragepy/issues/1942.
+                os.path.abspath(source)  # noqa: PTH100 #  see note below for why we're using `os.path.abspath`
+                if Path(source).is_dir()
+                else source
+                for source in self.cov_source
+            )
         config_file = Path(self.cov_config)
         if config_file.exists():
             os.environ['COV_CORE_CONFIG'] = os.fspath(config_file.resolve())