fix: workaround absolute paths appearing in coverage data (#524)

Issue reported here:
aspect-build/bazel-examples#404 (comment)

Now looks like

```
SF:src/__init__.py
DA:1,1
DA:2,1
```

Similar to
https://github.com/bazelbuild/rules_python/blob/466da1d9710289bfb01061b9be7bb124132996e0/python/private/python_bootstrap_template.txt#L336-L346
but ours is a bit more principled since we don't break the abstraction
of what coveragepy happens to do with the paths.


### Changes are visible to end-users: no

### Test plan

- Manual testing; please provide instructions so we can reproduce:
Just `bazel coverage src:test` and inspect the .dat file

.github/workflows/ci.yaml

@@ -76,6 +76,7 @@ jobs:
         uses: zgosalvez/github-actions-report-lcov@5989987f8058a03137e90bc16f9c0baaac5e069a # v4.1.22
         with:
           title-prefix: "e2e/use_release folder:"
+          working-directory: e2e/use_release
           # Point to the already-merged data file Bazel produces with --combined_report=lcov
           # Follows https://bazel.build/configure/coverage#running_coverage
           coverage-files: "${{ env.output_path }}/_coverage/_coverage_report.dat"

e2e/use_release/.coveragerc

@@ -0,0 +1,6 @@
+[run]
+# NB: this ought to put relative paths into the report files.
+# However Bazel actions run in a sandbox with symlinks, and coveragepy follows those out of the sandbox
+# resulting in the wrong paths.
+# See https://github.com/nedbat/coveragepy/issues/963 and https://github.com/aspect-build/rules_py/pull/524
+relative_files = False

e2e/use_release/BUILD.bazel

@@ -1,3 +1,5 @@
 load("@rules_uv//uv:pip.bzl", "pip_compile")
 
+exports_files([".coveragerc"])
+
 pip_compile(name = "pip_compile")

e2e/use_release/src/BUILD.bazel

@@ -12,6 +12,7 @@ py_binary(
 # TODO(alex): land https://github.com/aspect-build/rules_py/pull/401 and shorten this
 py_pytest_main(
     name = "__test__",
+    data = ["//:.coveragerc"],
     deps = [
         "@pip//coverage",
         "@pip//pytest",

py/private/pytest.py.tmpl

@@ -20,6 +20,8 @@ from typing import List
 import pytest
 # None means coverage wasn't enabled
 cov = None
+# For workaround of https://github.com/nedbat/coveragepy/issues/963
+coveragepy_absfile_mapping = {}
 
 # Since our py_test had InstrumentedFilesInfo, we know Bazel will hand us this environment variable.
 # https://bazel.build/rules/lib/providers/InstrumentedFilesInfo
@@ -28,9 +30,13 @@ if "COVERAGE_MANIFEST" in os.environ:
         import coverage
         # The lines are files that matched the --instrumentation_filter flag
         with open(os.getenv("COVERAGE_MANIFEST"), "r") as mf:
-            cov = coverage.Coverage(include = mf.read().splitlines())
+            manifest_entries = mf.read().splitlines()
+            cov = coverage.Coverage(include = manifest_entries)
+            # coveragepy incorrectly converts our entries by following symlinks
+            # record a mapping of their conversion so we can undo it later in reporting the coverage
+            coveragepy_absfile_mapping = {coverage.files.abs_file(mfe): mfe for mfe in manifest_entries}
         cov.start()
-    except Exception as e:
+    except ModuleNotFoundError as e:
         print("WARNING: python coverage setup failed. Do you need to include the 'coverage' library as a dependency of py_pytest_main?", e)
         pass
 
@@ -95,15 +101,22 @@ if __name__ == "__main__":
         # https://bazel.build/configure/coverage
         coverage_output_file = os.getenv("COVERAGE_OUTPUT_FILE")
 
-        # Workaround https://github.com/bazelbuild/bazel/issues/25118
-        # by removing 'end line number' from FN: records
         unfixed_dat = coverage_output_file + ".tmp"
         cov.lcov_report(outfile = unfixed_dat)
         cov.save()
 
         with open(unfixed_dat, "r") as unfixed:
           with open(coverage_output_file, "w") as output_file:
             for line in unfixed:
+              # Workaround https://github.com/nedbat/coveragepy/issues/963
+              # by mapping SF: records to un-do the symlink-following
+              if line.startswith('SF:'):
+                sourcefile = line[3:].rstrip()
+                if sourcefile in coveragepy_absfile_mapping:
+                    output_file.write(f"SF:{coveragepy_absfile_mapping[sourcefile]}\n")
+                    continue
+              # Workaround https://github.com/bazelbuild/bazel/issues/25118
+              # by removing 'end line number' from FN: records
               if line.startswith('FN:'):
                 parts = line[3:].split(",")  # Remove 'FN:' and split by commas
                 if len(parts) == 3: