feat: collect coverage data from Python subprocesses

examples/pytest/BUILD.bazel

@@ -170,3 +170,38 @@ sh_test(
         ":coverage_setup_test",
     ],
 )
+
+# ─── Subprocess coverage regression ────────────────────────────────────
+#
+# Verifies that coverage data is captured from Python subprocesses
+# (e.g. pytest-xdist workers, subprocess.Popen). The test calls
+# foo.subtract() in a subprocess; the LCOV should include DA hits for
+# both add() (in-process) and subtract() (subprocess).
+
+py_test(
+    name = "subprocess_coverage_setup_test",
+    srcs = ["subprocess_coverage_test.py"],
+    data = [":coverage_manifest"],
+    env = {
+        "COVERAGE_MANIFEST": "$(rootpath :coverage_manifest)",
+        "COVERAGE_OUTPUT_FILE": "subprocess_coverage_setup_test.lcov",
+    },
+    imports = ["../.."],
+    package_collisions = "warning",
+    pytest_main = True,
+    tags = ["manual"],
+    deps = [
+        ":lib",
+        "@pypi//coverage",
+        "@pypi//pytest",
+    ],
+)
+
+sh_test(
+    name = "subprocess_coverage_lcov_test",
+    srcs = ["run_subprocess_coverage_check.sh"],
+    data = [
+        ":coverage_manifest",
+        ":subprocess_coverage_setup_test",
+    ],
+)

examples/pytest/run_subprocess_coverage_check.sh

@@ -0,0 +1,40 @@
+#!/usr/bin/env bash
+# Verifies that subprocess coverage data is captured in the LCOV output.
+# The subprocess_coverage_test.py test calls foo.subtract() (line 5) in
+# a subprocess. If subprocess coverage propagation works, the LCOV
+# should contain a DA:5,<nonzero> record.
+
+set -euo pipefail
+
+LAUNCHER="$TEST_SRCDIR/_main/examples/pytest/subprocess_coverage_setup_test"
+MANIFEST="$TEST_SRCDIR/_main/examples/pytest/coverage_manifest.txt"
+
+[[ -x "$LAUNCHER" ]] || { echo "launcher not found: $LAUNCHER" >&2; exit 1; }
+[[ -f "$MANIFEST" ]] || { echo "manifest not found: $MANIFEST" >&2; exit 1; }
+
+LCOV="$(mktemp -d)/coverage.lcov"
+
+COVERAGE_MANIFEST="$MANIFEST" \
+  COVERAGE_OUTPUT_FILE="$LCOV" \
+  "$LAUNCHER"
+
+[[ -s "$LCOV" ]] || { echo "LCOV file empty or missing: $LCOV" >&2; exit 1; }
+
+# foo.py line 2 (add body) should be covered in-process.
+grep -qE '^DA:2,[1-9]' "$LCOV" || {
+  echo "Expected DA:2,<nonzero> (add function) not found in LCOV." >&2
+  echo "LCOV contents:" >&2
+  cat "$LCOV" >&2
+  exit 1
+}
+
+# foo.py line 5 (subtract body) should be covered via subprocess.
+grep -qE '^DA:5,[1-9]' "$LCOV" || {
+  echo "Expected DA:5,<nonzero> (subtract function, called in subprocess) not found." >&2
+  echo "This means subprocess coverage data was not captured." >&2
+  echo "LCOV contents:" >&2
+  cat "$LCOV" >&2
+  exit 1
+}
+
+echo "OK: subprocess coverage captured — both add() and subtract() have DA hits."

examples/pytest/subprocess_coverage_test.py

@@ -0,0 +1,28 @@
+"""Regression test: coverage data from Python subprocesses is captured.
+
+The test calls foo.subtract() in a subprocess. If subprocess coverage
+propagation works, the LCOV report will include a DA hit for line 5
+of foo.py (the subtract function body).
+"""
+
+import os
+import subprocess
+import sys
+
+from examples.pytest.foo import add
+
+
+def test_add_in_process():
+    assert add(1, 1) == 2
+
+
+def test_subtract_in_subprocess():
+    env = {**os.environ, "PYTHONPATH": os.pathsep.join(sys.path)}
+    result = subprocess.run(
+        [sys.executable, "-c", "from examples.pytest.foo import subtract; print(subtract(3, 1))"],
+        capture_output=True,
+        text=True,
+        env=env,
+    )
+    assert result.returncode == 0, result.stderr
+    assert result.stdout.strip() == "2"

py/private/pytest_main.py

@@ -37,11 +37,22 @@
         # The lines are files that matched the --instrumentation_filter flag
         with open(os.getenv("COVERAGE_MANIFEST"), "r") as mf:
             manifest_entries = mf.read().splitlines()
-            cov = coverage.Coverage(include = manifest_entries)
+            cov = coverage.Coverage(include = manifest_entries, data_suffix = True, config_file = False)
             # 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()
+        # Propagate coverage config to subprocesses (e.g. pytest-xdist
+        # workers).  We write a .pth file so that every Python subprocess
+        # sharing this venv calls coverage.process_startup() at interpreter
+        # init.  Combined with COVERAGE_PROCESS_CONFIG, the subprocess
+        # automatically starts its own Coverage instance and writes a
+        # per-process data file that we combine() during teardown.
+        import sysconfig
+        _subprocess_pth = os.path.join(sysconfig.get_path("purelib"), "_coverage_subprocess.pth")
+        with open(_subprocess_pth, "w") as _f:
+            _f.write("import coverage; coverage.process_startup()\n")
+        os.environ["COVERAGE_PROCESS_CONFIG"] = cov.config.serialize()
     except ModuleNotFoundError as e:
         print("WARNING: python coverage setup failed. Do you need to include the 'coverage' package as a dependency of py_pytest_main?", e)
         pass
@@ -128,13 +139,14 @@
         print("Ran pytest.main with " + str(args), file=sys.stderr)
     elif cov:
         cov.stop()
+        cov.save()
+        cov.combine()
         # https://bazel.build/configure/coverage
         coverage_output_file = os.getenv("COVERAGE_OUTPUT_FILE")
 
         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:

pyproject.toml

@@ -10,7 +10,7 @@ dependencies = [
    "colorama~=0.4.0",
    "click",
    "pytest",
-   "coverage",
+   "coverage>=7.10.3",
    "cowsay",
    "pkg",
    "snakesay",

requirements.in

@@ -4,7 +4,7 @@ django~=4.2.7
 colorama~=0.4.0
 click
 pytest
-coverage
+coverage>=7.10.3
 cowsay
 snakesay
 ftfy==6.2.0