initial commit

Author: alexeagle

README.md

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+Replacement for the 'bazel coverage' command:
+
+- we don't accidentally bust the analysis cache with 'coverage' flags differing from 'test'
+- forget having a Bazel-specific, Java-based, unmaintained LCOV merger and 'combined report' - that never should have been Bazel's job
+- developers really need git-aware diff to show "incremental coverage" - which of the lines I added or modified are covered by tests?

coverage.axl

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+"""
+Implement a 'coverage' task that wraps a test command.
+
+Gathers the coverage.dat files to power common developer workflows.
+"""
+
+# buildifier: disable=function-docstring
+def impl(ctx) -> int:
+    out = ctx.std.io.stdout
+    build = ctx.bazel.build(
+        "//...",
+        events = True,
+        bazel_flags = [
+            "--isatty=" + str(int(out.is_tty)),
+            # We probably want to abbreviate the following flags under
+            # "--config=coverage"
+            "--collect_code_coverage",
+            "--instrumentation_filter=",
+            # See https://github.com/bazel-contrib/bazelrc-preset.bzl/issues/83
+            "--experimental_fetch_all_coverage_outputs",
+            "--experimental_split_coverage_postprocessing",
+        ],
+        bazel_verb = "test"
+    );
+
+    for event in build.events():
+        # TODO: get the paths of produced (or cached) coverage report files
+        # TODO: populate a GITHUB_OUTPUT variable so we can use https://github.com/codecov/codecov-action
+        if event.type == "progress":
+            out.write(event.payload.stdout)
+            out.write(event.payload.stderr)
+
+    build.wait()
+    # TODO: get the delta of changed files from VCS, and render 'incremental' coverage as the default presentation
+
+    return 0
+
+coverage = task(
+    implementation = impl,
+    args = {
+        "targets": args.positional(minimum = 1, maximum = 30),
+    }
+)