improve test coverage (#4529)

Author: anonrig

.bazelrc

@@ -395,14 +395,25 @@ build:windows --copt='-Wno-unused-command-line-argument' --host_copt='-Wno-unuse
 # set the correct value. We currently don't check __cplusplus, but some dependencies do.
 build:windows --cxxopt='/Zc:__cplusplus' --host_cxxopt='/Zc:__cplusplus'
 
-# enable clang coverage: https://clang.llvm.org/docs/SourceBasedCodeCoverage.html
-build:clang-coverage --copt="-fprofile-instr-generate" --linkopt="-fprofile-instr-generate"
-build:clang-coverage --copt="-fcoverage-mapping" --linkopt="-fcoverage-mapping"
-# disabled due to clang: warning: argument unused during compilation: '--coverage'
-build:clang-coverage --copt='-Wno-unused-command-line-argument' --host_copt='-Wno-unused-command-line-argument'
+build:coverage --action_env=GCOV=llvm-profdata-19
+build:coverage --action_env=BAZEL_USE_LLVM_NATIVE_COVERAGE=1
+build:coverage --copt=-DNDEBUG
+build:coverage --combined_report=lcov
+build:coverage --strategy=TestRunner=remote,sandboxed,local
+build:coverage --strategy=CoverageReport=sandboxed,local
+build:coverage --experimental_use_llvm_covmap
+build:coverage --experimental_generate_llvm_lcov
+build:coverage --experimental_fetch_all_coverage_outputs
+build:coverage --collect_code_coverage
+build:coverage --instrumentation_filter="^//,-//external"
+build:coverage --remote_download_minimal
+build:coverage --define=tcmalloc=gperftools
+build:coverage --test_timeout=120,120,120,120
+build:coverage --instrument_test_targets
 
 # This config is defined internally and enabled on many machines.
 # Defining it as empty just so these machines can run build commands from the workerd repo
 build:rosetta-arm64 --define=rosetta_arm64_no_op=1
 
-coverage --test_timeout=120,120,120,120 --strategy=CoverageReport=local --instrument_test_targets --combined_report=lcov
+coverage --build_tests_only
+coverage --config=coverage

build/wd_cc_binary.bzl

@@ -42,4 +42,6 @@ def wd_cc_binary(
             "@//build/config:prebuilt_binaries_arm64": "@//:bin.arm64/tmp/{}/{}.aarch64-linux-gnu".format(pkg, prebuilt_binary_name),
             "//conditions:default": name,
         }),
+        # Propagate InstrumentedFilesInfo through the alias for coverage support
+        testonly = kwargs.get("testonly", False),
     )

build/wd_test.bzl

@@ -54,6 +54,7 @@ def wd_test(
     ] + args
 
     _wd_test(
+        src = src,
         name = name,
         data = data,
         args = args,
@@ -173,17 +174,43 @@ def _wd_test_impl(ctx):
         if default_runfiles:
             runfiles = runfiles.merge(default_runfiles)
 
+    # IMPORTANT: The workerd binary must be listed in dependency_attributes
+    # to ensure its transitive dependencies (all the C++ source files) are
+    # included in the coverage instrumentation. Without this, coverage data
+    # won't be collected for the actual workerd implementation code.
+    instrumented_files_info = coverage_common.instrumented_files_info(
+        ctx,
+        source_attributes = ["src", "data"],
+        dependency_attributes = ["workerd", "sidecar", "sidecar_supervisor"],
+        # Include all file types that might contain testable code
+        extensions = ["cc", "c++", "cpp", "cxx", "c", "h", "hh", "hpp", "hxx", "inc", "js", "ts", "mjs", "wd-test", "capnp"],
+    )
+
     return [
         DefaultInfo(
             executable = executable,
             runfiles = runfiles,
         ),
+        instrumented_files_info,
     ]
 
 _wd_test = rule(
     implementation = _wd_test_impl,
     test = True,
     attrs = {
+        # Implicit dependencies used by Bazel to generate coverage reports.
+        "_lcov_merger": attr.label(
+            default = configuration_field(fragment = "coverage", name = "output_generator"),
+            executable = True,
+            cfg = config.exec(exec_group = "test"),
+        ),
+        "_collect_cc_coverage": attr.label(
+            default = "@bazel_tools//tools/test:collect_cc_coverage",
+            executable = True,
+            cfg = config.exec(exec_group = "test"),
+        ),
+        # Source file
+        "src": attr.label(allow_single_file = True),
         # The workerd executable is used to run all tests
         "workerd": attr.label(
             allow_single_file = True,
@@ -216,7 +243,7 @@ _wd_test = rule(
             allow_single_file = True,
             executable = True,
             cfg = "exec",
-            default = "//src/workerd/api/node:sidecar-supervisor",
+            default = "//src/workerd/api/node/tests:sidecar-supervisor",
         ),
         "python_snapshot_test": attr.bool(),
         # A reference to the Windows platform label, needed for the implementation of wd_test

justfile

@@ -112,8 +112,8 @@ eslint:
   just stream-test //src/node:node@eslint
 
 coverage path="//...":
-  bazel coverage --nocache_test_results --config=clang-coverage --instrumentation_filter="^//,-//external" {{path}}
-  genhtml --branch-coverage --synthesize-missing --keep-going --output coverage "$(bazel info output_path)/_coverage/_coverage_report.dat"
+  bazel coverage {{path}}
+  genhtml --branch-coverage --output coverage "$(bazel info output_path)/_coverage/_coverage_report.dat"
   open coverage/index.html
 
 profile path: