Fix missing line coverage (bazelbuild#3956)

closes bazelbuild#3948

Author: UebelAndre

.bazelci/presubmit.yml

@@ -88,6 +88,12 @@ coverage_validation_post_shell_commands: &coverage_validation_post_shell_command
       ; 1>&2 cat bazel-out/_coverage/_coverage_report.dat \
       ; exit 1 \
       ; }
+  - |
+    grep -q '^LF:[1-9]' bazel-out/_coverage/_coverage_report.dat \
+    || { 1>&2 echo "Coverage report has no line data" \
+      ; 1>&2 head -50 bazel-out/_coverage/_coverage_report.dat \
+      ; exit 1 \
+      ; }
 split_coverage_postprocessing_shell_commands: &split_coverage_postprocessing_shell_commands
   - echo "coverage --experimental_fetch_all_coverage_outputs" >> user.bazelrc
   - echo "coverage --experimental_split_coverage_postprocessing" >> user.bazelrc

rust/private/rust.bzl

@@ -524,6 +524,13 @@ def _rust_test_impl(ctx):
 
         env["RUST_LLVM_COV"] = llvm_cov_path
         env["RUST_LLVM_PROFDATA"] = llvm_profdata_path
+
+        # Bazel's collect_coverage.sh checks both GENERATE_LLVM_LCOV and
+        # CC_CODE_COVERAGE_SCRIPT before invoking the coverage collector.
+        # These are not automatically wired for Starlark test rules so we
+        # must set them explicitly.
+        env["GENERATE_LLVM_LCOV"] = "1"
+        env["CC_CODE_COVERAGE_SCRIPT"] = ctx.executable._collect_cc_coverage.path
     components = "{}/{}".format(ctx.label.workspace_root, ctx.label.package).split("/")
     env["CARGO_MANIFEST_DIR"] = "/".join([c for c in components if c])
     providers.append(RunEnvironmentInfo(

rust/private/rustc.bzl

@@ -1653,6 +1653,12 @@ def rustc_compile_action(
     coverage_runfiles = []
     if toolchain.llvm_cov and ctx.configuration.coverage_enabled and crate_info.is_test:
         coverage_runfiles = [toolchain.llvm_cov, toolchain.llvm_profdata] + toolchain.llvm_lib
+        collect_cc_coverage = getattr(ctx.executable, "_collect_cc_coverage", None)
+        if not collect_cc_coverage:
+            collect_cc_coverage = getattr(ctx.file, "_collect_cc_coverage", None)
+
+        if collect_cc_coverage:
+            coverage_runfiles.append(collect_cc_coverage)
 
     experimental_use_coverage_metadata_files = toolchain._experimental_use_coverage_metadata_files
 

util/collect_coverage/BUILD.bazel

@@ -4,5 +4,14 @@ rust_binary(
     name = "collect_coverage",
     srcs = ["collect_coverage.rs"],
     edition = "2018",
+    # To ensure this tool is produced deterministically, debug info
+    # is stripped out. On macOS, full symbol stripping is needed to
+    # also remove N_OSO stab entries whose timestamps and sandbox
+    # paths vary between builds.
+    rustc_flags = select({
+        "@platforms//os:linux": ["-Cstrip=debuginfo"],
+        "@platforms//os:macos": ["-Cstrip=symbols"],
+        "//conditions:default": [],
+    }),
     visibility = ["//visibility:public"],
 )

util/collect_coverage/collect_coverage.rs

@@ -189,11 +189,12 @@ fn main() {
         .arg("-format=lcov")
         .arg("-instr-profile")
         .arg(&profdata_file)
-        .arg("-ignore-filename-regex='.*external/.+'")
-        .arg("-ignore-filename-regex='/tmp/.+'")
-        .arg(format!("-path-equivalence=.,'{}'", execroot.display()))
+        .arg("-ignore-filename-regex=.*external/.+")
+        .arg("-ignore-filename-regex=/tmp/.+")
+        .arg(format!("-path-equivalence=.,{}", execroot.display()))
         .arg(test_binary)
-        .stdout(process::Stdio::piped());
+        .stdout(process::Stdio::piped())
+        .stderr(process::Stdio::piped());
 
     debug_log!("Spawning {:#?}", llvm_cov_cmd);
     let child = llvm_cov_cmd
@@ -202,6 +203,18 @@ fn main() {
 
     let output = child.wait_with_output().expect("llvm-cov process failed");
 
+    if !output.status.success() {
+        let stderr = std::str::from_utf8(&output.stderr).unwrap_or("<non-utf8>");
+        if stderr.contains("no coverage data found") {
+            debug_log!("No coverage data found in binary; writing empty report");
+            fs::write(&coverage_output_file, "").unwrap();
+            fs::remove_file(&profdata_file).ok();
+            return;
+        }
+        eprintln!("llvm-cov export failed:\n{}", stderr);
+        process::exit(output.status.code().unwrap_or(1));
+    }
+
     // Parse the child process's stdout to a string now that it's complete.
     debug_log!("Parsing llvm-cov output");
     let report_str = std::str::from_utf8(&output.stdout).expect("Failed to parse llvm-cov output");