Introduce reproducible branch-based coverage

Reports may now be build via:

```nix
nix build .#nativelinkCoverageForHost
```

The `result` symlink then contains the contents of a webpage to view the
existing reports.

Reports are built in release mode to closely resemble production coverage
of the testsuite. This also means that most worker tests are ignored via
a new `nix` feature to make the testsuite run in nix sandboxes. It's not
ideal, but accurately reflects our production coverage guarantees. A
future resolution for this might be  to implement more elaborate mocking
functionality for `nativelink-worker`.

Coverage leverages nix caching, but not Bazel caching. While Bazel has
builtin support for coverage, the reports were not satisfactory as they
rely on outdated gcov toolchains with vague hermeticity guarantees and
unsatisfactory implementations for llvm-cov-based workflows (e.g. no
branch-based coverage for rust and no story around coverage for
heterogeneous code). Mid-term we should implement "fast development"
coverage via Bazel alongside the "slow production" coverage via Nix.

As next steps we should continuously publish the generated HTML pages
via the web infrastructure and add hooks to report regressions.

CONTRIBUTING.md

@@ -468,3 +468,13 @@ most automatically generated changelogs provide.
 
 NativeLink Code of Conduct is available in the
 [CODE_OF_CONDUCT](https://github.com/tracemachina/nativelink/tree/main/CODE_OF_CONDUCT.md) file.
+
+## Generating code coverage
+
+You can generate branch-based coverage reports via:
+
+```
+nix run .#nativelinkCoverageForHost
+```
+
+The `result` symlink contains a webpage with the visualized report.

Cargo.toml

@@ -32,6 +32,9 @@ name = "nativelink"
 enable_tokio_console = [
   "nativelink-util/enable_tokio_console"
 ]
+nix = [
+  "nativelink-worker/nix"
+]
 
 [dependencies]
 nativelink-error = { path = "nativelink-error" }

flake.nix

@@ -117,12 +117,21 @@
             ];
           };
 
+        nightlyRustFor = p:
+          p.rust-bin.nightly.${nightly-rust-version}.default.override {
+            extensions = ["llvm-tools"];
+            targets = [
+              "${nixSystemToRustTriple p.stdenv.targetPlatform.system}"
+            ];
+          };
+
         craneLibFor = p: (crane.mkLib p).overrideToolchain stableRustFor;
+        nightlyCraneLibFor = p: (crane.mkLib p).overrideToolchain nightlyRustFor;
 
         src = pkgs.lib.cleanSourceWith {
           src = (craneLibFor pkgs).path ./.;
           filter = path: type:
-            (builtins.match "^.*(data/SekienSkashita\.jpg|nativelink-config/README\.md)" path != null)
+            (builtins.match "^.*(data/SekienAkashita\.jpg|nativelink-config/README\.md)" path != null)
             || ((craneLibFor pkgs).filterCargoSources path type);
         };
 
@@ -184,6 +193,7 @@
 
         # Additional target for external dependencies to simplify caching.
         cargoArtifactsFor = p: (craneLibFor p).buildDepsOnly (commonArgsFor p);
+        nightlyCargoArtifactsFor = p: (craneLibFor p).buildDepsOnly (commonArgsFor p);
 
         nativelinkFor = p:
           (craneLibFor p).buildPackage ((commonArgsFor p)
@@ -334,6 +344,34 @@
             os = "linux";
           };
         };
+
+        nativelinkCoverageFor = p: let
+          coverageArgs =
+            (commonArgsFor p)
+            // {
+              # TODO(aaronmondal): For some reason we're triggering an edgecase where
+              #                    mimalloc builds against glibc headers in coverage
+              #                    builds. This leads to nonexistend __memcpy_chk and
+              #                    __memset_chk symbols if fortification is enabled.
+              #                    Our regular builds also have this issue, but we
+              #                    should investigate further.
+              hardeningDisable = ["fortify"];
+            };
+        in
+          (nightlyCraneLibFor p).cargoLlvmCov (coverageArgs
+            // {
+              cargoArtifacts = nightlyCargoArtifactsFor p;
+              cargoExtraArgs = builtins.concatStringsSep " " [
+                "--all"
+                "--locked"
+                "--features nix"
+                "--branch"
+                "--ignore-filename-regex '.*(genproto|vendor-cargo-deps|crates).*'"
+              ];
+              cargoLlvmCovExtraArgs = "--html --output-dir $out";
+            });
+
+        nativelinkCoverageForHost = nativelinkCoverageFor pkgs;
       in rec {
         _module.args.pkgs = let
           nixpkgs-patched = (import self.inputs.nixpkgs {inherit system;}).applyPatches {
@@ -366,6 +404,7 @@
               lre-cc
               native-cli
               nativelink
+              nativelinkCoverageForHost
               nativelink-aarch64-linux
               nativelink-debug
               nativelink-image

nativelink-metric/nativelink-metric-macro-derive/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "nativelink-metric-macro-derive"
-version = "0.4.0"
+version = "0.5.3"
 edition = "2021"
 
 [lib]

nativelink-worker/Cargo.toml

@@ -3,6 +3,9 @@ name = "nativelink-worker"
 version = "0.5.3"
 edition = "2021"
 
+[features]
+nix = []
+
 [dependencies]
 nativelink-error = { path = "../nativelink-error" }
 nativelink-proto = { path = "../nativelink-proto" }

nativelink-worker/tests/local_worker_test.rs

@@ -73,6 +73,7 @@ fn make_temp_path(data: &str) -> String {
     )
 }
 
+#[cfg_attr(feature = "nix", ignore)]
 #[nativelink_test]
 async fn platform_properties_smoke_test() -> Result<(), Error> {
     let mut platform_properties = HashMap::new();

nativelink-worker/tests/running_actions_manager_test.rs

@@ -989,6 +989,7 @@ async fn upload_files_from_above_cwd_test() -> Result<(), Box<dyn std::error::Er
 
 // Windows does not support symlinks.
 #[cfg(not(target_family = "windows"))]
+#[cfg_attr(feature = "nix", ignore)]
 #[nativelink_test]
 async fn upload_dir_and_symlink_test() -> Result<(), Box<dyn std::error::Error>> {
     const WORKER_ID: &str = "foo_worker_id";
@@ -1321,6 +1322,7 @@ async fn cleanup_happens_on_job_failure() -> Result<(), Box<dyn std::error::Erro
     Ok(())
 }
 
+#[cfg_attr(feature = "nix", ignore)]
 #[nativelink_test]
 async fn kill_ends_action() -> Result<(), Box<dyn std::error::Error>> {
     const WORKER_ID: &str = "foo_worker_id";
@@ -1429,6 +1431,7 @@ async fn kill_ends_action() -> Result<(), Box<dyn std::error::Error>> {
 // The wrapper script will print a constant string to stderr, and the test itself will
 // print to stdout. We then check the results of both to make sure the shell script was
 // invoked and the actual command was invoked under the shell script.
+#[cfg_attr(feature = "nix", ignore)]
 #[nativelink_test]
 async fn entrypoint_does_invoke_if_set() -> Result<(), Box<dyn std::error::Error>> {
     #[cfg(target_family = "unix")]
@@ -1572,6 +1575,7 @@ exit 0
     Ok(())
 }
 
+#[cfg_attr(feature = "nix", ignore)]
 #[nativelink_test]
 async fn entrypoint_injects_properties() -> Result<(), Box<dyn std::error::Error>> {
     #[cfg(target_family = "unix")]
@@ -1747,6 +1751,7 @@ exit 0
     Ok(())
 }
 
+#[cfg_attr(feature = "nix", ignore)]
 #[nativelink_test]
 async fn entrypoint_sends_timeout_via_side_channel() -> Result<(), Box<dyn std::error::Error>> {
     #[cfg(target_family = "unix")]
@@ -2268,6 +2273,7 @@ async fn action_result_has_used_in_message() -> Result<(), Box<dyn std::error::E
     Ok(())
 }
 
+#[cfg_attr(feature = "nix", ignore)]
 #[nativelink_test]
 async fn ensure_worker_timeout_chooses_correct_values() -> Result<(), Box<dyn std::error::Error>> {
     const WORKER_ID: &str = "foo_worker_id";
@@ -2681,6 +2687,7 @@ async fn worker_times_out() -> Result<(), Box<dyn std::error::Error>> {
     Ok(())
 }
 
+#[cfg_attr(feature = "nix", ignore)]
 #[nativelink_test]
 async fn kill_all_waits_for_all_tasks_to_finish() -> Result<(), Box<dyn std::error::Error>> {
     const WORKER_ID: &str = "foo_worker_id";
@@ -2841,6 +2848,7 @@ async fn kill_all_waits_for_all_tasks_to_finish() -> Result<(), Box<dyn std::err
 
 /// Regression Test for Issue #675
 #[cfg(target_family = "unix")]
+#[cfg_attr(feature = "nix", ignore)]
 #[nativelink_test]
 async fn unix_executable_file_test() -> Result<(), Box<dyn std::error::Error>> {
     const WORKER_ID: &str = "foo_worker_id";
@@ -3219,6 +3227,7 @@ async fn upload_with_single_permit() -> Result<(), Box<dyn std::error::Error>> {
     Ok(())
 }
 
+#[cfg_attr(feature = "nix", ignore)]
 #[nativelink_test]
 async fn running_actions_manager_respects_action_timeout() -> Result<(), Box<dyn std::error::Error>>
 {