hydra: Restore coverage job

Sometime ago we lost the coverage job in the midst of
meson migration. Until we have something like codecov
it'd be very useful to restore this job with the html
reports and historical metrics.

As a bonus we get more coverage metrics by switching to
LLVM tooling from LCOV.

ci/gha/tests/default.nix

@@ -2,6 +2,12 @@
   nixFlake ? builtins.getFlake ("git+file://" + toString ../../..),
   system ? builtins.currentSystem,
   pkgs ? nixFlake.inputs.nixpkgs.legacyPackages.${system},
+  nixComponents ? (
+    nixFlake.lib.makeComponents {
+      inherit pkgs;
+      inherit getStdenv;
+    }
+  ),
   getStdenv ? p: p.stdenv,
   componentTestsPrefix ? "",
   withSanitizers ? false,
@@ -64,18 +70,13 @@ let
 in
 
 rec {
-  nixComponents =
+  nixComponentsInstrumented = nixComponents.overrideScope (
-    (nixFlake.lib.makeComponents {
+    final: prev: {
-      inherit pkgs;
+      nix-store-tests = prev.nix-store-tests.override { withBenchmarks = true; };
-      inherit getStdenv;
-    }).overrideScope
-      (
-        final: prev: {
-          nix-store-tests = prev.nix-store-tests.override { withBenchmarks = true; };
 
-          mesonComponentOverrides = lib.composeManyExtensions componentOverrides;
+      mesonComponentOverrides = lib.composeManyExtensions componentOverrides;
-        }
+    }
-      );
+  );
 
   /**
     Top-level tests for the flake outputs, as they would be built by hydra.
@@ -120,15 +121,15 @@ rec {
       lib.concatMapAttrs (testName: test: {
         "${componentTestsPrefix}${pkgName}-${testName}" = test;
       }) (pkg.tests or { })
-    ) nixComponents)
+    ) nixComponentsInstrumented)
     // lib.optionalAttrs (pkgs.stdenv.hostPlatform == pkgs.stdenv.buildPlatform) {
-      "${componentTestsPrefix}nix-functional-tests" = nixComponents.nix-functional-tests;
+      "${componentTestsPrefix}nix-functional-tests" = nixComponentsInstrumented.nix-functional-tests;
     };
 
   codeCoverage =
     let
       componentsTestsToProfile =
-        (builtins.mapAttrs (n: v: nixComponents.${n}.tests.run) {
+        (builtins.mapAttrs (n: v: nixComponentsInstrumented.${n}.tests.run) {
           "nix-util-tests" = { };
           "nix-store-tests" = { };
           "nix-fetchers-tests" = { };
@@ -136,7 +137,7 @@ rec {
           "nix-flake-tests" = { };
         })
         // {
-          inherit (nixComponents) nix-functional-tests;
+          inherit (nixComponentsInstrumented) nix-functional-tests;
         };
 
       coverageProfileDrvs = lib.mapAttrs (
@@ -170,12 +171,13 @@ rec {
 
       coverageReports =
         let
-          nixComponentDrvs = lib.filter (lib.isDerivation) (lib.attrValues nixComponents);
+          nixComponentDrvs = lib.filter (lib.isDerivation) (lib.attrValues nixComponentsInstrumented);
         in
         pkgs.runCommand "code-coverage-report"
           {
             nativeBuildInputs = [
               pkgs.llvmPackages.libllvm
+              pkgs.jq
             ];
             __structuredAttrs = true;
             nixComponents = nixComponentDrvs;
@@ -201,6 +203,24 @@ rec {
               echo
             } >> $out/index.txt
 
+            llvm-cov export $arguments -instr-profile ${mergedProfdata} -format=text > $out/coverage.json
+
+            mkdir -p $out/nix-support
+
+            coverageTotals=$(jq ".data[0].totals" $out/coverage.json)
+
+            # Mostly inline from pkgs/build-support/setup-hooks/make-coverage-analysis-report.sh [1],
+            # which we can't use here, because we rely on LLVM's infra for source code coverage collection.
+            # [1]: https://github.com/NixOS/nixpkgs/blob/67bb48c4c8e327417d6d5aa7e538244b209e852b/pkgs/build-support/setup-hooks/make-coverage-analysis-report.sh#L16
+            declare -A metricsArray=(["lineCoverage"]="lines" ["functionCoverage"]="functions" ["branchCoverage"]="branches")
+
+            for metricName in "''\${!metricsArray[@]}"; do
+              key="''\${metricsArray[$metricName]}"
+              metric=$(echo "$coverageTotals" | jq ".$key.percent * 10 | round / 10")
+              echo "$metricName $metric %" >> $out/nix-support/hydra-metrics
+            done
+
+            echo "report coverage $out" >> $out/nix-support/hydra-build-products
           '';
     in
     assert withCoverage;

packaging/hydra.nix

@@ -223,10 +223,17 @@ in
   dockerImage = lib.genAttrs linux64BitSystems (system: self.packages.${system}.dockerImage);
 
   # # Line coverage analysis.
-  # coverage = nixpkgsFor.x86_64-linux.native.nix.override {
+  coverage =
-  #   pname = "nix-coverage";
+    (import ./../ci/gha/tests rec {
-  #   withCoverageChecks = true;
+      withCoverage = true;
-  # };
+      pkgs = nixpkgsFor.x86_64-linux.nativeForStdenv.clangStdenv;
+      nixComponents = pkgs.nixComponents2;
+      nixFlake = null;
+      getStdenv = p: p.clangStdenv;
+    }).codeCoverage.coverageReports.overrideAttrs
+      {
+        name = "nix-coverage"; # For historical consistency
+      };
 
   # Nix's manual
   manual = nixpkgsFor.x86_64-linux.native.nixComponents2.nix-manual;