Merge pull request #13998 from Mic92/fast-flake-check

nix flake check: Skip substitutable derivations

doc/manual/rl-next/faster-nix-flake-check.md

@@ -0,0 +1,9 @@
+---
+synopsis: "`nix flake check` now skips derivations that can be substituted"
+prs: [13574]
+---
+
+Previously, `nix flake check` would evaluate and build/substitute all
+derivations. Now, it will skip downloading derivations that can be substituted.
+This can drastically decrease the time invocations take in environments where
+checks may already be cached (like in CI).

src/nix/flake.cc

@@ -786,8 +786,32 @@ struct CmdFlakeCheck : FlakeCommand
         }
 
         if (build && !drvPaths.empty()) {
-            Activity act(*logger, lvlInfo, actUnknown, fmt("running %d flake checks", drvPaths.size()));
-            store->buildPaths(drvPaths);
+            // TODO: This filtering of substitutable paths is a temporary workaround until
+            // https://github.com/NixOS/nix/issues/5025 (union stores) is implemented.
+            //
+            // Once union stores are available, this code should be replaced with a proper
+            // union store configuration. Ideally, we'd use a union of multiple destination
+            // stores to preserve the current behavior where different substituters can
+            // cache different check results.
+            //
+            // For now, we skip building derivations whose outputs are already available
+            // via substitution, as `nix flake check` only needs to verify buildability,
+            // not actually produce the outputs.
+            auto missing = store->queryMissing(drvPaths);
+            // Only occurs if `drvPaths` contains a `DerivedPath::Opaque`, which should never happen
+            assert(missing.unknown.empty());
+
+            std::vector<DerivedPath> toBuild;
+            for (auto & path : missing.willBuild) {
+                toBuild.emplace_back(
+                    DerivedPath::Built{
+                        .drvPath = makeConstantStorePathRef(path),
+                        .outputs = OutputsSpec::All{},
+                    });
+            }
+
+            Activity act(*logger, lvlInfo, actUnknown, fmt("running %d flake checks", toBuild.size()));
+            store->buildPaths(toBuild);
         }
         if (hasErrors)
             throw Error("some errors were encountered during the evaluation");

tests/functional/flakes/check.sh

@@ -167,3 +167,23 @@ EOF
 if !isTestOnNixOS && $NIX_REMOTE != daemon; then
     expectStderr 100 nix flake check "$flakeDir" | grepQuiet 'builder failed with exit code 1'
 fi
+
+# Ensure non-substitutable (read: usually failed) checks are actually run
+# https://github.com/NixOS/nix/pull/13574
+cp "$config_nix" $flakeDir/
+cat > $flakeDir/flake.nix <<EOF
+{
+  outputs = { self }: with import ./config.nix; {
+    checks.${system}.expectedToFail = derivation {
+      name = "expected-to-fail";
+      inherit system;
+      builder = "not-a-real-file";
+    };
+  };
+}
+EOF
+
+# NOTE: Regex pattern is used for compatibility with older daemon versions
+# We also can't expect a specific status code. Earlier daemons return 1, but as of 2.31, we return 100
+checkRes=$(nix flake check $flakeDir 2>&1 && fail "nix flake check should have failed" || true)
+echo "$checkRes" | grepQuiet -E "builder( for .*)? failed with exit code 1"