nix flake check: Skip substitutable derivations

Since `nix flake check` doesn't produce a `result` symlink, it doesn't
actually need to build/substitute derivations that are already known
to have succeeded, i.e. that are substitutable.

This can speed up CI jobs in cases where the derivations have already
been built by other jobs. For instance, a command like

  nix flake check github:NixOS/hydra/aa62c7f7db31753f0cde690f8654dd1907fc0ce2

should no longer build anything because the outputs are already in
cache.nixos.org.

Based-on: DeterminateSystems#134
Based-on: https://gerrit.lix.systems/c/lix/+/3841
Co-authored-by: Eelco Dolstra <[email protected]>

Author: getchoo

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

@@ -783,8 +783,23 @@ struct CmdFlakeCheck : FlakeCommand
         }
 
         if (build && !drvPaths.empty()) {
-            Activity act(*logger, lvlInfo, actUnknown, fmt("running %d flake checks", drvPaths.size()));
-            store->buildPaths(drvPaths);
+            // This command doesn't need to actually substitute derivation outputs
+            // if they're missing but substitutable.
+            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

@@ -135,3 +135,21 @@ EOF
 
 checkRes=$(nix flake check --all-systems $flakeDir 2>&1 && fail "nix flake check --all-systems should have failed" || true)
 echo "$checkRes" | grepQuiet "formatter.system-1"
+
+# 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
+
+checkRes=$(nix flake check $flakeDir 2>&1 && fail "nix flake check should have failed" || true)
+echo "$checkRes" | grepQuiet "builder failed with exit code 1"