nix flake metadata: Show flake fingerprint

This is useful for testing/debugging and maybe for sharing eval caches
(since it tells you what file in ~/.cache/nix/eval-cache-v5 to copy).

(cherry picked from commit 1ff186fc6e)

src/nix/flake.cc

@@ -232,6 +232,8 @@ struct CmdFlakeMetadata : FlakeCommand, MixJSON
                 j["lastModified"] = *lastModified;
             j["path"] = storePath;
             j["locks"] = lockedFlake.lockFile.toJSON().first;
+            if (auto fingerprint = lockedFlake.getFingerprint(store))
+                j["fingerprint"] = fingerprint->to_string(HashFormat::Base16, false);
             logger->cout("%s", j.dump());
         } else {
             logger->cout(
@@ -264,6 +266,10 @@ struct CmdFlakeMetadata : FlakeCommand, MixJSON
                 logger->cout(
                     ANSI_BOLD "Last modified:" ANSI_NORMAL " %s",
                     std::put_time(std::localtime(&*lastModified), "%F %T"));
+            if (auto fingerprint = lockedFlake.getFingerprint(store))
+                logger->cout(
+                    ANSI_BOLD "Fingerprint:" ANSI_NORMAL "   %s",
+                    fingerprint->to_string(HashFormat::Base16, false));
 
             if (!lockedFlake.lockFile.root->inputs.empty())
                 logger->cout(ANSI_BOLD "Inputs:" ANSI_NORMAL);

tests/functional/flakes/flakes.sh

@@ -189,6 +189,7 @@ json=$(nix flake metadata flake1 --json | jq .)
 [[ -d $(echo "$json" | jq -r .path) ]]
 [[ $(echo "$json" | jq -r .lastModified) = $(git -C "$flake1Dir" log -n1 --format=%ct) ]]
 hash1=$(echo "$json" | jq -r .revision)
+[[ -n $(echo "$json" | jq -r .fingerprint) ]]
 
 echo foo > "$flake1Dir/foo"
 git -C "$flake1Dir" add $flake1Dir/foo

tests/nixos/tarball-flakes.nix

@@ -66,6 +66,9 @@ in
     # Check that we got redirected to the immutable URL.
     assert info["locked"]["url"] == "http://localhost/stable/${nixpkgs.rev}.tar.gz"
 
+    # Check that we got a fingerprint for caching.
+    assert info["fingerprint"]
+
     # Check that we got the rev and revCount attributes.
     assert info["revision"] == "${nixpkgs.rev}"
     assert info["revCount"] == 1234