From 1ff186fc6e99e57501fee9291e7013ea88d8720a Mon Sep 17 00:00:00 2001
From: Eelco Dolstra <edolstra@gmail.com>
Date: Wed, 3 Jul 2024 16:37:26 +0200
Subject: [PATCH] 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).
---
 src/nix/flake.cc                  | 6 ++++++
 tests/functional/flakes/flakes.sh | 1 +
 tests/nixos/tarball-flakes.nix    | 3 +++
 3 files changed, 10 insertions(+)

diff --git a/src/nix/flake.cc b/src/nix/flake.cc
index fb7ea6211..84c659023 100644
--- a/src/nix/flake.cc
+++ b/src/nix/flake.cc
@@ -233,6 +233,8 @@ 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(
@@ -265,6 +267,10 @@ 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);
diff --git a/tests/functional/flakes/flakes.sh b/tests/functional/flakes/flakes.sh
index c3cb2c661..26b91eda7 100755
--- a/tests/functional/flakes/flakes.sh
+++ b/tests/functional/flakes/flakes.sh
@@ -195,6 +195,7 @@ 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 $flake1Dir/foo
diff --git a/tests/nixos/tarball-flakes.nix b/tests/nixos/tarball-flakes.nix
index 2e7f98b5e..bc0ddc3f6 100644
--- a/tests/nixos/tarball-flakes.nix
+++ b/tests/nixos/tarball-flakes.nix
@@ -70,6 +70,9 @@ in
     # Check that we got redirected to the immutable URL.
     assert info["locked"]["url"] == "http://localhost/stable/${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