From 29049d26533fb9077b0214fad276804784e02e45 Mon Sep 17 00:00:00 2001
From: Eelco Dolstra <edolstra@gmail.com>
Date: Mon, 4 Mar 2024 19:21:31 +0100
Subject: [PATCH 1/2] Implement getFingerprint() for store paths

---
 src/libfetchers/path.cc | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/src/libfetchers/path.cc b/src/libfetchers/path.cc
index 276fd1b36..8231492e8 100644
--- a/src/libfetchers/path.cc
+++ b/src/libfetchers/path.cc
@@ -147,6 +147,20 @@ struct PathInputScheme : InputScheme
         return {std::move(*storePath), input};
     }
 
+    std::optional<std::string> getFingerprint(ref<Store> store, const Input & input) const override
+    {
+        /* If this path is in the Nix store, use the hash of the
+           store object and the subpath. */
+        auto path = getAbsPath(input);
+        try {
+            auto [storePath, subPath] = store->toStorePath(path.abs());
+            auto info = store->queryPathInfo(storePath);
+            return fmt("path:%s:%s", info->narHash.to_string(HashFormat::Base16, false), subPath);
+        } catch (Error &) {
+            return std::nullopt;
+        }
+    }
+
     std::optional<ExperimentalFeature> experimentalFeature() const override
     {
         return Xp::Flakes;

From 6558da45f5497eb54cc42866f81a3660862056ff Mon Sep 17 00:00:00 2001
From: Eelco Dolstra <edolstra@gmail.com>
Date: Mon, 4 Mar 2024 19:22:23 +0100
Subject: [PATCH 2/2] LockedFlake::getFingerprint(): Use
 Input::getFingerprint()

---
 src/libcmd/installables.cc |  4 ++--
 src/libexpr/flake/flake.cc | 15 +++++++--------
 src/libexpr/flake/flake.hh |  2 +-
 3 files changed, 10 insertions(+), 11 deletions(-)

diff --git a/src/libcmd/installables.cc b/src/libcmd/installables.cc
index d87d7b9b1..6db9bf9a1 100644
--- a/src/libcmd/installables.cc
+++ b/src/libcmd/installables.cc
@@ -443,10 +443,10 @@ ref<eval_cache::EvalCache> openEvalCache(
     EvalState & state,
     std::shared_ptr<flake::LockedFlake> lockedFlake)
 {
-    auto fingerprint = lockedFlake->getFingerprint();
+    auto fingerprint = lockedFlake->getFingerprint(state.store);
     return make_ref<nix::eval_cache::EvalCache>(
         evalSettings.useEvalCache && evalSettings.pureEval
-            ? std::optional { std::cref(fingerprint) }
+            ? fingerprint
             : std::nullopt,
         state,
         [&state, lockedFlake]()
diff --git a/src/libexpr/flake/flake.cc b/src/libexpr/flake/flake.cc
index fd9341504..4a69bb381 100644
--- a/src/libexpr/flake/flake.cc
+++ b/src/libexpr/flake/flake.cc
@@ -926,18 +926,17 @@ static RegisterPrimOp r4({
 
 }
 
-Fingerprint LockedFlake::getFingerprint() const
+std::optional<Fingerprint> LockedFlake::getFingerprint(ref<Store> store) const
 {
+    if (lockFile.isUnlocked()) return std::nullopt;
+
+    auto fingerprint = flake.lockedRef.input.getFingerprint(store);
+    if (!fingerprint) return std::nullopt;
+
     // FIXME: as an optimization, if the flake contains a lock file
     // and we haven't changed it, then it's sufficient to use
     // flake.sourceInfo.storePath for the fingerprint.
-    return hashString(HashAlgorithm::SHA256,
-        fmt("%s;%s;%d;%d;%s",
-            flake.path.to_string(),
-            flake.lockedRef.subdir,
-            flake.lockedRef.input.getRevCount().value_or(0),
-            flake.lockedRef.input.getLastModified().value_or(0),
-            lockFile));
+    return hashString(HashAlgorithm::SHA256, fmt("%s;%s;%s", *fingerprint, flake.lockedRef.subdir, lockFile));
 }
 
 Flake::~Flake() { }
diff --git a/src/libexpr/flake/flake.hh b/src/libexpr/flake/flake.hh
index 48907813f..1ba085f0f 100644
--- a/src/libexpr/flake/flake.hh
+++ b/src/libexpr/flake/flake.hh
@@ -119,7 +119,7 @@ struct LockedFlake
      */
     std::map<ref<Node>, SourcePath> nodePaths;
 
-    Fingerprint getFingerprint() const;
+    std::optional<Fingerprint> getFingerprint(ref<Store> store) const;
 };
 
 struct LockFlags