From a60a1f09b2f667d10ff0873b9fd5a70bad3febfd Mon Sep 17 00:00:00 2001
From: Guillaume Maudoux <guillaume.maudoux@tweag.io>
Date: Fri, 19 Apr 2024 16:21:51 +0200
Subject: [PATCH 1/3] Reuse eval caches and related values when possible

---
 src/libcmd/installables.cc | 22 +++++++++++++++-------
 src/libexpr/eval.hh        |  8 ++++++++
 2 files changed, 23 insertions(+), 7 deletions(-)

diff --git a/src/libcmd/installables.cc b/src/libcmd/installables.cc
index ed6772377..5683e1afa 100644
--- a/src/libcmd/installables.cc
+++ b/src/libcmd/installables.cc
@@ -444,12 +444,10 @@ ref<eval_cache::EvalCache> openEvalCache(
     std::shared_ptr<flake::LockedFlake> lockedFlake)
 {
     auto fingerprint = lockedFlake->getFingerprint(state.store);
-    return make_ref<nix::eval_cache::EvalCache>(
-        evalSettings.useEvalCache && evalSettings.pureEval
-            ? fingerprint
-            : std::nullopt,
-        state,
-        [&state, lockedFlake]()
+    auto hash = evalSettings.useEvalCache && evalSettings.pureEval
+        ? fingerprint
+        : std::nullopt;
+    auto rootLoader = [&state, lockedFlake]()
         {
             /* For testing whether the evaluation cache is
                complete. */
@@ -465,7 +463,17 @@ ref<eval_cache::EvalCache> openEvalCache(
             assert(aOutputs);
 
             return aOutputs->value;
-        });
+        };
+
+    if (hash) {
+        auto search = state.evalCaches.find(hash.value());
+        if (search == state.evalCaches.end()) {
+            search = state.evalCaches.emplace(hash.value(), make_ref<nix::eval_cache::EvalCache>(hash, state, rootLoader)).first;
+        }
+        return search->second;
+    } else {
+        return make_ref<nix::eval_cache::EvalCache>(hash, state, rootLoader);
+    }
 }
 
 Installables SourceExprCommand::parseInstallables(
diff --git a/src/libexpr/eval.hh b/src/libexpr/eval.hh
index 3477f6c46..508361301 100644
--- a/src/libexpr/eval.hh
+++ b/src/libexpr/eval.hh
@@ -34,6 +34,9 @@ class StorePath;
 struct SingleDerivedPath;
 enum RepairFlag : bool;
 struct MemoryInputAccessor;
+namespace eval_cache {
+    class EvalCache;
+}
 
 
 /**
@@ -282,6 +285,11 @@ public:
         return *new EvalErrorBuilder<T>(*this, args...);
     }
 
+    /**
+     * A cache for evaluation caches, so as to reuse the same root value if possible
+     */
+    std::map<const Hash, ref<eval_cache::EvalCache>> evalCaches;
+
 private:
 
     /* Cache for calls to addToStore(); maps source paths to the store

From 1c4e392c64939ca961458cfb15a9da1e4d883b22 Mon Sep 17 00:00:00 2001
From: Guillaume Maudoux <layus.on@gmail.com>
Date: Thu, 25 Apr 2024 00:44:47 +0200
Subject: [PATCH 2/3] Compute fingerprint only if needed

As per Eelco's review comments

Co-authored-by: Eelco Dolstra <edolstra@gmail.com>
---
 src/libcmd/installables.cc | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/src/libcmd/installables.cc b/src/libcmd/installables.cc
index 5683e1afa..e5c981629 100644
--- a/src/libcmd/installables.cc
+++ b/src/libcmd/installables.cc
@@ -443,9 +443,8 @@ ref<eval_cache::EvalCache> openEvalCache(
     EvalState & state,
     std::shared_ptr<flake::LockedFlake> lockedFlake)
 {
-    auto fingerprint = lockedFlake->getFingerprint(state.store);
-    auto hash = evalSettings.useEvalCache && evalSettings.pureEval
-        ? fingerprint
+    auto fingerprint = evalSettings.useEvalCache && evalSettings.pureEval
+        ? lockedFlake->getFingerprint(state.store)
         : std::nullopt;
     auto rootLoader = [&state, lockedFlake]()
         {
@@ -472,7 +471,7 @@ ref<eval_cache::EvalCache> openEvalCache(
         }
         return search->second;
     } else {
-        return make_ref<nix::eval_cache::EvalCache>(hash, state, rootLoader);
+        return make_ref<nix::eval_cache::EvalCache>(std::nullopt, state, rootLoader);
     }
 }
 

From 19cc50dcbf58dd52a82308ab1733d2d7764db9d1 Mon Sep 17 00:00:00 2001
From: Guillaume Maudoux <layus.on@gmail.com>
Date: Thu, 25 Apr 2024 00:47:46 +0200
Subject: [PATCH 3/3] fixup: Compute fingerprint only if needed

---
 src/libcmd/installables.cc | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/src/libcmd/installables.cc b/src/libcmd/installables.cc
index e5c981629..b93c7f7e8 100644
--- a/src/libcmd/installables.cc
+++ b/src/libcmd/installables.cc
@@ -464,10 +464,10 @@ ref<eval_cache::EvalCache> openEvalCache(
             return aOutputs->value;
         };
 
-    if (hash) {
-        auto search = state.evalCaches.find(hash.value());
+    if (fingerprint) {
+        auto search = state.evalCaches.find(fingerprint.value());
         if (search == state.evalCaches.end()) {
-            search = state.evalCaches.emplace(hash.value(), make_ref<nix::eval_cache::EvalCache>(hash, state, rootLoader)).first;
+            search = state.evalCaches.emplace(fingerprint.value(), make_ref<nix::eval_cache::EvalCache>(fingerprint, state, rootLoader)).first;
         }
         return search->second;
     } else {