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] 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