From 5592bb717beb7afa43a232a13e78d2c62a794fb1 Mon Sep 17 00:00:00 2001
From: John Ericson <John.Ericson@Obsidian.Systems>
Date: Sat, 27 Sep 2025 16:30:36 -0400
Subject: [PATCH] Implement realisation operations on dummy store

---
 src/libstore-tests/dummy-store.cc             | 15 +++++++++++++--
 src/libstore/dummy-store.cc                   | 19 ++++++++++++++++---
 .../include/nix/store/dummy-store-impl.hh     | 12 ++++++++++++
 src/libstore/include/nix/store/dummy-store.hh |  2 ++
 src/libutil/include/nix/util/hash.hh          | 19 +++++++++++++++++++
 5 files changed, 62 insertions(+), 5 deletions(-)

diff --git a/src/libstore-tests/dummy-store.cc b/src/libstore-tests/dummy-store.cc
index b841d7890..3dd8137a3 100644
--- a/src/libstore-tests/dummy-store.cc
+++ b/src/libstore-tests/dummy-store.cc
@@ -1,6 +1,6 @@
 #include <gtest/gtest.h>
 
-#include "nix/store/dummy-store.hh"
+#include "nix/store/dummy-store-impl.hh"
 #include "nix/store/globals.hh"
 #include "nix/store/realisation.hh"
 
@@ -13,7 +13,7 @@ TEST(DummyStore, realisation_read)
     auto store = [] {
         auto cfg = make_ref<DummyStoreConfig>(StoreReference::Params{});
         cfg->readOnly = false;
-        return cfg->openStore();
+        return cfg->openDummyStore();
     }();
 
     auto drvHash = Hash::parseExplicitFormatUnprefixed(
@@ -22,6 +22,17 @@ TEST(DummyStore, realisation_read)
     auto outputName = "foo";
 
     EXPECT_EQ(store->queryRealisation({drvHash, outputName}), nullptr);
+
+    UnkeyedRealisation value{
+        .outPath = StorePath{"g1w7hy3qg1w7hy3qg1w7hy3qg1w7hy3q-foo.drv"},
+    };
+
+    store->buildTrace.insert({drvHash, {{outputName, make_ref<UnkeyedRealisation>(value)}}});
+
+    auto value2 = store->queryRealisation({drvHash, outputName});
+
+    ASSERT_TRUE(value2);
+    EXPECT_EQ(*value2, value);
 }
 
 } // namespace nix
diff --git a/src/libstore/dummy-store.cc b/src/libstore/dummy-store.cc
index 209be3ce9..509b7a0b1 100644
--- a/src/libstore/dummy-store.cc
+++ b/src/libstore/dummy-store.cc
@@ -3,6 +3,7 @@
 #include "nix/util/callback.hh"
 #include "nix/util/memory-source-accessor.hh"
 #include "nix/store/dummy-store-impl.hh"
+#include "nix/store/realisation.hh"
 
 #include <boost/unordered/concurrent_flat_map.hpp>
 
@@ -251,7 +252,10 @@ struct DummyStoreImpl : DummyStore
 
     void registerDrvOutput(const Realisation & output) override
     {
-        unsupported("registerDrvOutput");
+        auto ref = make_ref<UnkeyedRealisation>(output);
+        buildTrace.insert_or_visit({output.id.drvHash, {{output.id.outputName, ref}}}, [&](auto & kv) {
+            kv.second.insert_or_assign(output.id.outputName, make_ref<UnkeyedRealisation>(output));
+        });
     }
 
     void narFromPath(const StorePath & path, Sink & sink) override
@@ -267,9 +271,18 @@ struct DummyStoreImpl : DummyStore
     }
 
     void queryRealisationUncached(
-        const DrvOutput &, Callback<std::shared_ptr<const UnkeyedRealisation>> callback) noexcept override
+        const DrvOutput & drvOutput, Callback<std::shared_ptr<const UnkeyedRealisation>> callback) noexcept override
     {
-        callback(nullptr);
+        bool visited = false;
+        buildTrace.cvisit(drvOutput.drvHash, [&](const auto & kv) {
+            if (auto it = kv.second.find(drvOutput.outputName); it != kv.second.end()) {
+                visited = true;
+                callback(it->second.get_ptr());
+            }
+        });
+
+        if (!visited)
+            callback(nullptr);
     }
 
     std::shared_ptr<SourceAccessor> getFSAccessor(const StorePath & path, bool requireValidPath) override
diff --git a/src/libstore/include/nix/store/dummy-store-impl.hh b/src/libstore/include/nix/store/dummy-store-impl.hh
index e05bb94ff..4c9f54e98 100644
--- a/src/libstore/include/nix/store/dummy-store-impl.hh
+++ b/src/libstore/include/nix/store/dummy-store-impl.hh
@@ -30,6 +30,18 @@ struct DummyStore : virtual Store
      */
     boost::concurrent_flat_map<StorePath, PathInfoAndContents> contents;
 
+    /**
+     * The build trace maps the pair of a content-addressing (fixed or
+     * floating) derivations an one of its output to a
+     * (content-addressed) store object.
+     *
+     * It is [curried](https://en.wikipedia.org/wiki/Currying), so we
+     * instead having a single output with a `DrvOutput` key, we have an
+     * outer map for the derivation, and inner maps for the outputs of a
+     * given derivation.
+     */
+    boost::concurrent_flat_map<Hash, std::map<std::string, ref<UnkeyedRealisation>>> buildTrace;
+
     DummyStore(ref<const Config> config)
         : Store{*config}
         , config(config)
diff --git a/src/libstore/include/nix/store/dummy-store.hh b/src/libstore/include/nix/store/dummy-store.hh
index 95c09078c..d371c4e51 100644
--- a/src/libstore/include/nix/store/dummy-store.hh
+++ b/src/libstore/include/nix/store/dummy-store.hh
@@ -3,6 +3,8 @@
 
 #include "nix/store/store-api.hh"
 
+#include <boost/unordered/concurrent_flat_map.hpp>
+
 namespace nix {
 
 struct DummyStore;
diff --git a/src/libutil/include/nix/util/hash.hh b/src/libutil/include/nix/util/hash.hh
index 571b6acca..0b16b423c 100644
--- a/src/libutil/include/nix/util/hash.hh
+++ b/src/libutil/include/nix/util/hash.hh
@@ -222,3 +222,22 @@ public:
 };
 
 } // namespace nix
+
+template<>
+struct std::hash<nix::Hash>
+{
+    std::size_t operator()(const nix::Hash & hash) const noexcept
+    {
+        assert(hash.hashSize > sizeof(size_t));
+        return *reinterpret_cast<const std::size_t *>(&hash.hash);
+    }
+};
+
+namespace nix {
+
+inline std::size_t hash_value(const Hash & hash)
+{
+    return std::hash<Hash>{}(hash);
+}
+
+} // namespace nix