Use MemorySourceAccessor in DummyStore

Add `read-only` setting to `dummy://` store for back compat.

Test by changing an existing test to use this instead, fixing a TODO.

Co-Authored-By: HaeNoe <git@haenoe.party>
Co-authored-by: Eelco Dolstra <edolstra@gmail.com>

src/libfetchers-tests/git.cc

@@ -1,5 +1,6 @@
 #include "nix/store/store-open.hh"
 #include "nix/store/globals.hh"
+#include "nix/store/dummy-store.hh"
 #include "nix/fetchers/fetch-settings.hh"
 #include "nix/fetchers/fetchers.hh"
 #include "nix/fetchers/git-utils.hh"
@@ -179,10 +180,11 @@ TEST_F(GitTest, submodulePeriodSupport)
     // 6) Commit the addition in super
     commitAll(super.get(), "Add submodule with branch='.'");
 
-    // TODO: Use dummy:// store with MemorySourceAccessor.
+    auto store = [] {
-    Path storeTmpDir = createTempDir();
+        auto cfg = make_ref<DummyStoreConfig>(StoreReference::Params{});
-    auto storeTmpDirAutoDelete = AutoDelete(storeTmpDir, true);
+        cfg->readOnly = false;
-    ref<Store> store = openStore(storeTmpDir);
+        return cfg->openStore();
+    }();
 
     auto settings = fetchers::Settings{};
     auto input = fetchers::Input::fromAttrs(

src/libstore/dummy-store.cc

@@ -1,5 +1,7 @@
 #include "nix/store/store-registration.hh"
+#include "nix/util/archive.hh"
 #include "nix/util/callback.hh"
+#include "nix/util/memory-source-accessor.hh"
 #include "nix/store/dummy-store.hh"
 
 namespace nix {
@@ -17,9 +19,12 @@ struct DummyStore : virtual Store
 
     ref<const Config> config;
 
+    ref<MemorySourceAccessor> contents;
+
     DummyStore(ref<const Config> config)
         : Store{*config}
         , config(config)
+        , contents(make_ref<MemorySourceAccessor>())
     {
     }
 
@@ -47,8 +52,8 @@ struct DummyStore : virtual Store
         unsupported("addToStore");
     }
 
-    virtual StorePath addToStoreFromDump(
+    StorePath addToStoreFromDump(
-        Source & dump,
+        Source & source,
         std::string_view name,
         FileSerialisationMethod dumpMethod = FileSerialisationMethod::NixArchive,
         ContentAddressMethod hashMethod = FileIngestionMethod::NixArchive,
@@ -56,7 +61,45 @@ struct DummyStore : virtual Store
         const StorePathSet & references = StorePathSet(),
         RepairFlag repair = NoRepair) override
     {
-        unsupported("addToStore");
+        if (config->readOnly)
+            unsupported("addToStoreFromDump");
+
+        auto temp = make_ref<MemorySourceAccessor>();
+
+        {
+            MemorySink tempSink{*temp};
+
+            // TODO factor this out into `restorePath`, same todo on it.
+            switch (dumpMethod) {
+            case FileSerialisationMethod::NixArchive:
+                parseDump(tempSink, source);
+                break;
+            case FileSerialisationMethod::Flat: {
+                // Replace root dir with file so next part succeeds.
+                temp->root = MemorySourceAccessor::File::Regular{};
+                tempSink.createRegularFile(CanonPath::root, [&](auto & sink) { source.drainInto(sink); });
+                break;
+            }
+            }
+        }
+
+        auto hash = hashPath({temp, CanonPath::root}, hashMethod.getFileIngestionMethod(), hashAlgo).first;
+
+        auto desc = ContentAddressWithReferences::fromParts(
+            hashMethod,
+            hash,
+            {
+                .others = references,
+                // caller is not capable of creating a self-reference, because
+                // this is content-addressed without modulus
+                .self = false,
+            });
+
+        auto dstPath = makeFixedOutputPathFromCA(name, desc);
+
+        contents->open(CanonPath(printStorePath(dstPath)), std::move(temp->root));
+
+        return dstPath;
     }
 
     void narFromPath(const StorePath & path, Sink & sink) override
@@ -72,7 +115,7 @@ struct DummyStore : virtual Store
 
     virtual ref<SourceAccessor> getFSAccessor(bool requireValidPath) override
     {
-        return makeEmptySourceAccessor();
+        return this->contents;
     }
 };
 

src/libstore/dummy-store.md

@@ -2,9 +2,11 @@ R"(
 
 **Store URL format**: `dummy://`
 
-This store type represents a store that contains no store paths and
+This store type represents a store in memory.
-cannot be written to. It's useful when you want to use the Nix
+Store objects can be read and written, but only so long as the store is open.
-evaluator when no actual Nix store exists, e.g.
+Once the store is closed, all data will be forgoton.
+
+It's useful when you want to use the Nix evaluator when no actual Nix store exists, e.g.
 
 ```console
 # nix eval --store dummy:// --expr '1 + 2'

src/libstore/include/nix/store/dummy-store.hh

@@ -13,6 +13,15 @@ struct DummyStoreConfig : public std::enable_shared_from_this<DummyStoreConfig>,
             throw UsageError("`%s` store URIs must not contain an authority part %s", scheme, authority);
     }
 
+    Setting<bool> readOnly{
+        this,
+        true,
+        "read-only",
+        R"(
+          Make any sort of write fail instead of succeeding.
+          No additional memory will be used, because no information needs to be stored.
+        )"};
+
     static const std::string name()
     {
         return "Dummy Store";