Document the new store hierarchy

src/libstore/build.cc

@@ -2883,7 +2883,7 @@ struct RestrictedStore : public LocalFSStore
     DerivationGoal & goal;
 
     RestrictedStore(const Params & params, ref<LocalStore> next, DerivationGoal & goal)
-        : StoreConfig(params), LocalFSStoreConfig(params), Store(params), LocalFSStore(params), next(next), goal(goal)
+        : StoreConfig(params), Store(params), LocalFSStore(params), next(next), goal(goal)
     { }
 
     Path getRealStoreDir() override

src/libstore/dummy-store.cc

@@ -2,7 +2,7 @@
 
 namespace nix {
 
-struct DummyStoreConfig : StoreConfig {
+struct DummyStoreConfig : virtual StoreConfig {
     using StoreConfig::StoreConfig;
 };
 
@@ -14,7 +14,6 @@ struct DummyStore : public Store, public virtual DummyStoreConfig
 
     DummyStore(const Params & params)
         : StoreConfig(params)
-        , DummyStoreConfig(params)
         , Store(params)
     {
     }

src/libstore/http-binary-cache-store.cc

@@ -33,9 +33,7 @@ public:
         const Path & _cacheUri,
         const Params & params)
         : StoreConfig(params)
-        , BinaryCacheStoreConfig(params)
         , BinaryCacheStore(params)
-        , HttpBinaryCacheStoreConfig(params)
         , cacheUri(scheme + "://" + _cacheUri)
     {
         if (cacheUri.back() == '/')

src/libstore/legacy-ssh-store.cc

@@ -45,7 +45,6 @@ struct LegacySSHStore : public Store, public virtual LegacySSHStoreConfig
 
     LegacySSHStore(const string & scheme, const string & host, const Params & params)
         : StoreConfig(params)
-        , LegacySSHStoreConfig(params)
         , Store(params)
         , host(host)
         , connections(make_ref<Pool<Connection>>(

src/libstore/local-binary-cache-store.cc

@@ -22,8 +22,6 @@ public:
         const Path & binaryCacheDir,
         const Params & params)
         : StoreConfig(params)
-        , BinaryCacheStoreConfig(params)
-        , LocalBinaryCacheStoreConfig(params)
         , BinaryCacheStore(params)
         , binaryCacheDir(binaryCacheDir)
     {

src/libstore/local-store.cc

@@ -44,8 +44,6 @@ namespace nix {
 LocalStore::LocalStore(const Params & params)
     : StoreConfig(params)
     , Store(params)
-    , LocalFSStoreConfig(params)
-    , LocalStoreConfig(params)
     , LocalFSStore(params)
     , realStoreDir_{this, false, rootDir != "" ? rootDir + "/nix/store" : storeDir, "real",
         "physical path to the Nix store"}

src/libstore/remote-store.cc

@@ -126,9 +126,6 @@ ref<RemoteStore::Connection> RemoteStore::openConnectionWrapper()
 UDSRemoteStore::UDSRemoteStore(const Params & params)
     : StoreConfig(params)
     , Store(params)
-    , LocalFSStoreConfig(params)
-    , RemoteStoreConfig(params)
-    , UDSRemoteStoreConfig(params)
     , LocalFSStore(params)
     , RemoteStore(params)
 {

src/libstore/s3-binary-cache-store.cc

@@ -201,8 +201,6 @@ struct S3BinaryCacheStoreImpl : public S3BinaryCacheStore, virtual S3BinaryCache
         const std::string & bucketName,
         const Params & params)
         : StoreConfig(params)
-        , BinaryCacheStoreConfig(params)
-        , S3BinaryCacheStoreConfig(params)
         , S3BinaryCacheStore(params)
         , bucketName(bucketName)
         , s3Helper(profile, region, scheme, endpoint)

src/libstore/ssh-store.cc

@@ -25,9 +25,7 @@ public:
     SSHStore([[maybe_unused]] const std::string & scheme, const std::string & host, const Params & params)
         : StoreConfig(params)
         , Store(params)
-        , RemoteStoreConfig(params)
         , RemoteStore(params)
-        , SSHStoreConfig(params)
         , host(host)
         , master(
             host,

src/libstore/store-api.hh

@@ -24,6 +24,31 @@
 
 namespace nix {
 
+/**
+ * About the class hierarchy of the store implementations:
+ *
+ * Each store type `Foo` consists of two classes:
+ *
+ * 1. A class `FooConfig : virtual StoreConfig` that contains the configuration
+ *   for the store
+ *
+ *   It should only contain members of type `const Setting<T>` (or subclasses
+ *   of it) and inherit the constructors of `StoreConfig`
+ *   (`using StoreConfig::StoreConfig`).
+ *
+ * 2. A class `Foo : virtual Store, virtual FooConfig` that contains the
+ *   implementation of the store.
+ *
+ *   This class is expected to have a constructor `Foo(const Params & params)`
+ *   that calls `StoreConfig(params)` (otherwise you're gonna encounter an
+ *   `assertion failure` when trying to instantiate it).
+ *
+ * You can then register the new store using:
+ *
+ * ```
+ * cpp static RegisterStoreImplementation<Foo, FooConfig> regStore;
+ * ```
+ */
 
 MakeError(SubstError, Error);
 MakeError(BuildError, Error); // denotes a permanent build failure
@@ -148,7 +173,26 @@ struct BuildResult
 struct StoreConfig : public Config
 {
     using Config::Config;
-    StoreConfig() = delete;
+
+    /**
+     * When constructing a store implementation, we pass in a map `params` of
+     * parameters that's supposed to initialize the associated config.
+     * To do that, we must use the `StoreConfig(StringMap & params)`
+     * constructor, so we'd like to `delete` its default constructor to enforce
+     * it.
+     *
+     * However, actually deleting it means that all the subclasses of
+     * `StoreConfig` will have their default constructor deleted (because it's
+     * supposed to call the deleted default constructor of `StoreConfig`). But
+     * because we're always using virtual inheritance, the constructors of
+     * child classes will never implicitely call this one, so deleting it will
+     * be more painful than anything else.
+     *
+     * So we `assert(false)` here to ensure at runtime that the right
+     * constructor is always called without having to redefine a custom
+     * constructor for each `*Config` class.
+     */
+    StoreConfig() { assert(false); }
 
 
     const PathSetting storeDir_{this, false, settings.nixStore,