Separate the instantiation and initialisation of the stores

Add a new `init()` method to the `Store` class that is supposed to
handle all the effectful initialisation needed to set-up the store.
The constructor should remain side-effect free and just initialize the
c++ data structure.

The goal behind that is that we can create “dummy” instances of each
store to query static properties about it (the parameters it accepts for
example)

src/libstore/binary-cache-store.hh

@@ -58,7 +58,7 @@ public:
 
 public:
 
-    virtual void init();
+    virtual void init() override;
 
 private:
 

src/libstore/dummy-store.cc

@@ -5,6 +5,10 @@ namespace nix {
 struct DummyStore : public Store
 {
     DummyStore(const std::string uri, const Params & params)
+        : DummyStore(params)
+    { }
+
+    DummyStore(const Params & params)
         : Store(params)
     { }
 

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

@@ -23,6 +23,12 @@ private:
 
 public:
 
+    HttpBinaryCacheStore(
+        const Params & params)
+        : HttpBinaryCacheStore("dummy", params)
+    {
+    }
+
     HttpBinaryCacheStore(
         const Path & _cacheUri,
         const Params & params)

src/libstore/legacy-ssh-store.cc

@@ -37,6 +37,9 @@ struct LegacySSHStore : public Store
 
     static std::vector<std::string> uriPrefixes() { return {"ssh"}; }
 
+    LegacySSHStore(const Params & params)
+        : LegacySSHStore("dummy", params)
+    {}
 
     LegacySSHStore(const string & host, const Params & params)
         : Store(params)

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

@@ -12,6 +12,12 @@ private:
 
 public:
 
+    LocalBinaryCacheStore(
+        const Params & params)
+        : LocalBinaryCacheStore("dummy", params)
+    {
+    }
+
     LocalBinaryCacheStore(
         const Path & binaryCacheDir,
         const Params & params)

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

@@ -192,6 +192,9 @@ struct S3BinaryCacheStoreImpl : public S3BinaryCacheStore
 
     S3Helper s3Helper;
 
+    S3BinaryCacheStoreImpl(const Params & params)
+        : S3BinaryCacheStoreImpl("dummy-bucket", params) {}
+
     S3BinaryCacheStoreImpl(
         const std::string & bucketName,
         const Params & params)

src/libstore/ssh-store.cc

@@ -17,6 +17,12 @@ public:
     const Setting<Path> remoteProgram{(Store*) this, "nix-daemon", "remote-program", "path to the nix-daemon executable on the remote system"};
     const Setting<std::string> remoteStore{(Store*) this, "", "remote-store", "URI of the store on the remote system"};
 
+    SSHStore(
+        const Params & params)
+        : SSHStore("dummy", params)
+    {
+    }
+
     SSHStore(const std::string & host, const Params & params)
         : Store(params)
         , RemoteStore(params)

src/libstore/store-api.cc

@@ -1094,8 +1094,9 @@ ref<Store> openStore(const std::string & uri_,
     }
 
     for (auto implem : *Implementations::registered) {
-        auto store = implem.open(uri, params);
+        auto store = implem.create(uri, params);
         if (store) {
+            store->init();
             store->warnUnknownSettings();
             return ref<Store>(store);
         }

src/libstore/store-api.hh

@@ -200,9 +200,12 @@ protected:
 
     Store(const Params & params);
 
-    std::shared_ptr<Config> getConfig();
-
 public:
+    /**
+     * Perform any necessary effectful operation to make the store up and
+     * running
+     */
+    virtual void init() {};
 
     virtual ~Store() { }
 
@@ -749,7 +752,8 @@ std::list<ref<Store>> getDefaultSubstituters();
 
 struct StoreFactory
 {
-    std::function<std::shared_ptr<Store> (const std::string & uri, const Store::Params & params)> open;
+    std::function<std::shared_ptr<Store> (const std::string & uri, const Store::Params & params)> create;
+    std::function<std::shared_ptr<Store> (const Store::Params & params)> createDummy;
 };
 struct Implementations
 {
@@ -760,10 +764,14 @@ struct Implementations
     {
         if (!registered) registered = new std::vector<StoreFactory>();
         StoreFactory factory{
-            .open =
+            .create =
                 ([](const std::string & uri, const Store::Params & params)
                  -> std::shared_ptr<Store>
                  { return std::make_shared<T>(uri, params); }),
+            .createDummy =
+                ([](const Store::Params & params)
+                 -> std::shared_ptr<Store>
+                 { return std::make_shared<T>(params); })
         };
         registered->push_back(factory);
     }