refactor: Extract simply, awkwardly Store::queryPathInfoFromClientCache

This is useful for determining quickly which substituters to query.

An alternative would be for users to invoke the narinfo cache db directly,
so why do we need this change?

 - It is easier to use. I believe Nix itself should also use it.

 - This way, the narinfo cache db remains an implementation detail.

 - Callers get to use the in-memory cache as well.

src/libstore/store-api.cc

@@ -685,20 +685,19 @@ static bool goodStorePath(const StorePath & expected, const StorePath & actual)
         && (expected.name() == Store::MissingName || expected.name() == actual.name());
 }
 
-
-void Store::queryPathInfo(const StorePath & storePath,
-    Callback<ref<const ValidPathInfo>> callback) noexcept
+bool Store::queryPathInfoFromClientCache(const StorePath & storePath,
+    Callback<ref<const ValidPathInfo>> & callback)
 {
     auto hashPart = std::string(storePath.hashPart());
 
-    try {
     {
         auto res = state.lock()->pathInfoCache.get(std::string(storePath.to_string()));
         if (res && res->isKnownNow()) {
             stats.narInfoReadAverted++;
             if (!res->didExist())
                 throw InvalidPath("path '%s' is not valid", printStorePath(storePath));
-                return callback(ref<const ValidPathInfo>(res->value));
+            callback(ref<const ValidPathInfo>(res->value));
+            return true;
         }
     }
 
@@ -714,10 +713,23 @@ void Store::queryPathInfo(const StorePath & storePath,
                     !goodStorePath(storePath, res.second->path))
                     throw InvalidPath("path '%s' is not valid", printStorePath(storePath));
             }
-                return callback(ref<const ValidPathInfo>(res.second));
+            callback(ref<const ValidPathInfo>(res.second));
+            return true;
         }
     }
 
+    return false;
+}
+
+
+void Store::queryPathInfo(const StorePath & storePath,
+    Callback<ref<const ValidPathInfo>> callback) noexcept
+{
+    auto hashPart = std::string(storePath.hashPart());
+
+    try {
+        if (queryPathInfoFromClientCache(storePath, callback))
+            return;
     } catch (...) { return callback.rethrow(); }
 
     auto callbackPtr = std::make_shared<decltype(callback)>(std::move(callback));

src/libstore/store-api.hh

@@ -282,6 +282,18 @@ public:
     void queryPathInfo(const StorePath & path,
         Callback<ref<const ValidPathInfo>> callback) noexcept;
 
+    /**
+     * NOTE: this is not the final interface - to be modified in next commit.
+     *
+     * Asynchronous version that only queries the local narinfo cache and not
+     * the actual store.
+     *
+     * @return true if the path was known and the callback invoked
+     * @return false if the path was not known and the callback not invoked
+     * @throw InvalidPathError if the path is known to be invalid
+     */
+    bool queryPathInfoFromClientCache(const StorePath & path, Callback<ref<const ValidPathInfo>> & callback);
+
     /**
      * Query the information about a realisation.
      */