Store::addToStore bail out on no-op earlier in many cases

`BinaryCacheStore` and `LocalStore` already had this optimization. Now,
all the implementations do.

src/libstore/binary-cache-store.cc

@@ -296,8 +296,8 @@ void BinaryCacheStore::addToStore(
     const ValidPathInfo & info, Source & narSource, RepairFlag repair, CheckSigsFlag checkSigs)
 {
     if (!repair && isValidPath(info.path)) {
-        // FIXME: copyNAR -> null sink
+        NullFileSystemObjectSink s;
-        narSource.drain();
+        parseDump(s, narSource);
         return;
     }
 

src/libstore/dummy-store.cc

@@ -160,6 +160,10 @@ struct DummyStoreImpl : DummyStore
     void
     addToStore(const ValidPathInfo & info, const SourcePath & path, RepairFlag repair, CheckSigsFlag checkSigs) override
     {
+        if (!repair && isValidPath(info.path)) {
+            return;
+        }
+
         if (config->readOnly)
             unsupported("addToStore");
 

src/libstore/legacy-ssh-store.cc

@@ -145,6 +145,12 @@ void LegacySSHStore::queryPathInfoUncached(
 
 void LegacySSHStore::addToStore(const ValidPathInfo & info, Source & source, RepairFlag repair, CheckSigsFlag checkSigs)
 {
+    if (!repair && isValidPath(info.path)) {
+        NullFileSystemObjectSink s;
+        parseDump(s, source);
+        return;
+    }
+
     debug("adding path '%s' to remote host '%s'", printStorePath(info.path), config->authority.host);
 
     auto conn(connections->get());

src/libstore/remote-store.cc

@@ -424,6 +424,12 @@ StorePath RemoteStore::addToStoreFromDump(
 
 void RemoteStore::addToStore(const ValidPathInfo & info, Source & source, RepairFlag repair, CheckSigsFlag checkSigs)
 {
+    if (!repair && isValidPath(info.path)) {
+        NullFileSystemObjectSink s;
+        parseDump(s, source);
+        return;
+    }
+
     auto conn(getConnection());
 
     conn->to << WorkerProto::Op::AddToStoreNar << printStorePath(info.path)

src/libstore/store-api.cc

@@ -86,6 +86,12 @@ StorePath Store::followLinksToStorePath(std::string_view path) const
 
 void Store::addToStore(const ValidPathInfo & info, Source & narSource, RepairFlag repair, CheckSigsFlag checkSigs)
 {
+    if (!repair && isValidPath(info.path)) {
+        NullFileSystemObjectSink s;
+        parseDump(s, narSource);
+        return;
+    }
+
     auto temp = make_ref<MemorySourceAccessor>();
     MemorySink tempSink{*temp};
     parseDump(tempSink, narSource);
@@ -94,6 +100,10 @@ void Store::addToStore(const ValidPathInfo & info, Source & narSource, RepairFla
 
 void Store::addToStore(const ValidPathInfo & info, const SourcePath & path, RepairFlag repair, CheckSigsFlag checkSigs)
 {
+    if (!repair && isValidPath(info.path)) {
+        return;
+    }
+
     auto sink = sourceToSink([&](Source & source) { addToStore(info, source, repair, checkSigs); });
     dumpPath(path, *sink, FileSerialisationMethod::NixArchive);
 }