command/file: "albumart" tries to send larger chunks if available

If we only receive very little data from the InputStream, try a second
Read() call to get more data.  This works around tiny reads at input
buffer boundaries with the io_uring input plugin.  These tiny reads
are inefficient, and we can afford to wait one more low-level I/O
iteration to finish (but not more).

Closes https://github.com/MusicPlayerDaemon/MPD/issues/2186

NEWS

@@ -1,4 +1,6 @@
 ver 0.23.17 (not yet released)
+* protocol
+  - "albumart" tries to send larger chunks if available
 * storage
   - nfs: require libnfs 4.0 or later
 * database

src/command/FileCommands.cxx

@@ -215,7 +215,20 @@ read_stream_art(Response &r, const std::string_view art_directory,
 	if (buffer_size > 0) {
 		std::unique_lock<Mutex> lock(is->mutex);
 		is->Seek(lock, offset);
+
+		const bool was_ready = is->IsReady();
+
 		read_size = is->Read(lock, buffer.get(), buffer_size);
+
+		if (was_ready && read_size < buffer_size / 2)
+			/* the InputStream was ready before, but we
+			   got only very little data; probably just
+			   some data left in the buffer without doing
+			   any I/O; let's wait for the next low-level
+			   read to complete to get more data for the
+			   client */
+			read_size += is->Read(lock, buffer.get() + read_size,
+					      buffer_size - read_size);
 	}
 
 	r.Fmt("size: {}\n", art_file_size);