diff --git a/src/DecoderBuffer.cxx b/src/DecoderBuffer.cxx
index 78a2ccdc6..4a5125bc5 100644
--- a/src/DecoderBuffer.cxx
+++ b/src/DecoderBuffer.cxx
@@ -130,6 +130,12 @@ decoder_buffer_fill(DecoderBuffer *buffer)
 	return true;
 }
 
+size_t
+decoder_buffer_available(const DecoderBuffer *buffer)
+{
+	return buffer->length - buffer->consumed;;
+}
+
 const void *
 decoder_buffer_read(const DecoderBuffer *buffer, size_t *length_r)
 {
diff --git a/src/DecoderBuffer.hxx b/src/DecoderBuffer.hxx
index 0eaf0c4f3..65c6e0d2e 100644
--- a/src/DecoderBuffer.hxx
+++ b/src/DecoderBuffer.hxx
@@ -77,6 +77,13 @@ decoder_buffer_clear(DecoderBuffer *buffer);
 bool
 decoder_buffer_fill(DecoderBuffer *buffer);
 
+/**
+ * How many bytes are stored in the buffer?
+ */
+gcc_pure
+size_t
+decoder_buffer_available(const DecoderBuffer *buffer);
+
 /**
  * Reads data from the buffer.  This data is not yet consumed, you
  * have to call decoder_buffer_consume() to do that.  The returned