lib: Add i_stream_nonseekable_try_seek()

This can be used by istreams to more easily implement seeking backwards when
it has to be done by first seeking back to offset 0 and reading from there.

src/lib/istream-private.h

@@ -39,6 +39,9 @@ struct istream_private {
 
 	size_t buffer_size, max_buffer_size, init_buffer_size;
 	size_t skip, pos, try_alloc_limit;
+	/* If seeking backwards within the buffer, the next read() will
+	   return again pos..high_pos */
+	size_t high_pos;
 
 	struct istream *parent; /* for filter streams */
 	uoff_t parent_start_offset;
@@ -98,6 +101,12 @@ void i_stream_free_buffer(struct istream_private *stream);
 ssize_t i_stream_read_copy_from_parent(struct istream *istream);
 void i_stream_default_seek_nonseekable(struct istream_private *stream,
 				       uoff_t v_offset, bool mark);
+/* Returns FALSE if seeking must be done by starting from the beginning.
+   The caller is then expected to reset the stream and call this function
+   again, which should work then. If TRUE is returned, the seek was either
+   successfully done or stream_errno is set. */
+bool i_stream_nonseekable_try_seek(struct istream_private *stream,
+				   uoff_t v_offset);
 
 /* Default snapshot handling: use memarea if it exists, otherwise snapshot
    parent stream. */

src/lib/istream.c

@@ -303,7 +303,15 @@ ssize_t i_stream_read_memarea(struct istream *stream)
 		i_stream_seek(_stream->parent, _stream->parent_expected_offset);
 
 	old_size = _stream->pos - _stream->skip;
-	ret = _stream->read(_stream);
+	if (_stream->pos < _stream->high_pos) {
+		/* we're here because we seeked back within the read buffer. */
+		ret = _stream->high_pos - _stream->pos;
+		_stream->pos = _stream->high_pos;
+		_stream->high_pos = 0;
+	} else {
+		_stream->high_pos = 0;
+		ret = _stream->read(_stream);
+	}
 	i_assert(old_size <= _stream->pos - _stream->skip);
 	switch (ret) {
 	case -2:
@@ -1030,6 +1038,34 @@ void i_stream_default_seek_nonseekable(struct istream_private *stream,
 	}
 }
 
+bool i_stream_nonseekable_try_seek(struct istream_private *stream,
+				   uoff_t v_offset)
+{
+	uoff_t start_offset = stream->istream.v_offset - stream->skip;
+
+	if (v_offset < start_offset) {
+		/* have to seek backwards */
+		i_stream_seek(stream->parent, stream->parent_start_offset);
+		stream->parent_expected_offset = stream->parent_start_offset;
+		stream->skip = stream->pos = 0;
+		stream->istream.v_offset = 0;
+		stream->high_pos = 0;
+		return FALSE;
+	}
+
+	if (v_offset <= start_offset + stream->pos) {
+		/* seeking backwards within what's already cached */
+		stream->skip = v_offset - start_offset;
+		stream->istream.v_offset = v_offset;
+		stream->high_pos = stream->pos;
+		stream->pos = stream->skip;
+	} else {
+		/* read forward */
+		i_stream_default_seek_nonseekable(stream, v_offset, FALSE);
+	}
+	return TRUE;
+}
+
 static int
 i_stream_default_stat(struct istream_private *stream, bool exact)
 {