libio: Move istream_t precache logic into backend implementation

The end goal is to remove direct buffer access from the istream_t
interfaces and make that opaque. For the tar implementation, this
already safes us needless buffer copying, as we essentially allow
the user to read-through from the underlying stream.

Signed-off-by: David Oberhollenzer <david.oberhollenzer@sigma-star.at>

1 files changed, 15 insertions, 0 deletions

diff --git a/lib/io/src/xfrm/istream.c b/lib/io/src/xfrm/istream.c
index 4a1ad77..b301209 100644
--- a/lib/io/src/xfrm/istream.c
+++ b/lib/io/src/xfrm/istream.c
@@ -20,6 +20,21 @@ static int xfrm_precache(istream_t *base)
 	istream_xfrm_t *xfrm = (istream_xfrm_t *)base;
 	int ret;
 
+	if (base->buffer_offset >= base->buffer_used) {
+		base->buffer_offset = 0;
+		base->buffer_used = 0;
+	} else if (base->buffer_offset > 0) {
+		memmove(base->buffer,
+			base->buffer + base->buffer_offset,
+			base->buffer_used - base->buffer_offset);
+
+		base->buffer_used -= base->buffer_offset;
+		base->buffer_offset = 0;
+	}
+
+	if (base->eof)
+		return 0;
+
 	ret = istream_precache(xfrm->wrapped);
 	if (ret != 0)
 		return ret;