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, 14 insertions, 1 deletions

diff --git a/lib/io/src/mem.c b/lib/io/src/mem.c
index 4e9f9af..d305a83 100644
--- a/lib/io/src/mem.c
+++ b/lib/io/src/mem.c
@@ -26,8 +26,21 @@ typedef struct {
 static int mem_in_precache(istream_t *strm)
 {
 	mem_istream_t *mem = (mem_istream_t *)strm;
-	size_t diff = mem->bufsz - strm->buffer_used;
+	size_t diff;
+
+	if (strm->buffer_offset >= strm->buffer_used) {
+		strm->buffer_offset = 0;
+		strm->buffer_used = 0;
+	} else if (strm->buffer_offset > 0) {
+		memmove(strm->buffer,
+			strm->buffer + strm->buffer_offset,
+			strm->buffer_used - strm->buffer_offset);
+
+		strm->buffer_used -= strm->buffer_offset;
+		strm->buffer_offset = 0;
+	}
 
+	diff = mem->bufsz - strm->buffer_used;
 	if (diff > mem->size)
 		diff = mem->size;