block/qcow2-threads: fix qcow2_decompress

On success path we return what inflate() returns instead of 0. And it
most probably works for Z_STREAM_END as it is positive, but is
definitely broken for Z_BUF_ERROR.

While being here, switch to errno return code, to be closer to
qcow2_compress API (and usual expectations).

Revert condition in if to be more positive. Drop dead initialization of
ret.

Cc: qemu-stable@nongnu.org # v4.0
Fixes: 341926ab
Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20200302150930.16218-1-vsementsov@virtuozzo.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>

block/qcow2-threads.c

@@ -128,12 +128,12 @@ static ssize_t qcow2_compress(void *dest, size_t dest_size,
  * @src - source buffer, @src_size bytes
  *
  * Returns: 0 on success
- *          -1 on fail
+ *          -EIO on fail
  */
 static ssize_t qcow2_decompress(void *dest, size_t dest_size,
                                 const void *src, size_t src_size)
 {
-    int ret = 0;
+    int ret;
     z_stream strm;
 
     memset(&strm, 0, sizeof(strm));
@@ -144,17 +144,19 @@ static ssize_t qcow2_decompress(void *dest, size_t dest_size,
 
     ret = inflateInit2(&strm, -12);
     if (ret != Z_OK) {
-        return -1;
+        return -EIO;
     }
 
     ret = inflate(&strm, Z_FINISH);
-    if ((ret != Z_STREAM_END && ret != Z_BUF_ERROR) || strm.avail_out != 0) {
+    if ((ret == Z_STREAM_END || ret == Z_BUF_ERROR) && strm.avail_out == 0) {
         /*
          * We approve Z_BUF_ERROR because we need @dest buffer to be filled, but
          * @src buffer may be processed partly (because in qcow2 we know size of
          * compressed data with precision of one sector)
          */
-        ret = -1;
+        ret = 0;
+    } else {
+        ret = -EIO;
     }
 
     inflateEnd(&strm);