gunzip: fix ordering of boundary callbacks

The boundary callback should not happen until all the uncompressed
data has been consumed. This previously seems to have worked
because normally gzip library returns "no error" instead of the
"stream end" if we extract exactly the amount of bytes remaining
in the archive. (Perhaps this was changed in new zlib.) In any
case, verification was broken with some apks due to this callback
ordering issue.

src/gunzip.c

@@ -27,6 +27,7 @@ struct apk_gzip_istream {
 	apk_multipart_cb cb;
 	void *cbctx;
 	void *cbprev;
+	apk_blob_t cbarg;
 };
 
 static ssize_t gzi_read(void *stream, void *ptr, size_t size)
@@ -48,6 +49,18 @@ static ssize_t gzi_read(void *stream, void *ptr, size_t size)
 	gis->zs.next_out  = ptr;
 
 	while (gis->zs.avail_out != 0 && gis->err == 0) {
+		if (!APK_BLOB_IS_NULL(gis->cbarg)) {
+			r = gis->cb(gis->cbctx,
+				    gis->err ? APK_MPART_END : APK_MPART_BOUNDARY,
+				    gis->cbarg);
+			if (r > 0)
+				r = -ECANCELED;
+			if (r != 0) {
+				gis->err = r;
+				goto ret;
+			}
+			gis->cbarg = APK_BLOB_NULL;
+		}
 		if (gis->zs.avail_in == 0) {
 			apk_blob_t blob;
 
@@ -86,19 +99,22 @@ static ssize_t gzi_read(void *stream, void *ptr, size_t size)
 			    gis->zs.avail_in == 0)
 				gis->err = 1;
 			if (gis->cb != NULL) {
+				gis->cbarg = APK_BLOB_PTR_LEN(gis->cbprev, (void *) gis->zs.next_in - gis->cbprev); 
+				gis->cbprev = gis->zs.next_in;
+			}
+			/* If we hit end of the bitstream (not end
+			 * of just this gzip), we need to do the
+			 * callback here, as we won't be called again.
+			 * For boundaries it should be postponed to not
+			 * be called until next gzip read is started. */
+			if (gis->err) {
 				r = gis->cb(gis->cbctx,
 					    gis->err ? APK_MPART_END : APK_MPART_BOUNDARY,
-					    APK_BLOB_PTR_LEN(gis->cbprev, (void *) gis->zs.next_in - gis->cbprev));
+					    gis->cbarg);
 				if (r > 0)
 					r = -ECANCELED;
-				if (r != 0) {
-					gis->err = r;
-					goto ret;
-				}
-				gis->cbprev = gis->zs.next_in;
-			}
-			if (gis->err)
 				goto ret;
+			}
 			inflateEnd(&gis->zs);
 			if (inflateInit2(&gis->zs, 15+32) != Z_OK)
 				return -ENOMEM;