block: Replace bdrv_get_format() by bdrv_get_format_name()

So callers don't need to know anything about maximum name length.
Returning a pointer is safe, because the name string lives as long as
the block driver it names, and block drivers don't die.

Requested by Peter Maydell.
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block.c

@@ -1035,7 +1035,8 @@ void bdrv_append(BlockDriverState *bs_new, BlockDriverState *bs_top)
      * swapping bs_new and bs_top contents. */
     tmp.backing_hd = bs_new;
     pstrcpy(tmp.backing_file, sizeof(tmp.backing_file), bs_top->filename);
-    bdrv_get_format(bs_top, tmp.backing_format, sizeof(tmp.backing_format));
+    pstrcpy(tmp.backing_format, sizeof(tmp.backing_format),
+            bs_top->drv ? bs_top->drv->format_name : "");
 
     /* swap contents of the fixed new bs and the current top */
     *bs_new = *bs_top;
@@ -2428,13 +2429,9 @@ int bdrv_set_key(BlockDriverState *bs, const char *key)
     return ret;
 }
 
-void bdrv_get_format(BlockDriverState *bs, char *buf, int buf_size)
+const char *bdrv_get_format_name(BlockDriverState *bs)
 {
-    if (!bs->drv) {
-        buf[0] = '\0';
-    } else {
-        pstrcpy(buf, buf_size, bs->drv->format_name);
-    }
+    return bs->drv ? bs->drv->format_name : NULL;
 }
 
 void bdrv_iterate_format(void (*it)(void *opaque, const char *name),

block.h

@@ -296,7 +296,7 @@ int bdrv_is_inserted(BlockDriverState *bs);
 int bdrv_media_changed(BlockDriverState *bs);
 void bdrv_lock_medium(BlockDriverState *bs, bool locked);
 void bdrv_eject(BlockDriverState *bs, bool eject_flag);
-void bdrv_get_format(BlockDriverState *bs, char *buf, int buf_size);
+const char *bdrv_get_format_name(BlockDriverState *bs);
 BlockDriverState *bdrv_find(const char *name);
 BlockDriverState *bdrv_next(BlockDriverState *bs);
 void bdrv_iterate(void (*it)(void *opaque, BlockDriverState *bs),

qemu-img.c

@@ -1107,7 +1107,7 @@ static int img_info(int argc, char **argv)
     int c;
     const char *filename, *fmt;
     BlockDriverState *bs;
-    char fmt_name[128], size_buf[128], dsize_buf[128];
+    char size_buf[128], dsize_buf[128];
     uint64_t total_sectors;
     int64_t allocated_size;
     char backing_filename[1024];
@@ -1139,7 +1139,6 @@ static int img_info(int argc, char **argv)
     if (!bs) {
         return 1;
     }
-    bdrv_get_format(bs, fmt_name, sizeof(fmt_name));
     bdrv_get_geometry(bs, &total_sectors);
     get_human_readable_size(size_buf, sizeof(size_buf), total_sectors * 512);
     allocated_size = bdrv_get_allocated_file_size(bs);
@@ -1153,7 +1152,7 @@ static int img_info(int argc, char **argv)
            "file format: %s\n"
            "virtual size: %s (%" PRId64 " bytes)\n"
            "disk size: %s\n",
-           filename, fmt_name, size_buf,
+           filename, bdrv_get_format_name(bs), size_buf,
            (total_sectors * 512),
            dsize_buf);
     if (bdrv_is_encrypted(bs)) {