[PATCH for 5.2 1/1] qemu-io: add -V flag for read sub-command

[Denis V. Lunev]

The problem this patch is trying to address is libguestfs behavior on the
appliance startup. It starts supporting to use root=UUID definition in
the kernel command line of its root filesystem using
    file --  /usr/lib64/guestfs/appliance/root
This works fine with RAW image, but we are using QCOW2 as a storage to
save a bit of file space and in this case we get
    QEMU QCOW Image (v3), 1610612736 bytes
instead of UUID of the root filesystem.

The solution is very simple - we should dump first 256k of the image file
like the follows
    qemu-io -c "read -V 0 256k" appliance | file -
which will provide correct result for all possible types of the appliance
storage.

Unfortunately, additional option for qemu-io is the only and the simplest
solution as '-v' creates very specific output, which requires to be
parsed. 'qemu-img dd of=/dev/stdout' does not work and the fix would be
much more intrusive.

Signed-off-by: Denis V. Lunev <den@openvz.org>
CC: Andrey Shinkevich <andrey.shinkevich@virtuozzo.com>
CC: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
CC: Kevin Wolf <kwolf@redhat.com>
CC: Max Reitz <mreitz@redhat.com>
CC: Richard W.M. Jones <rjones@redhat.com>
---
P.S. Patch to libguestfs will follow.

 qemu-io-cmds.c | 17 +++++++++++++----
 1 file changed, 13 insertions(+), 4 deletions(-)

diff --git a/qemu-io-cmds.c b/qemu-io-cmds.c
index baeae86d8c..7aae9726cd 100644
--- a/qemu-io-cmds.c
+++ b/qemu-io-cmds.c
@@ -718,7 +718,7 @@ static const cmdinfo_t read_cmd = {
     .cfunc      = read_f,
     .argmin     = 2,
     .argmax     = -1,
-    .args       = "[-abCqv] [-P pattern [-s off] [-l len]] off len",
+    .args       = "[-abCqvV] [-P pattern [-s off] [-l len]] off len",
     .oneline    = "reads a number of bytes at a specified offset",
     .help       = read_help,
 };
@@ -728,6 +728,7 @@ static int read_f(BlockBackend *blk, int argc, char **argv)
     struct timespec t1, t2;
     bool Cflag = false, qflag = false, vflag = false;
     bool Pflag = false, sflag = false, lflag = false, bflag = false;
+    bool vrawflag = true;
     int c, cnt, ret;
     char *buf;
     int64_t offset;
@@ -737,7 +738,7 @@ static int read_f(BlockBackend *blk, int argc, char **argv)
     int pattern = 0;
     int64_t pattern_offset = 0, pattern_count = 0;
 
-    while ((c = getopt(argc, argv, "bCl:pP:qs:v")) != -1) {
+    while ((c = getopt(argc, argv, "bCl:pP:qs:vV")) != -1) {
         switch (c) {
         case 'b':
             bflag = true;
@@ -777,6 +778,9 @@ static int read_f(BlockBackend *blk, int argc, char **argv)
         case 'v':
             vflag = true;
             break;
+        case 'V':
+            vrawflag = true;
+            break;
         default:
             qemuio_command_usage(&read_cmd);
             return -EINVAL;
@@ -869,10 +873,15 @@ static int read_f(BlockBackend *blk, int argc, char 
**argv)
     if (vflag) {
         dump_buffer(buf, offset, count);
     }
+    if (vrawflag) {
+        write(STDOUT_FILENO, buf, count);
+    }
 
     /* Finally, report back -- -C gives a parsable format */
-    t2 = tsub(t2, t1);
-    print_report("read", &t2, offset, count, total, cnt, Cflag);
+    if (!vrawflag) {
+        t2 = tsub(t2, t1);
+        print_report("read", &t2, offset, count, total, cnt, Cflag);
+    }
 
 out:
     qemu_io_free(buf);
-- 
2.17.1