[Bug 1865048] Re: qemu-img --force-share does not disable file locking

[Max Reitz]

Hi Olaf,

Every “node” in the block graph corresponds to some driver.  A driver
can be a protocol or a format driver (or a filter driver, but that isn’t
important here).  In your example, there is only a single node, for a
protocol driver (namely “file”).  You need a format driver node on top
to interpret the image format.

If you use file.driver=file,file.filename=foo.qcow2,file.locking=off,
then that specifies the options driver, filename, and locking for a node
under another node’s “file” link.  So this has to create two nodes.  The
node on top (for which no options are specified) should default to being
a format node whose format is probed.

Of course you can also give options to the top (format) node, like e.g.
the driver.  (In fact, you should probably give the driver, because
format probing is considered dangerous.)

Then it would look like this:
driver=qcow2,file.driver=file,file.filename=foo.qcow2,file.locking=off

(Or, in JSON, but that only works with qemu’s -blockdev (but I think it’s 
better for visualizing the resulting block graph:
 {"node-name": "some-node-name",
  "driver": "qcow2",
  "file": {
      "driver": "file",
      "filename": "foo.qcow2",
      "locking": false
  } })


Hope that helps,

Max

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https://bugs.launchpad.net/bugs/1865048

Title:
  qemu-img --force-share does not disable file locking

Status in QEMU:
  Invalid

Bug description:
  The new option "--force-share" for qemu-img does not disable file
  locking.

  I tried it with version qemu-img version 2.11.1(Debian 1:2.11+dfsg-
  1ubuntu7.21~cloud0) and I traced the source code of the current git
  trunk.

  Sample to demonstrate:

  # strace qemu-img info --force-share testfile.qcow2   2>&1 | grep F_RDLCK
  fcntl(11, F_OFD_SETLK, {l_type=F_RDLCK, l_whence=SEEK_SET, l_start=100, 
l_len=1}) = 0
  fcntl(11, F_OFD_SETLK, {l_type=F_RDLCK, l_whence=SEEK_SET, l_start=100, 
l_len=1}) = 0
  fcntl(11, F_OFD_SETLK, {l_type=F_RDLCK, l_whence=SEEK_SET, l_start=100, 
l_len=1}) = 0
  fcntl(11, F_OFD_SETLK, {l_type=F_RDLCK, l_whence=SEEK_SET, l_start=100, 
l_len=1}) = 0
  fcntl(11, F_OFD_SETLK, {l_type=F_RDLCK, l_whence=SEEK_SET, l_start=100, 
l_len=1}) = 0
  fcntl(11, F_OFD_SETLK, {l_type=F_RDLCK, l_whence=SEEK_SET, l_start=100, 
l_len=1}) = 0
  fcntl(11, F_OFD_SETLK, {l_type=F_RDLCK, l_whence=SEEK_SET, l_start=100, 
l_len=1}) = 0
  fcntl(11, F_OFD_SETLK, {l_type=F_RDLCK, l_whence=SEEK_SET, l_start=100, 
l_len=1}) = 0
  fcntl(11, F_OFD_SETLK, {l_type=F_RDLCK, l_whence=SEEK_SET, l_start=100, 
l_len=1}) = 0

  I traced the passing of the --force-share option through the source
  code (I used commit 6c599282f8 as of Mon Feb 17 13:32:25 2020 +0000)

  qemu-img.c:img_info()
          force_share = true;
  qemu-img.c:collect_image_info_list(force_share)
  qemu-img.c:img_open(force_share)
  qemu-img.c:img_open_file(force_share)
          qdict_put_bool(options, BDRV_OPT_FORCE_SHARE, true);
  block/block-backend.c:blk_new_open(options)
  block.c:bdrv_open(options)
  block.c:bdrv_open_inheritoptions()
  block.c:bdrv_open_common(options)
          bs->force_share = qemu_opt_get_bool(opts, BDRV_OPT_FORCE_SHARE, 
false);
  block.c:bdrv_open_driver(bs)
  include/block/block_int.h:int (*bdrv_file_open)(BlockDriverState *bs, QDict 
*options, int flags,
  block/file-posix.c:.bdrv_file_open = raw_open,
  block/file-posix.c:raw_open_common(bs)
          locking = qapi_enum_parse(&OnOffAuto_lookup,
                                qemu_opt_get(opts, "locking"),
                                ON_OFF_AUTO_AUTO, &local_err);
          ignoring bs->force_share

  At the end, bs->force_share is ignored in determining the locking
  value.

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