virt-v2v/SOURCES/0035-v2v-Allow-output-to-block-devices-RHBZ-1868690.patch

From 704e86cb3bd4ddc3b7c207967f0413b4637be1f3 Mon Sep 17 00:00:00 2001
From: "Richard W.M. Jones" <rjones@redhat.com>
Date: Tue, 1 Sep 2020 14:44:17 +0100
Subject: [PATCH] v2v: Allow output to block devices (RHBZ#1868690).

We previously implicitly supported writing to block devices instead of
local files, but there were several problems:

* Block devices could be deleted, especially if virt-v2v failed during
  a conversion.

* Block devices could be overwritten by a file with the same name,
  although I believe this is just an observed consequence of the
  previous point, or at least I was not able to reproduce this until
  virt-v2v failed for another reason and then I noticed that because
  the block device was deleted, the next run overwrote it with a file.

* It was not documented anywhere how to do it.

This commit makes the small code change needed to allow virt-v2v to
write to a block device, only for existing outputs which write to
local files (ie. using TargetFile).  Also it avoids deleting block
devices accidentally on failure.

Note this commit intentionally does not prevent you from writing qcow2
to a block device.  RHV uses this so it is a thing that people do.

(cherry picked from commit 9a5974fa3bc038e5e5dbb9605a6db77d06e7bf77)
---
 docs/virt-v2v.pod | 33 ++++++++++++++++++++++++++++++
 v2v/v2v.ml        | 51 ++++++++++++++++++++++++++++-------------------
 2 files changed, 63 insertions(+), 21 deletions(-)

diff --git a/docs/virt-v2v.pod b/docs/virt-v2v.pod
index af69d633..50b0bc8e 100644
--- a/docs/virt-v2v.pod
+++ b/docs/virt-v2v.pod
@@ -1378,8 +1378,41 @@ require either a special user and/or for you to source a script that
 sets authentication environment variables.  Consult the Glance
 documentation.
 
+=item Writing to block devices
+
+This normally requires root.  See the next section.
+
 =back
 
+=head2 Writing to block devices
+
+Some output modes write to local files.  In general these modes also
+let you write to block devices, but before you run virt-v2v you may
+have to arrange for symbolic links to the desired block devices in the
+output directory.
+
+For example if using I<-o local -os /dir> then virt-v2v would normally
+create files called:
+
+ /dir/name-sda     # first disk
+ /dir/name-sdb     # second disk
+ ...
+ /dir/name.xml     # metadata
+
+If you wish the disks to be written to block devices then you would
+need to create F</dir/I<name>-sda> (etc) as symlinks to the block
+devices:
+
+ # lvcreate -L 10G -n VolumeForDiskA VG
+ # lvcreate -L 6G -n VolumeForDiskB VG
+ # ln -sf /dev/VG/VolumeForDiskA /dir/name-sda
+ # ln -sf /dev/VG/VolumeForDiskB /dir/name-sdb
+
+Note that you must precreate the correct number of block devices of
+the correct size.  Typically I<-of raw> has to be used too, but other
+formats such as qcow2 can be useful occasionally so virt-v2v does not
+force you to use raw on block devices.
+
 =head2 Minimal XML for -i libvirtxml option
 
 When using the I<-i libvirtxml> option, you have to supply some
diff --git a/v2v/v2v.ml b/v2v/v2v.ml
index a58ff433..1f8d0138 100644
--- a/v2v/v2v.ml
+++ b/v2v/v2v.ml
@@ -681,7 +681,10 @@ and copy_targets cmdline targets input output =
         fun t ->
           match t.target_file with
           | TargetURI _ -> ()
-          | TargetFile s -> try unlink s with _ -> ()
+          | TargetFile filename ->
+             if not (is_block_device filename) then (
+               try unlink filename with _ -> ()
+             )
       ) targets
     )
   );
@@ -711,27 +714,33 @@ and copy_targets cmdline targets input output =
 
       (match t.target_file with
        | TargetFile filename ->
-          (* It turns out that libguestfs's disk creation code is
-           * considerably more flexible and easier to use than
-           * qemu-img, so create the disk explicitly using libguestfs
-           * then pass the 'qemu-img convert -n' option so qemu reuses
-           * the disk.
-           *
-           * Also we allow the output mode to actually create the disk
-           * image.  This lets the output mode set ownership and
-           * permissions correctly if required.
+          (* As a special case, allow output to a block device or
+           * symlink to a block device.  In this case we don't
+           * create/overwrite the block device.  (RHBZ#1868690).
            *)
-          (* What output preallocation mode should we use? *)
-          let preallocation =
-            match t.target_format, cmdline.output_alloc with
-            | ("raw"|"qcow2"), Sparse -> Some "sparse"
-            | ("raw"|"qcow2"), Preallocated -> Some "full"
-            | _ -> None (* ignore -oa flag for other formats *) in
-          let compat =
-            match t.target_format with "qcow2" -> Some "1.1" | _ -> None in
-          output#disk_create filename t.target_format
-                             t.target_overlay.ov_virtual_size
-                             ?preallocation ?compat
+          if not (is_block_device filename) then (
+            (* It turns out that libguestfs's disk creation code is
+             * considerably more flexible and easier to use than
+             * qemu-img, so create the disk explicitly using libguestfs
+             * then pass the 'qemu-img convert -n' option so qemu reuses
+             * the disk.
+             *
+             * Also we allow the output mode to actually create the disk
+             * image.  This lets the output mode set ownership and
+             * permissions correctly if required.
+             *)
+            (* What output preallocation mode should we use? *)
+            let preallocation =
+              match t.target_format, cmdline.output_alloc with
+              | ("raw"|"qcow2"), Sparse -> Some "sparse"
+              | ("raw"|"qcow2"), Preallocated -> Some "full"
+              | _ -> None (* ignore -oa flag for other formats *) in
+            let compat =
+              match t.target_format with "qcow2" -> Some "1.1" | _ -> None in
+            output#disk_create filename t.target_format
+                               t.target_overlay.ov_virtual_size
+                               ?preallocation ?compat
+          )
 
        | TargetURI _ ->
           (* XXX For the moment we assume that qemu URI outputs