[Libguestfs] [v2v PATCH] RHV outputs: limit copied disk count to 23

[Laszlo Ersek]

We currently support virtio-blk (commonly) or IDE (unusually) for exposing
disks to the converted guest; refer to "guestcaps.gcaps_block_bus" in
"lib/create_ovf.ml". When using virtio-blk (i.e., in the common case), RHV
can deal with at most 23 disks, as it plugs each virtio-blk device in a
separate slot on the PCI(e) root bus; and the other slots are reserved for
various purposes. When a domain has too many disks, the problem only
becomes apparent once the copying finishes and an import is attempted.
Modify the RHV outputs to fail relatively early when a domain has more
than 23 disks that need to be copied.

Notes:

- With IDE, the theoretical limit may even be as low as 4. However, in the
  "Output_module.setup" function, we don't have access to
  "guestcaps.gcaps_block_bus", and in practice the IDE limitation has not
  caused surprises. So for now stick with 23, assuming virtio-blk.
  Modifying the "Output_module.setup" parameter list just for this seems
  overkill.

- We could move the new check to an even earlier step, namely
  "Output_module.parse_options", due to the v2v directory deliberately
  existing (and having been populated with input sockets) at that time.
  However, even discounting the fact that "parse_options" is not a good
  name for including this kind of step, "parse_options" does not have
  access to the v2v directory name, and modifying the signature just for
  this is (again) overkill.

- By adding the check to "Output_module.setup", we waste *some* effort
  (namely, the conversion occurs between "parse_options" and "setup"),
  but: (a) the "rhv-disk-uuid" count check (against the disk count) is
  already being done in the rhv-upload module's "setup" function, (b) in
  practice the slowest step ought to be the copying, and placing the new
  check in "setup" is early enough to prevent that.

Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2051564
Signed-off-by: Laszlo Ersek <lersek at redhat.com>
---

Notes:
    This patch can be tested easily by replacing 23 with 0 in all three
    affected output modules -- then the following test cases all fail in
    "make check":
    
    FAIL: test-v2v-o-rhv.sh
    FAIL: test-v2v-o-vdsm-options.sh
    FAIL: test-v2v-o-rhv-upload.sh
    
    > [  11.3] Setting up the destination: -o rhv
    > virt-v2v: error: this output module doesn't support copying more than 0 disks
    
    > [   9.0] Setting up the destination: -o vdsm
    > virt-v2v: error: this output module doesn't support copying more than 0 disks
    
    > [  11.2] Setting up the destination: -o rhv-upload -oc https://example.com/ovirt-engine/api -os Storage
    > virt-v2v: error: this output module doesn't support copying more than 0 disks

 output/output.mli           | 7 +++++++
 output/output.ml            | 5 +++++
 output/output_rhv.ml        | 1 +
 output/output_rhv_upload.ml | 1 +
 output/output_vdsm.ml       | 1 +
 5 files changed, 15 insertions(+)

diff --git a/output/output.mli b/output/output.mli
index 533a0c51d31c..2dec8ccdc690 100644
--- a/output/output.mli
+++ b/output/output.mli
@@ -76,6 +76,13 @@ val get_disks : string -> (int * int64) list
 (** Examines the v2v directory and opens each input socket (in0 etc),
     returning a list of input disk index and size. *)
 
+val bail_if_disk_count_gt : string -> int -> unit
+(** This function lets an output module enforce a maximum disk count.
+    [bail_if_disk_count_gt dir n] checks whether the domain has more than [n]
+    disks that need to be copied, by examining the existence of input NBD socket
+    "in[n]" in the v2v directory [dir].  If the socket exists, [error] is
+    called. *)
+
 val output_to_local_file : ?changeuid:((unit -> unit) -> unit) ->
                            Types.output_allocation ->
                            string -> string -> int64 -> string ->
diff --git a/output/output.ml b/output/output.ml
index 10e685c46926..0c4b437997d4 100644
--- a/output/output.ml
+++ b/output/output.ml
@@ -64,6 +64,11 @@ let get_disks dir =
   in
   loop [] 0
 
+let bail_if_disk_count_gt dir n =
+  let socket = sprintf "%s/in%d" dir n in
+  if Sys.file_exists socket then
+    error (f_"this output module doesn't support copying more than %d disks") n
+
 let output_to_local_file ?(changeuid = fun f -> f ())
       output_alloc output_format filename size socket =
   (* Check nbdkit is installed and has the required plugin. *)
diff --git a/output/output_rhv.ml b/output/output_rhv.ml
index 119207fdc065..a0c0be270755 100644
--- a/output/output_rhv.ml
+++ b/output/output_rhv.ml
@@ -56,6 +56,7 @@ module RHV = struct
     (options.output_alloc, options.output_format, output_name, output_storage)
 
   let rec setup dir options source =
+    bail_if_disk_count_gt dir 23;
     let disks = get_disks dir in
     let output_alloc, output_format, output_name, output_storage = options in
 
diff --git a/output/output_rhv_upload.ml b/output/output_rhv_upload.ml
index 828996b36261..6a9abf4eecdf 100644
--- a/output/output_rhv_upload.ml
+++ b/output/output_rhv_upload.ml
@@ -133,6 +133,7 @@ after their uploads (if you do, you must supply one for each disk):
     else PCRE.matches (Lazy.force rex_uuid) uuid
 
   let rec setup dir options source =
+    bail_if_disk_count_gt dir 23;
     let disks = get_disks dir in
     let output_conn, output_format,
         output_password, output_name, output_storage,
diff --git a/output/output_vdsm.ml b/output/output_vdsm.ml
index a1e8c2465810..02a2b5817fbc 100644
--- a/output/output_vdsm.ml
+++ b/output/output_vdsm.ml
@@ -119,6 +119,7 @@ For each disk you must supply one of each of these options:
      compat, ovf_flavour)
 
   let setup dir options source =
+    bail_if_disk_count_gt dir 23;
     let disks = get_disks dir in
     let output_alloc, output_format,
         output_name, output_storage,
-- 
2.19.1.3.g30247aa5d201