[Libguestfs] [PATCH] v2v: ova: Don't rely on qemu-img version, test "offset" and "size" features.

[Richard W.M. Jones]

See:

https://www.redhat.com/archives/libguestfs/2017-February/msg00064.html
---
 v2v/input_ova.ml |  5 ++---
 v2v/utils.ml     | 51 +++++++++++++++++++++++++++++++--------------------
 v2v/utils.mli    |  7 +++----
 3 files changed, 36 insertions(+), 27 deletions(-)

diff --git a/v2v/input_ova.ml b/v2v/input_ova.ml
index 3c11cd0..3beaffc 100644
--- a/v2v/input_ova.ml
+++ b/v2v/input_ova.ml
@@ -90,9 +90,8 @@ object
         match detect_file_type ova with
         | `Tar ->
           (* Normal ovas are tar file (not compressed). *)
-          let qmajor, qminor = qemu_img_version () in
-          if qmajor > 2 || (qmajor == 2 && qminor >= 8) then (
-            (* If QEMU is 2.8 or newer we don't have to extract everything.
+          if qemu_img_supports_offset_and_size () then (
+            (* In newe QEMU we don't have to extract everything.
              * We can access disks inside the tar archive directly.
              *)
             untar_metadata ova tmpdir;
diff --git a/v2v/utils.ml b/v2v/utils.ml
index 6a3074e..09d7af5 100644
--- a/v2v/utils.ml
+++ b/v2v/utils.ml
@@ -91,26 +91,37 @@ let du filename =
   | line::_ -> Int64.of_string line
   | [] -> invalid_arg filename
 
-let qemu_img_version () =
-  let lines = external_command "qemu-img --version" in
-  match lines with
-  | [] -> error (f_"'qemu-img --version' returned no output")
-  | line :: _ ->
-      let rex = Str.regexp
-        "qemu-img version \\([0-9]+\\)\\.\\([0-9]+\\)" in
-      if Str.string_match rex line 0 then (
-        try
-          int_of_string (Str.matched_group 1 line),
-          int_of_string (Str.matched_group 2 line)
-        with Failure _ ->
-          warning (f_"failed to parse qemu-img version(%S), assuming 0.9")
-            line;
-          0, 9
-      ) else (
-        warning (f_"failed to read qemu-img version(%S), assuming 0.9")
-          line;
-        0, 9
-      )
+let qemu_img_supports_offset_and_size () =
+  (* We actually attempt to create a qcow2 file with a raw backing
+   * file that has an offset and size.
+   *)
+  let tmp1 = Filename.temp_file "v2vqemuimgtst" ".img"
+  and tmp2 = Filename.temp_file "v2vqemuimgtst" ".img" in
+  Unix.truncate tmp2 1024;
+
+  let json = [
+      "file", JSON.Dict [
+        "driver", JSON.String "raw";
+        "offset", JSON.Int 512;
+        "size", JSON.Int 512;
+        "file", JSON.Dict [
+          "filename", JSON.String tmp2
+        ]
+      ]
+  ] in
+
+  let cmd =
+    sprintf "qemu-img create %s -f qcow2 -b 'json:%s'%s"
+            (quote tmp1)
+            (quote (JSON.string_of_doc ~fmt:JSON.Compact json))
+            (if verbose () then "" else " >/dev/null 2>&1") in
+
+  let r = Sys.command cmd in
+
+  Unix.unlink tmp1;
+  Unix.unlink tmp2;
+
+  r = 0
 
 let find_file_in_tar tar filename =
   let lines = external_command (sprintf "tar tRvf %s" (Filename.quote tar)) in
diff --git a/v2v/utils.mli b/v2v/utils.mli
index eb47d03..b75baa7 100644
--- a/v2v/utils.mli
+++ b/v2v/utils.mli
@@ -51,10 +51,9 @@ val du : string -> int64
     This can raise either [Failure] or [Invalid_argument] in case
     of errors. *)
 
-val qemu_img_version : unit -> int * int
-(** Returns version of qemu-img as a tuple [(major, minor)].
-
-    In case of error [(0,9)] is returned. *)
+val qemu_img_supports_offset_and_size : unit -> bool
+(** Return true iff [qemu-img] supports the ["offset"] and ["size"]
+    parameters to open a subset of a file. *)
 
 val find_file_in_tar : string -> string -> int64 * int64
 (** [find_file_in_tar tar filename] looks up file in [tar] archive and returns
-- 
2.10.2