[Libguestfs] [PATCH] v2v: -o null: support older qemu-img (RHBZ#1580309)

[Pino Toscano]

Commit 4699c7b6e126e07c95b67fb95df58aed87a680dd converted the null
output to use the null-co qemu driver with a JSON URL syntax --
especially the latter is only available in newer versions of qemu.

Even if this output mode is mostly for testing, check at runtime whether
the null-co + JSON way is possible, falling back to the creation of
thrown-away temporary files as before.
---
 v2v/output_null.ml | 60 +++++++++++++++++++++++++++++++++++++---------
 1 file changed, 49 insertions(+), 11 deletions(-)

diff --git a/v2v/output_null.ml b/v2v/output_null.ml
index b93d53dc5..5a5473c0d 100644
--- a/v2v/output_null.ml
+++ b/v2v/output_null.ml
@@ -42,7 +42,37 @@ open Utils
  * size instead.
  *)
 
+let qemu_supports_null_co_device () =
+  (* We actually attempt to convert a raw file to the null-co device. *)
+  let tmp = Filename.temp_file "v2vqemunullcotst" ".img" in
+  Unix.truncate tmp 1024;
+
+  let json = [
+    "file.driver", JSON.String "null-co";
+    "file.size", JSON.String "1E";
+  ] in
+
+  let cmd =
+    sprintf "qemu-img convert -n -f raw -O raw %s json:%s >/dev/null%s"
+            (quote tmp)
+            (quote (JSON.string_of_doc ~fmt:JSON.Compact json))
+            (if verbose () then "" else " 2>&1") in
+  debug "%s" cmd;
+  let r = 0 = Sys.command cmd in
+  Unix.unlink tmp;
+  debug "qemu-img supports the null-co device: %b" r;
+  r
+
 class output_null =
+  (* Create a temporary directory which is always deleted at exit,
+   * so we can put the drives there in case qemu does not support
+   * the null-co device.
+   *)
+  let tmpdir =
+    let base_dir = (open_guestfs ())#get_cachedir () in
+    let t = Mkdtemp.temp_dir ~base_dir "null." in
+    rmdir_on_exit t;
+    t in
 object
   inherit output
 
@@ -51,19 +81,27 @@ object
   method supported_firmware = [ TargetBIOS; TargetUEFI ]
 
   method prepare_targets source targets =
-    let json_params = [
-      "file.driver", JSON.String "null-co";
-      "file.size", JSON.String "1E";
-    ] in
-    let target_file = TargetURI ("json:" ^ JSON.string_of_doc json_params) in
+    if qemu_supports_null_co_device () then (
+      let json_params = [
+        "file.driver", JSON.String "null-co";
+        "file.size", JSON.String "1E";
+      ] in
+      let target_file = TargetURI ("json:" ^ JSON.string_of_doc json_params) in
 
-    (* While it's not intended that output drivers can set the
-     * target_format field (thus overriding the -of option), in
-     * this special case of -o null it is reasonable.
-     *)
-    let target_format = "raw" in
+      (* While it's not intended that output drivers can set the
+       * target_format field (thus overriding the -of option), in
+       * this special case of -o null it is reasonable.
+       *)
+      let target_format = "raw" in
 
-    List.map (fun t -> { t with target_file; target_format }) targets
+      List.map (fun t -> { t with target_file; target_format }) targets
+    ) else (
+      List.map (
+        fun t ->
+          let target_file = tmpdir // t.target_overlay.ov_sd in
+          { t with target_file = TargetFile target_file }
+      ) targets
+    )
 
   method create_metadata _ _ _ _ _ _ = ()
 end
-- 
2.17.0