v2v: Check if there is sufficient free space for conversion (RHBZ#1139543).

Before trying to perform the conversion, estimate if there is
sufficient free space on the guest filesystem(s) for the conversion to
be successful.

If not, you will see an error similar to this one:

[   6.0] Checking for sufficient free disk space in the guest
virt-v2v: error: not enough free space for conversion on filesystem '/'.
4354048 bytes free < 20000000 bytes needed

v2v/v2v.ml

@@ -179,6 +179,10 @@ let rec main () =
   msg (f_"Inspecting the overlay");
   let inspect = inspect_source g root_choice in
 
+  (* Check there is enough free space to perform conversion. *)
+  msg (f_"Checking for sufficient free disk space in the guest");
+  check_free_space g;
+
   (* Conversion. *)
   let guestcaps =
     (match inspect.i_product_name with
@@ -381,4 +385,35 @@ and inspect_source g root_choice =
     i_apps = apps;
     i_apps_map = apps_map; }
 
+(* Conversion can fail if there is no space on the guest filesystems
+ * (RHBZ#1139543).  To avoid this situation, check there is some
+ * headroom.  Mainly we care about the root filesystem.
+ *)
+and check_free_space g =
+  let mps = g#mountpoints () in
+  List.iter (
+    fun (_, mp) ->
+      (* bfree = free blocks for root user *)
+      let { G.bfree = bfree; bsize = bsize } = g#statvfs mp in
+      let free_bytes = bfree *^ bsize in
+      let needed_bytes =
+        match mp with
+        | "/" ->
+          (* We may install some packages, and they would usually go
+           * on the root filesystem.
+           *)
+          20_000_000L
+        | "/boot" ->
+          (* We usually regenerate the initramfs, which has a typical size
+           * of 20-30MB.  Hence:
+           *)
+          50_000_000L
+        | _ ->
+          (* For everything else, just make sure there is some free space. *)
+          10_000_000L in
+      if free_bytes < needed_bytes then
+        error (f_"not enough free space for conversion on filesystem '%s'.  %Ld bytes free < %Ld bytes needed")
+          mp free_bytes needed_bytes
+  ) mps
+
 let () = run_main_and_handle_errors ~prog main