output: Report space on the target filesystem

For output modes which use the common 'create_local_output_disks'
function (those that produce '*-sda' files), report how much space is
available on the filesystem hosting that output file.  This is because
we've had failures where there wasn't enough space on the target
filesystem, so this just collects the information ahead of time in
case we see those kinds of failures again.

Thanks: Martin Necas, Alex Kalenyuk

Author: rwmjones

output/output.ml

@@ -177,7 +177,17 @@ let create_local_output_disks dir
       | Sparse -> None in
     List.iter (
       fun (size, filename) ->
-        g#disk_create ?preallocation filename output_format size
+        g#disk_create ?preallocation filename output_format size;
+
+        (* We've had issues with there not being enough space to write
+         * the disk image.  Run df on the output filename.  df follows
+         * symlinks and reports the space on the filesystem.  But don't
+         * fail here if df cannot be run.
+         *)
+        if verbose () then (
+          let cmd = sprintf "df %s 1>&2" (quote filename) in
+          ignore (Sys.command cmd)
+        )
     ) (List.combine input_sizes output_disk_names);
 
     let socket = sprintf "%s/out0" dir in