diff --git a/sparsify/sparsify.ml b/sparsify/sparsify.ml
index b12440653..e79fe7896 100644
--- a/sparsify/sparsify.ml
+++ b/sparsify/sparsify.ml
@@ -31,68 +31,69 @@ external statvfs_free_space : string -> int64 =
 
 let () = Random.self_init ()
 
-(* Command line argument parsing. *)
 let prog = Filename.basename Sys.executable_name
 let error fs = error ~prog fs
 
-let indisk, outdisk, check_tmpdir, compress, convert, debug_gc,
-  format, ignores, machine_readable,
-  option, quiet, verbose, trace, zeroes =
-  let display_version () =
-    printf "virt-sparsify %s\n" Config.package_version;
-    exit 0
-  in
+let main () =
+  (* Command line argument parsing. *)
+  let indisk, outdisk, check_tmpdir, compress, convert, debug_gc,
+    format, ignores, machine_readable,
+    option, quiet, verbose, trace, zeroes =
+    let display_version () =
+      printf "virt-sparsify %s\n" Config.package_version;
+      exit 0
+    in
 
-  let add xs s = xs := s :: !xs in
+    let add xs s = xs := s :: !xs in
 
-  let check_tmpdir = ref `Warn in
-  let set_check_tmpdir = function
-    | "ignore" | "i" -> check_tmpdir := `Ignore
-    | "continue" | "cont" | "c" -> check_tmpdir := `Continue
-    | "warn" | "warning" | "w" -> check_tmpdir := `Warn
-    | "fail" | "f" | "error" -> check_tmpdir := `Fail
-    | str ->
-      eprintf (f_"--check-tmpdir: unknown argument `%s'\n") str;
-      exit 1
-  in
+    let check_tmpdir = ref `Warn in
+    let set_check_tmpdir = function
+      | "ignore" | "i" -> check_tmpdir := `Ignore
+      | "continue" | "cont" | "c" -> check_tmpdir := `Continue
+      | "warn" | "warning" | "w" -> check_tmpdir := `Warn
+      | "fail" | "f" | "error" -> check_tmpdir := `Fail
+      | str ->
+        eprintf (f_"--check-tmpdir: unknown argument `%s'\n") str;
+        exit 1
+    in
 
-  let compress = ref false in
-  let convert = ref "" in
-  let debug_gc = ref false in
-  let format = ref "" in
-  let ignores = ref [] in
-  let machine_readable = ref false in
-  let option = ref "" in
-  let quiet = ref false in
-  let verbose = ref false in
-  let trace = ref false in
-  let zeroes = ref [] in
+    let compress = ref false in
+    let convert = ref "" in
+    let debug_gc = ref false in
+    let format = ref "" in
+    let ignores = ref [] in
+    let machine_readable = ref false in
+    let option = ref "" in
+    let quiet = ref false in
+    let verbose = ref false in
+    let trace = ref false in
+    let zeroes = ref [] in
 
-  let ditto = " -\"-" in
-  let argspec = Arg.align [
-    "--check-tmpdir", Arg.String set_check_tmpdir,  "ignore|..." ^ " " ^ s_"Check there is enough space in $TMPDIR";
-    "--compress", Arg.Set compress,         " " ^ s_"Compressed output format";
-    "--convert", Arg.Set_string convert,    s_"format" ^ " " ^ s_"Format of output disk (default: same as input)";
-    "--debug-gc", Arg.Set debug_gc,         " " ^ s_"Debug GC and memory allocations";
-    "--format",  Arg.Set_string format,     s_"format" ^ " " ^ s_"Format of input disk";
-    "--ignore",  Arg.String (add ignores),  s_"fs" ^ " " ^ s_"Ignore filesystem";
-    "--long-options", Arg.Unit display_long_options, " " ^ s_"List long options";
-    "--machine-readable", Arg.Set machine_readable, " " ^ s_"Make output machine readable";
-    "-o",        Arg.Set_string option,     s_"option" ^ " " ^ s_"Add qemu-img options";
-    "-q",        Arg.Set quiet,             " " ^ s_"Quiet output";
-    "--quiet",   Arg.Set quiet,             ditto;
-    "-v",        Arg.Set verbose,           " " ^ s_"Enable debugging messages";
-    "--verbose", Arg.Set verbose,           ditto;
-    "-V",        Arg.Unit display_version,  " " ^ s_"Display version and exit";
-    "--version", Arg.Unit display_version,  ditto;
-    "-x",        Arg.Set trace,             " " ^ s_"Enable tracing of libguestfs calls";
-    "--zero",    Arg.String (add zeroes),   s_"fs" ^ " " ^ s_"Zero filesystem";
-  ] in
-  long_options := argspec;
-  let disks = ref [] in
-  let anon_fun s = disks := s :: !disks in
-  let usage_msg =
-    sprintf (f_"\
+    let ditto = " -\"-" in
+    let argspec = Arg.align [
+      "--check-tmpdir", Arg.String set_check_tmpdir,  "ignore|..." ^ " " ^ s_"Check there is enough space in $TMPDIR";
+      "--compress", Arg.Set compress,         " " ^ s_"Compressed output format";
+      "--convert", Arg.Set_string convert,    s_"format" ^ " " ^ s_"Format of output disk (default: same as input)";
+      "--debug-gc", Arg.Set debug_gc,         " " ^ s_"Debug GC and memory allocations";
+      "--format",  Arg.Set_string format,     s_"format" ^ " " ^ s_"Format of input disk";
+      "--ignore",  Arg.String (add ignores),  s_"fs" ^ " " ^ s_"Ignore filesystem";
+      "--long-options", Arg.Unit display_long_options, " " ^ s_"List long options";
+      "--machine-readable", Arg.Set machine_readable, " " ^ s_"Make output machine readable";
+      "-o",        Arg.Set_string option,     s_"option" ^ " " ^ s_"Add qemu-img options";
+      "-q",        Arg.Set quiet,             " " ^ s_"Quiet output";
+      "--quiet",   Arg.Set quiet,             ditto;
+      "-v",        Arg.Set verbose,           " " ^ s_"Enable debugging messages";
+      "--verbose", Arg.Set verbose,           ditto;
+      "-V",        Arg.Unit display_version,  " " ^ s_"Display version and exit";
+      "--version", Arg.Unit display_version,  ditto;
+      "-x",        Arg.Set trace,             " " ^ s_"Enable tracing of libguestfs calls";
+      "--zero",    Arg.String (add zeroes),   s_"fs" ^ " " ^ s_"Zero filesystem";
+    ] in
+    long_options := argspec;
+    let disks = ref [] in
+    let anon_fun s = disks := s :: !disks in
+    let usage_msg =
+      sprintf (f_"\
 %s: sparsify a virtual machine disk
 
  virt-sparsify [--options] indisk outdisk
@@ -100,118 +101,114 @@ let indisk, outdisk, check_tmpdir, compress, convert, debug_gc,
 A short summary of the options is given below.  For detailed help please
 read the man page virt-sparsify(1).
 ")
-      prog in
-  Arg.parse argspec anon_fun usage_msg;
+        prog in
+    Arg.parse argspec anon_fun usage_msg;
 
-  (* Dereference the rest of the args. *)
-  let check_tmpdir = !check_tmpdir in
-  let compress = !compress in
-  let convert = match !convert with "" -> None | str -> Some str in
-  let debug_gc = !debug_gc in
-  let format = match !format with "" -> None | str -> Some str in
-  let ignores = List.rev !ignores in
-  let machine_readable = !machine_readable in
-  let option = match !option with "" -> None | str -> Some str in
-  let quiet = !quiet in
-  let verbose = !verbose in
-  let trace = !trace in
-  let zeroes = List.rev !zeroes in
+    (* Dereference the rest of the args. *)
+    let check_tmpdir = !check_tmpdir in
+    let compress = !compress in
+    let convert = match !convert with "" -> None | str -> Some str in
+    let debug_gc = !debug_gc in
+    let format = match !format with "" -> None | str -> Some str in
+    let ignores = List.rev !ignores in
+    let machine_readable = !machine_readable in
+    let option = match !option with "" -> None | str -> Some str in
+    let quiet = !quiet in
+    let verbose = !verbose in
+    let trace = !trace in
+    let zeroes = List.rev !zeroes in
 
-  (* No arguments and machine-readable mode?  Print out some facts
-   * about what this binary supports.
-   *)
-  if !disks = [] && machine_readable then (
-    printf "virt-sparsify\n";
-    printf "linux-swap\n";
-    printf "zero\n";
-    printf "check-tmpdir\n";
-    let g = new G.guestfs () in
-    g#add_drive "/dev/null";
-    g#launch ();
-    if g#feature_available [| "ntfsprogs"; "ntfs3g" |] then
-      printf "ntfs\n";
-    if g#feature_available [| "btrfs" |] then
-      printf "btrfs\n";
-    exit 0
-  );
+    (* No arguments and machine-readable mode?  Print out some facts
+     * about what this binary supports.
+     *)
+    if !disks = [] && machine_readable then (
+      printf "virt-sparsify\n";
+      printf "linux-swap\n";
+      printf "zero\n";
+      printf "check-tmpdir\n";
+      let g = new G.guestfs () in
+      g#add_drive "/dev/null";
+      g#launch ();
+      if g#feature_available [| "ntfsprogs"; "ntfs3g" |] then
+        printf "ntfs\n";
+      if g#feature_available [| "btrfs" |] then
+        printf "btrfs\n";
+      exit 0
+    );
 
-  (* Verify we got exactly 2 disks. *)
-  let indisk, outdisk =
-    match List.rev !disks with
-    | [indisk; outdisk] -> indisk, outdisk
-    | _ ->
+    (* Verify we got exactly 2 disks. *)
+    let indisk, outdisk =
+      match List.rev !disks with
+      | [indisk; outdisk] -> indisk, outdisk
+      | _ ->
         error "usage is: %s [--options] indisk outdisk" prog in
 
-  (* Simple-minded check that the user isn't trying to use the
-   * same disk for input and output.
+    (* Simple-minded check that the user isn't trying to use the
+     * same disk for input and output.
+     *)
+    if indisk = outdisk then
+      error (f_"you cannot use the same disk image for input and output");
+
+    (* The input disk must be an absolute path, so we can store the name
+     * in the overlay disk.
+     *)
+    let indisk =
+      if not (Filename.is_relative indisk) then
+        indisk
+      else
+        Sys.getcwd () // indisk in
+
+    (* Check the output is not a block or char special (RHBZ#1056290). *)
+    if is_block_device outdisk then
+      error (f_"output '%s' cannot be a block device, it must be a regular file")
+        outdisk;
+
+    if is_char_device outdisk then
+      error (f_"output '%s' cannot be a character device, it must be a regular file")
+        outdisk;
+
+    indisk, outdisk, check_tmpdir, compress, convert,
+      debug_gc, format, ignores, machine_readable,
+      option, quiet, verbose, trace, zeroes in
+
+  (* Once we have got past argument parsing and start to create
+   * temporary files (including the potentially massive overlay file), we
+   * need to catch SIGINT (^C) and exit cleanly so the temporary file
+   * goes away.  Note that we don't delete temporaries in the signal
+   * handler.
    *)
-  if indisk = outdisk then
-    error (f_"you cannot use the same disk image for input and output");
-
-  (* The input disk must be an absolute path, so we can store the name
-   * in the overlay disk.
-   *)
-  let indisk =
-    if not (Filename.is_relative indisk) then
-      indisk
-    else
-      Sys.getcwd () // indisk in
-
-  (* Check the output is not a block or char special (RHBZ#1056290). *)
-  if is_block_device outdisk then
-    error (f_"output '%s' cannot be a block device, it must be a regular file")
-      outdisk;
-
-  if is_char_device outdisk then
-    error (f_"output '%s' cannot be a character device, it must be a regular file")
-      outdisk;
-
-  indisk, outdisk, check_tmpdir, compress, convert,
-    debug_gc, format, ignores, machine_readable,
-    option, quiet, verbose, trace, zeroes
-
-(* Once we have got past argument parsing and start to create
- * temporary files (including the potentially massive overlay file), we
- * need to catch SIGINT (^C) and exit cleanly so the temporary file
- * goes away.  Note that we don't delete temporaries in the signal
- * handler.
- *)
-let () =
   let do_sigint _ = exit 1 in
-  Sys.set_signal Sys.sigint (Sys.Signal_handle do_sigint)
+  Sys.set_signal Sys.sigint (Sys.Signal_handle do_sigint);
 
-(* What should the output format be?  If the user specified an
- * input format, use that, else detect it from the source image.
- *)
-let output_format =
-  match convert with
-  | Some fmt -> fmt             (* user specified output conversion *)
-  | None ->
-    match format with
-    | Some fmt -> fmt           (* user specified input format, use that *)
+  (* What should the output format be?  If the user specified an
+   * input format, use that, else detect it from the source image.
+   *)
+  let output_format =
+    match convert with
+    | Some fmt -> fmt           (* user specified output conversion *)
     | None ->
-      (* Don't know, so we must autodetect. *)
-      match (new G.guestfs ())#disk_format indisk  with
-      | "unknown" ->
-        error (f_"cannot detect input disk format; use the --format parameter")
-      | fmt -> fmt
+      match format with
+      | Some fmt -> fmt    (* user specified input format, use that *)
+      | None ->
+        (* Don't know, so we must autodetect. *)
+        match (new G.guestfs ())#disk_format indisk  with
+        | "unknown" ->
+          error (f_"cannot detect input disk format; use the --format parameter")
+        | fmt -> fmt in
 
-(* Compression is not supported by raw output (RHBZ#852194). *)
-let () =
+  (* Compression is not supported by raw output (RHBZ#852194). *)
   if output_format = "raw" && compress then
-    error (f_"--compress cannot be used for raw output.  Remove this option or use --convert qcow2.")
+    error (f_"--compress cannot be used for raw output.  Remove this option or use --convert qcow2.");
 
-(* Get virtual size of the input disk. *)
-let virtual_size = (new G.guestfs ())#disk_virtual_size indisk
-let () =
+  (* Get virtual size of the input disk. *)
+  let virtual_size = (new G.guestfs ())#disk_virtual_size indisk in
   if not quiet then
     printf (f_"Input disk virtual size = %Ld bytes (%s)\n%!")
-      virtual_size (human_size virtual_size)
+      virtual_size (human_size virtual_size);
 
-(* Check there is enough space in $TMPDIR. *)
-let tmpdir = Filename.temp_dir_name
+  (* Check there is enough space in $TMPDIR. *)
+  let tmpdir = Filename.temp_dir_name in
 
-let () =
   let print_warning () =
     let free_space = statvfs_free_space tmpdir in
     let extra_needed = virtual_size -^ free_space in
@@ -236,7 +233,7 @@ You can ignore this warning or change it to a hard failure using the
     ) else false
   in
 
-  match check_tmpdir with
+  (match check_tmpdir with
   | `Ignore -> ()
   | `Continue -> ignore (print_warning ())
   | `Warn ->
@@ -249,57 +246,54 @@ You can ignore this warning or change it to a hard failure using the
       eprintf "Exiting because --check-tmpdir=fail was set.\n%!";
       exit 2
     )
+  );
 
-let () =
   if not quiet then
-    printf (f_"Create overlay file in %s to protect source disk ...\n%!") tmpdir
+    printf (f_"Create overlay file in %s to protect source disk ...\n%!") tmpdir;
 
-(* Create the temporary overlay file. *)
-let overlaydisk =
-  let tmp = Filename.temp_file "sparsify" ".qcow2" in
-  unlink_on_exit tmp;
+  (* Create the temporary overlay file. *)
+  let overlaydisk =
+    let tmp = Filename.temp_file "sparsify" ".qcow2" in
+    unlink_on_exit tmp;
 
-  (* Create it with the indisk as the backing file. *)
-  (* XXX Old code used to:
-   * - detect if compat=1.1 was supported
-   * - add lazy_refcounts option
-   *)
-  (new G.guestfs ())#disk_create
-    ~backingfile:indisk ?backingformat:format ~compat:"1.1"
-    tmp "qcow2" Int64.minus_one;
+    (* Create it with the indisk as the backing file. *)
+    (* XXX Old code used to:
+     * - detect if compat=1.1 was supported
+     * - add lazy_refcounts option
+     *)
+    (new G.guestfs ())#disk_create
+      ~backingfile:indisk ?backingformat:format ~compat:"1.1"
+      tmp "qcow2" Int64.minus_one;
 
-  tmp
+    tmp in
 
-let () =
   if not quiet then
-    printf (f_"Examine source disk ...\n%!")
+    printf (f_"Examine source disk ...\n%!");
 
-(* Connect to libguestfs. *)
-let g =
-  let g = new G.guestfs () in
-  if trace then g#set_trace true;
-  if verbose then g#set_verbose true;
+  (* Connect to libguestfs. *)
+  let g =
+    let g = new G.guestfs () in
+    if trace then g#set_trace true;
+    if verbose then g#set_verbose true;
 
-  (* Note that the temporary overlay disk is always qcow2 format. *)
-  g#add_drive ~format:"qcow2" ~readonly:false ~cachemode:"unsafe" overlaydisk;
+    (* Note that the temporary overlay disk is always qcow2 format. *)
+    g#add_drive ~format:"qcow2" ~readonly:false ~cachemode:"unsafe" overlaydisk;
 
-  if not quiet then Progress.set_up_progress_bar ~machine_readable g;
-  g#launch ();
+    if not quiet then Progress.set_up_progress_bar ~machine_readable g;
+    g#launch ();
 
-  g
+    g in
 
-(* Modify SIGINT handler (set first above) to cancel the handle. *)
-let () =
+  (* Modify SIGINT handler (set first above) to cancel the handle. *)
   let do_sigint _ =
     g#user_cancel ();
     exit 1
   in
-  Sys.set_signal Sys.sigint (Sys.Signal_handle do_sigint)
+  Sys.set_signal Sys.sigint (Sys.Signal_handle do_sigint);
 
-(* Write zeroes for non-ignored filesystems that we are able to mount,
- * and selected swap partitions.
- *)
-let () =
+  (* Write zeroes for non-ignored filesystems that we are able to mount,
+   * and selected swap partitions.
+   *)
   let filesystems = g#list_filesystems () in
   let filesystems = List.map fst filesystems in
   let filesystems = List.sort compare filesystems in
@@ -356,10 +350,9 @@ let () =
 
         g#umount_all ()
       )
-  ) filesystems
+  ) filesystems;
 
-(* Fill unused space in volume groups. *)
-let () =
+  (* Fill unused space in volume groups. *)
   let vgs = g#vgs () in
   let vgs = Array.to_list vgs in
   let vgs = List.sort compare vgs in
@@ -382,22 +375,19 @@ let () =
           g#lvremove lvdev
         )
       )
-  ) vgs
+  ) vgs;
 
-(* Don't need libguestfs now. *)
-let () =
+  (* Don't need libguestfs now. *)
   g#shutdown ();
-  g#close ()
+  g#close ();
 
-(* Modify SIGINT handler (set first above) to just exit. *)
-let () =
+  (* Modify SIGINT handler (set first above) to just exit. *)
   let do_sigint _ = exit 1 in
-  Sys.set_signal Sys.sigint (Sys.Signal_handle do_sigint)
+  Sys.set_signal Sys.sigint (Sys.Signal_handle do_sigint);
 
-(* Now run qemu-img convert which copies the overlay to the
- * destination and automatically does sparsification.
- *)
-let () =
+  (* Now run qemu-img convert which copies the overlay to the
+   * destination and automatically does sparsification.
+   *)
   if not quiet then
     printf (f_"Copy to destination and make sparse ...\n%!");
 
@@ -412,16 +402,42 @@ let () =
   if verbose then
     printf "%s\n%!" cmd;
   if Sys.command cmd <> 0 then
-    error (f_"external command failed: %s") cmd
+    error (f_"external command failed: %s") cmd;
 
-(* Finished. *)
-let () =
+  (* Finished. *)
   if not quiet then (
     print_newline ();
     wrap (s_"Sparsify operation completed with no errors.  Before deleting the old disk, carefully check that the target disk boots and works correctly.\n");
   );
 
   if debug_gc then
-    Gc.compact ();
+    Gc.compact ()
 
-  exit 0
+let () =
+  try main ()
+  with
+  | Unix.Unix_error (code, fname, "") -> (* from a syscall *)
+    eprintf (f_"%s: error: %s: %s\n") prog fname (Unix.error_message code);
+    exit 1
+  | Unix.Unix_error (code, fname, param) -> (* from a syscall *)
+    eprintf (f_"%s: error: %s: %s: %s\n") prog fname (Unix.error_message code)
+      param;
+    exit 1
+  | G.Error msg ->                      (* from libguestfs *)
+    eprintf (f_"%s: libguestfs error: %s\n") prog msg;
+    exit 1
+  | Failure msg ->                      (* from failwith/failwithf *)
+    eprintf (f_"%s: failure: %s\n") prog msg;
+    exit 1
+  | Invalid_argument msg ->             (* probably should never happen *)
+    eprintf (f_"%s: internal error: invalid argument: %s\n") prog msg;
+    exit 1
+  | Assert_failure (file, line, char) -> (* should never happen *)
+    eprintf (f_"%s: internal error: assertion failed at %s, line %d, char %d\n") prog file line char;
+    exit 1
+  | Not_found ->                        (* should never happen *)
+    eprintf (f_"%s: internal error: Not_found exception was thrown\n") prog;
+    exit 1
+  | exn ->                              (* something not matched above *)
+    eprintf (f_"%s: exception: %s\n") prog (Printexc.to_string exn);
+    exit 1