[Libguestfs] [PATCH v2 3/4] common/mltools: allow fd for machine readable output

[Pino Toscano]

Allow to specify a file descriptor for the machine readable output.

Use the same assumption as done in v2v, i.e. that Unix.file_descr is
simply the int file descriptor.
---
 common/mltools/test-machine-readable.sh |  7 +++++++
 common/mltools/tools_utils.ml           | 11 ++++++++++-
 lib/guestfs.pod                         |  5 +++++
 3 files changed, 22 insertions(+), 1 deletion(-)

diff --git a/common/mltools/test-machine-readable.sh b/common/mltools/test-machine-readable.sh
index 1162c58e9..824460e6d 100755
--- a/common/mltools/test-machine-readable.sh
+++ b/common/mltools/test-machine-readable.sh
@@ -65,3 +65,10 @@ test $($t --machine-readable=stream:stdout |& wc -l) -eq 3
 # Output "stream:stderr".
 $t --machine-readable=stream:stderr 2>&1 >/dev/null | grep 'machine-readable'
 test $($t --machine-readable=stream:stderr 2>&1 >/dev/null | wc -l) -eq 2
+
+# Output "fd:".
+fn="$tmpdir/fdfile"
+exec 4>"$fn"
+$t --machine-readable=fd:4
+exec 4>&-
+test $(cat "$fn" | wc -l) -eq 1
diff --git a/common/mltools/tools_utils.ml b/common/mltools/tools_utils.ml
index ade4cb37f..35478f39e 100644
--- a/common/mltools/tools_utils.ml
+++ b/common/mltools/tools_utils.ml
@@ -41,6 +41,7 @@ type machine_readable_output_type =
   | NoOutput
   | Channel of out_channel
   | File of string
+  | Fd of int
 let machine_readable_output = ref NoOutput
 let machine_readable_channel = ref None
 let machine_readable () =
@@ -50,7 +51,10 @@ let machine_readable () =
         match !machine_readable_output with
         | NoOutput -> None
         | Channel chan -> Some chan
-        | File f -> Some (open_out f) in
+        | File f -> Some (open_out f)
+        | Fd fd ->
+          (* Note that Unix.file_descr is really just an int. *)
+          Some (Unix.out_channel_of_descr (Obj.magic fd)) in
       machine_readable_channel := chan
     );
     !machine_readable_channel
@@ -296,6 +300,11 @@ let create_standard_options argspec ?anon_fun ?(key_opts = false) ?(machine_read
           | n ->
             error (f_"invalid output stream for --machine-readable: %s") fmt in
         machine_readable_output := Channel chan
+      | "fd" ->
+        (try
+          machine_readable_output := Fd (int_of_string outname)
+        with Failure _ ->
+          error (f_"invalid output fd for --machine-readable: %s") fmt)
       | n ->
         error (f_"invalid output for --machine-readable: %s") fmt
       )
diff --git a/lib/guestfs.pod b/lib/guestfs.pod
index 53cece2da..f11028466 100644
--- a/lib/guestfs.pod
+++ b/lib/guestfs.pod
@@ -3287,6 +3287,11 @@ The possible values are:
 
 =over 4
 
+=item B<fd:>I<fd>
+
+The output goes to the specified I<fd>, which is a file descriptor
+already opened for writing.
+
 =item B<file:>F<filename>
 
 The output goes to the specified F<filename>.
-- 
2.20.1