add CCUnix.async_call for spawning and communicating with subprocess

src/data/CCMixtbl.mli

@@ -25,7 +25,7 @@ OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
 (** {1 Hash Table with Heterogeneous Keys}
 
-From https://github.com/mjambon/mixtbl , thanks to him.
+From https://github.com/mjambon/mixtbl (thanks to him).
 Example:
 
 {[

src/unix/CCUnix.ml

@@ -113,3 +113,36 @@ let call ?(bufsize=2048) ?(stdin=`Str "") ?(env=[||]) cmd =
         end
     )
 
+type line = string
+
+type async_call_result =
+  < stdout:line gen;
+    stderr:line gen;
+    stdin:line -> unit; (* send a line *)
+    close_in:unit; (* close stdin *)
+    close_err:unit;
+    close_out:unit;
+    wait:Unix.process_status;  (* block until the process ends *)
+  >
+
+let async_call ?(env=[||]) cmd =
+  (* render the command *)
+  let buf = Buffer.create 256 in
+  kbprintf' buf cmd
+    (fun buf ->
+       let cmd = Buffer.contents buf in
+       let oc, ic, errc = Unix.open_process_full cmd env in
+       object
+         method stdout () =
+           try Some (input_line oc)
+           with End_of_file -> None
+         method stderr () =
+           try Some (input_line errc)
+           with End_of_file -> None
+         method stdin l = output_string ic l; output_char ic '\n'
+         method close_in = close_out ic
+         method close_out = close_in oc
+         method close_err = close_in errc
+         method wait = Unix.close_process_full (oc, ic, errc)
+       end
+    )

src/unix/CCUnix.mli

@@ -73,5 +73,27 @@ val call : ?bufsize:int ->
   (call "echo %s" "a'b'c")#stdout = "abc\n"
 *)
 
+type line = string
+
+type async_call_result =
+  < stdout:line gen;
+    stderr:line gen;
+    stdin:line -> unit; (* send a line *)
+    close_in:unit; (* close stdin *)
+    close_err:unit;
+    close_out:unit;
+    wait:Unix.process_status;  (* block until the process ends *)
+  >
+(** A subprocess for interactive usage (read/write channels line by line)
+    @since NEXT_RELEASE *)
+
+val async_call : ?env:string array ->
+                 ('a, Buffer.t, unit, async_call_result) format4 ->
+                 'a
+(** Spawns a subprocess, like {!call}, but the subprocess's channels are
+    line generators and line sinks (for stdin).
+    if [p] is [async_call "cmd"], then [p#wait] waits for the subprocess
+    to die. Channels can be closed independently.
+    @since NEXT_RELEASE *)