mllib: move which and its exception from dib

Rename it from "tool" to "executable" in the process, but otherwise
it is just code motion (with minimal adjustments in dib).

dib/dib.ml

@@ -297,7 +297,7 @@ $cmd \"$@\"
   (try
     let loc = which "dib-run-parts" in
     do_cp loc (destdir // "fake-bin")
-  with Tool_not_found _ ->
+  with Executable_not_found _ ->
     let fake_dib_run_parts = "\
 #!/bin/sh
 echo \"Please install dib-run-parts on the host\"

dib/utils.ml

@@ -21,8 +21,6 @@ open Common_utils
 
 open Printf
 
-exception Tool_not_found of string (* tool *)
-
 let quote = Filename.quote
 
 let unit_GB howmany =
@@ -97,21 +95,9 @@ let rec remove_dups = function
   | [] -> []
   | x :: xs -> x :: (remove_dups (List.filter ((<>) x) xs))
 
-let which tool =
-  let paths = String.nsplit ":" (Sys.getenv "PATH") in
-  let paths = filter_map (
-    fun p ->
-      let path = p // tool in
-      try Unix.access path [Unix.X_OK]; Some path
-      with Unix.Unix_error _ -> None
-  ) paths in
-  match paths with
-  | [] -> raise (Tool_not_found tool)
-  | x :: _ -> x
-
 let require_tool tool =
   try ignore (which tool)
-  with Tool_not_found tool ->
+  with Executable_not_found tool ->
     error (f_"%s needed but not found") tool
 
 let do_cp src destdir =

mllib/common_utils.ml

@@ -142,6 +142,8 @@ module String = struct
       )
 end
 
+exception Executable_not_found of string (* executable *)
+
 let (//) = Filename.concat
 
 let ( +^ ) = Int64.add
@@ -316,6 +318,18 @@ let protect ~f ~finally =
   finally ();
   match r with Either ret -> ret | Or exn -> raise exn
 
+let which executable =
+  let paths = String.nsplit ":" (Sys.getenv "PATH") in
+  let paths = filter_map (
+    fun p ->
+      let path = p // executable in
+      try Unix.access path [Unix.X_OK]; Some path
+      with Unix.Unix_error _ -> None
+  ) paths in
+  match paths with
+  | [] -> raise (Executable_not_found executable)
+  | x :: _ -> x
+
 (* Program name. *)
 let prog = Filename.basename Sys.executable_name
 

mllib/common_utils.mli

@@ -78,6 +78,10 @@ module String : sig
 end
 (** Override the String module from stdlib. *)
 
+(** Exception thrown by [which] when the specified executable is not found
+    in [$PATH]. *)
+exception Executable_not_found of string (* executable *)
+
 val ( // ) : string -> string -> string
 (** Concatenate directory and filename. *)
 
@@ -379,3 +383,8 @@ val inspect_mount_root_ro : Guestfs.guestfs -> string -> unit
 
 val is_btrfs_subvolume : Guestfs.guestfs -> string -> bool
 (** Checks if a filesystem is a btrfs subvolume. *)
+
+val which : string -> string
+(** Return the full path of the specified executable from [$PATH].
+
+    Throw [Executable_not_found] if not available. *)