feat(exec): spill long command lines into response files

lib/exec.ml

@@ -82,3 +82,39 @@ let which name =
     | Some path ->
         String.split_on_char ':' path
         |> List.exists (fun dir -> dir <> "" && Sys.file_exists (Filename.concat dir name))
+
+(* execve measures the arguments and the environment together against ARG_MAX,
+   so what a command line may use is the limit less the environment it inherits.
+   Staying under a fraction of that leaves room for the pointer table and for an
+   environment that grows between the check and the spawn. *)
+let arg_max =
+  lazy
+    (match capture [| "getconf"; "ARG_MAX" |] with
+    | 0, out -> ( match int_of_string_opt (String.trim out) with Some n when n > 0 -> n | _ -> 131072)
+    | _ -> 131072
+    | exception _ -> 131072)
+
+let entry_size s = String.length s + 1 + 8
+
+let budget =
+  lazy
+    (let env = Array.fold_left (fun n s -> n + entry_size s) 0 (Unix.environment ()) in
+     max 8192 ((Lazy.force arg_max - env) * 7 / 8))
+
+let too_long cmd = Array.fold_left (fun n s -> n + entry_size s) 0 cmd > Lazy.force budget
+
+(* A GNU response file holds one argument per line, quoted so that spaces and
+   backslashes in a path survive the second round of parsing. The compiler
+   drivers and binutils expand @file before they read anything else, so the tool
+   sees the same argument list either way. *)
+let quote_arg s =
+  let b = Buffer.create (String.length s + 2) in
+  Buffer.add_char b '"';
+  String.iter (fun c -> if c = '"' || c = '\\' then Buffer.add_char b '\\'; Buffer.add_char b c) s;
+  Buffer.add_char b '"';
+  Buffer.contents b
+
+let response path cmd =
+  let args = Array.to_list (Array.sub cmd 1 (Array.length cmd - 1)) in
+  Fs.write path (String.concat "\n" (List.map quote_arg args) ^ "\n");
+  [| cmd.(0); "@" ^ path |]