eio_main: make EIO_BACKEND handling more uniform

lib_eio_linux/eio_linux.ml

@@ -1318,15 +1318,18 @@ let with_uring ~queue_depth ?polling_timeout ?(fallback=no_fallback) fn =
   match Uring.create ~queue_depth ?polling_timeout () with
   | exception Unix.Unix_error(Unix.ENOSYS, _, _) -> fallback (`Msg "io_uring is not available on this system")
   | uring ->
-    match fn uring with
-    | x -> Uring.exit uring; x
-    | exception ex ->
-      let bt = Printexc.get_raw_backtrace () in
-      begin
-        try Uring.exit uring
-        with ex2 -> Log.warn (fun f -> f "Uring.exit failed (%a) while handling another error" Fmt.exn ex2)
-      end;
-      Printexc.raise_with_backtrace ex bt
+    let probe = Uring.get_probe uring in
+    if not (Uring.op_supported probe Uring.Op.shutdown) then
+      fallback (`Msg "Linux >= 5.11 is required for io_uring support")
+    else match fn uring with
+      | x -> Uring.exit uring; x
+      | exception ex ->
+        let bt = Printexc.get_raw_backtrace () in
+        begin
+          try Uring.exit uring
+          with ex2 -> Log.warn (fun f -> f "Uring.exit failed (%a) while handling another error" Fmt.exn ex2)
+        end;
+        Printexc.raise_with_backtrace ex bt
 
 let rec run : type a.
   ?queue_depth:int -> ?n_blocks:int -> ?block_size:int -> ?polling_timeout:int -> ?fallback:(_ -> a) -> (_ -> a) -> a =

lib_main/dune

@@ -1,7 +1,11 @@
 (library
   (name eio_main)
   (public_name eio_main)
-  (libraries eio_luv
-             (select eio_main.ml from
-              (eio_linux -> eio_main.linux.ml)
-              (          -> eio_main.default.ml))))
+  (libraries
+    (select linux_backend.ml from
+            (eio_linux -> linux_backend.enabled.ml)
+            (          -> linux_backend.disabled.ml))
+    (select luv_backend.ml from
+            (eio_luv -> luv_backend.enabled.ml)
+            (        -> luv_backend.disabled.ml))
+    ))

lib_main/eio_main.ml

@@ -0,0 +1,12 @@
+let force run fn =
+  run ~fallback:(fun (`Msg msg) -> failwith msg) fn
+
+let run fn =
+  match Sys.getenv_opt "EIO_BACKEND" with
+  | Some ("io-uring" | "linux") -> force Linux_backend.run fn
+  | Some "luv" -> force Luv_backend.run fn
+  | None | Some "" ->
+    Linux_backend.run fn ~fallback:(fun _ ->
+        force Luv_backend.run fn
+      )
+  | Some x -> Fmt.failwith "Unknown Eio backend %S (from $EIO_BACKEND)" x

lib_main/eio_main.mli

@@ -19,4 +19,4 @@ val run : (Eio.Stdenv.t -> 'a) -> 'a
 
     On recent-enough versions of Linux, it will use {!Eio_linux.run}.
     You can override this by setting the $EIO_BACKEND environment variable to
-    either "io-uring" or "luv". *)
+    either "linux" or "luv". *)

lib_main/linux_backend.disabled.ml

@@ -0,0 +1 @@
+let run ~fallback _ = fallback (`Msg "The io_uring backend was disabled at compile-time")

lib_main/linux_backend.enabled.ml

@@ -0,0 +1 @@
+let run ~fallback fn = Eio_linux.run ~fallback (fun env -> fn (env :> Eio.Stdenv.t))

lib_main/luv_backend.disabled.ml

@@ -0,0 +1 @@
+let run ~fallback _ = fallback (`Msg "The Luv backend was disabled at compile-time")

lib_main/luv_backend.enabled.ml

@@ -0,0 +1 @@
+let run ~fallback:_ fn = Eio_luv.run (fun env -> fn (env :> Eio.Stdenv.t))