Merge connect_from_config into connect with optional arguments

Existing code which calls `Block.connect path` will still work.

New code can now request different behaviour by supplying the optional
arguments ?buffered and ?sync.

This patch also renames `get_config` to `to_config` so we also have:

  val to_config: t -> Config.t
  (** [to_config t] returns the configuration of a device *)

  val of_config: Config.t -> [ `Ok of t | `Error of error ] io
  (** [of_config config] creates a fresh device from [config] *)

Signed-off-by: David Scott <dave@recoil.org>

lib/block.ml

@@ -100,7 +100,7 @@ type t = {
   use_fsync_on_flush: bool;
 }
 
-let get_config { config } = config
+let to_config { config } = config
 
 module Result = struct
   type ('a, 'b) result = [
@@ -138,7 +138,7 @@ let get_file_size filename fd =
       (`Unknown
          (Printf.sprintf "get_file_size %s: neither a file nor a block device" filename))
 
-let connect_from_config ({ Config.buffered; sync; path } as config) =
+let of_config ({ Config.buffered; sync; path } as config) =
   let openfile, use_fsync_after_write = match buffered, is_win32 with
     | true, _ -> Raw.openfile_buffered, false
     | false, false -> Raw.openfile_unbuffered, false
@@ -178,10 +178,12 @@ let remove_prefix prefix x =
   then true, String.sub x prefix' (x' - prefix')
   else false, x
 
-let connect name =
-  let buffered, path = remove_prefix buffered_prefix name in
-  let config = { Config.buffered; sync = false; path } in
-  connect_from_config config
+let connect ?buffered ?sync name =
+  let legacy_buffered, path = remove_prefix buffered_prefix name in
+  (* Keep support for the legacy buffered: prefix until version 3.x.y *)
+  let buffered = if legacy_buffered then Some true else buffered in
+  let config = Config.create ?buffered ?sync name in
+  of_config config
 
 let disconnect t = match t.fd with
   | Some fd ->

lib/block.mli

@@ -32,8 +32,6 @@ val blkgetsize: string -> Unix.file_descr -> [ `Ok of int64 | `Error of error ]
     given by [fd]. [path] is only used to construct a human-readable error
     message. *)
 
-val connect : string -> [`Ok of t | `Error of error] io
-
 module Config: sig
   type t = {
     buffered: bool; (** true if I/O hits the OS disk caches, false if "direct" *)
@@ -54,9 +52,11 @@ module Config: sig
   (** Parse the result of a previous [to_string] invocation *)
 end
 
-val connect_from_config : Config.t -> [`Ok of t | `Error of error] io
-(** [connect_from_config config] connects to the block device described by
-    [config]. *)
+val connect : ?buffered:bool -> ?sync:bool -> string -> [`Ok of t | `Error of error] io
+(** [connect ?buffered ?sync path] connects to a block device on the filesystem
+    at [path]. By default I/O is unbuffered and fully synchronous. These defaults
+    can be changed by supplying the optional arguments [~buffered:true] and
+    [~sync:false] *)
 
 val resize : t -> int64 -> [ `Ok of unit | `Error of error ] io
 (** [resize t new_size_sectors] attempts to resize the connected device
@@ -76,5 +76,8 @@ val seek_mapped: t -> int64 -> [ `Ok of int64 | `Error of error ] io
     device which may have data in it (typically this is the next mapped
     region) *)
 
-val get_config: t -> Config.t
-(** [get_config t] returns the configuration of a device *)
+val to_config: t -> Config.t
+(** [to_config t] returns the configuration of a device *)
+
+val of_config: Config.t -> [ `Ok of t | `Error of error ] io
+(** [of_config config] creates a fresh device from [config] *)