Don't pollute the logs with some errors (#6518)

It's an in-house joke that most of `xapi`'s errors are "expected"
(either because they're handled by the caller after being logged or
because they are frequent and benign) and knowing which ones are unusual
and actually critical takes a lot of learning. Try to remove at least
some of the most obvious ones from the toolstack startup/VM resume,
either removing the logs completely or downgrading it to `debug` when
appropriate.

Manually tested in the appropriate scenarios.

Author: last-genius

ocaml/libs/xapi-stdext/lib/xapi-stdext-unix/unixext.ml

@@ -17,6 +17,11 @@ exception Unix_error of int
 
 let _exit = Unix._exit
 
+let raise_with_preserved_backtrace exn f =
+  let bt = Printexc.get_raw_backtrace () in
+  f () ;
+  Printexc.raise_with_backtrace exn bt
+
 (** remove a file, but doesn't raise an exception if the file is already removed *)
 let unlink_safe file =
   try Unix.unlink file with (* Unix.Unix_error (Unix.ENOENT, _ , _)*) _ -> ()

ocaml/libs/xapi-stdext/lib/xapi-stdext-unix/unixext.mli

@@ -15,6 +15,10 @@
 
 val _exit : int -> unit
 
+val raise_with_preserved_backtrace : exn -> (unit -> unit) -> 'b
+(** A wrapper that preserves the backtrace (otherwise erased by calling
+    formatting functions, for example) *)
+
 val unlink_safe : string -> unit
 
 val mkdir_safe : string -> Unix.file_perm -> unit

ocaml/networkd/lib/network_utils.ml

@@ -162,7 +162,8 @@ module Sysfs = struct
     with
     | End_of_file ->
         ""
-    | Unix.Unix_error (Unix.EINVAL, _, _) ->
+    | Unix.Unix_error (Unix.EINVAL, _, _) | Unix.Unix_error (Unix.ENOENT, _, _)
+      ->
         (* The device is not yet up *)
         raise (Network_error (Read_error file))
     | exn ->

ocaml/xapi/xapi_vgpu_type.ml

@@ -1033,7 +1033,9 @@ module Nvidia_compat = struct
           read_configs ac tl
       )
     in
-    let conf_files = Array.to_list (Sys.readdir conf_dir) in
+    let conf_files =
+      try Array.to_list (Sys.readdir conf_dir) with Sys_error _ -> []
+    in
     debug "Reading NVIDIA vGPU config files %s/{%s}" conf_dir
       (String.concat ", " conf_files) ;
     read_configs []

ocaml/xenopsd/xc/xenguestHelper.ml

@@ -200,13 +200,14 @@ let rec non_debug_receive ?(debug_callback = fun s -> debug "%s" s) cnx =
 
 (* Dump memory statistics on failure *)
 let non_debug_receive ?debug_callback cnx =
-  let debug_memory () =
+  let debug_memory log_type =
     Xenctrl.with_intf (fun xc ->
         let open Memory in
         let open Int64 in
         let open Xenctrl in
         let p = Xenctrl.physinfo xc in
-        error "Memory F %Ld KiB S %Ld KiB T %Ld MiB"
+        (match log_type with Syslog.Debug -> debug | _ -> error)
+          "Memory F %Ld KiB S %Ld KiB T %Ld MiB"
           (p.free_pages |> of_nativeint |> kib_of_pages)
           (p.scrub_pages |> of_nativeint |> kib_of_pages)
           (p.total_pages |> of_nativeint |> mib_of_pages_free)
@@ -215,10 +216,18 @@ let non_debug_receive ?debug_callback cnx =
   try
     match non_debug_receive ?debug_callback cnx with
     | Error y as x ->
-        error "Received: %s" y ; debug_memory () ; x
+        error "Received: %s" y ; debug_memory Syslog.Err ; x
     | x ->
         x
-  with e -> debug_memory () ; raise e
+  with
+  | End_of_file as e ->
+      Unixext.raise_with_preserved_backtrace e (fun () ->
+          debug_memory Syslog.Debug
+      )
+  | e ->
+      Unixext.raise_with_preserved_backtrace e (fun () ->
+          debug_memory Syslog.Err
+      )
 
 (** For the simple case where we just want the successful result, return it. If
     we get an error message (or suspend) then throw an exception. *)