[tracing] CP-43769: instrument serialize_auth with a Named_mutex

Instrument 'serialize_auth' with a Named_mutex.
This function is a bottleneck both for succesful and failed authentication calls, and we need to be able to see it.

Eventually we should allow authentication to proceed in parallel, once we made our PAM library thread-safe:
PAM itself will apply throttling, currently both XAPI and PAM do this.
The throttling might still be useful for AD to prevent brute-force attacks.

Signed-off-by: Edwin Török <[email protected]>

ocaml/xapi/xapi_session.ml

@@ -255,7 +255,7 @@ let local_superuser = "root"
 
 let xapi_internal_originator = "xapi"
 
-let serialize_auth = Mutex.create ()
+let serialize_auth = Locking_helpers.Named_mutex.create "serialize_auth"
 
 let wipe_string_contents str =
   for i = 0 to Bytes.length str - 1 do
@@ -272,13 +272,13 @@ let wipe_params_after_fn params fn =
   with e -> wipe params ; raise e
 
 let do_external_auth ~__context uname pwd =
-  with_lock serialize_auth (fun () ->
+  Locking_helpers.Named_mutex.execute ~__context serialize_auth (fun () ->
       (Ext_auth.d ()).authenticate_username_password uname
         (Bytes.unsafe_to_string pwd)
   )
 
 let do_local_auth ~__context uname pwd =
-  with_lock serialize_auth (fun () ->
+  Locking_helpers.Named_mutex.execute ~__context serialize_auth (fun () ->
       try Pam.authenticate uname (Bytes.unsafe_to_string pwd)
       with Failure msg ->
         raise
@@ -288,7 +288,7 @@ let do_local_auth ~__context uname pwd =
   )
 
 let do_local_change_password ~__context uname newpwd =
-  with_lock serialize_auth (fun () ->
+  Locking_helpers.Named_mutex.execute ~__context serialize_auth (fun () ->
       Pam.change_password uname (Bytes.unsafe_to_string newpwd)
   )