xapi_message: Implement proper expression handling in get_all_records_where (xapi-project#6411)

Messages are not stored in the database, so their handling is done
through Custom_actions. The custom implementation of
get_all_records_where, in particular, used to ignore the query
expression altogether (as if expr = true).

Expand the existing handler to evaluate the query and match against the
messages retrieved and parsed from disk.

An alternative approach would have instead special cased filtering code
in the database itself, but in my attempts this was awkward and
duplicated quite a lot of code.

With this it is possible to filter messages properly now:
```
>>> session.xenapi.message.get_all_records_where('field "name" = "VM_STARTED"')
<list of messages with name = VM_STARTED>
>>> session.xenapi.message.get_all_records_where('field "name" = "VM_STARTED" and field "obj_uuid" = "3c61111f-1d67-7db4-95a5-f0287aff57bf"')
<list of messages with name = VM_STARTED and obj_uuid with such value>
```

Also update the documentation to match new behaviour.

Closes xapi-project#6340

Author: last-genius

ocaml/idl/datamodel.ml

@@ -8976,7 +8976,7 @@ module Message = struct
   let get_all_records_where =
     call ~name:"get_all_records_where"
       ~lifecycle:[(Published, rel_orlando, "")]
-      ~params:[(String, "expr", "The expression to match (not currently used)")]
+      ~params:[(String, "expr", "The expression to match")]
       ~flags:[`Session]
       ~result:(Map (Ref _message, Record _message), "The messages")
       ~allowed_roles:_R_READ_ONLY ()
@@ -10543,12 +10543,12 @@ let all_system =
 (**
    These are the pairs of (object, field) which are bound together in
    the database schema.
-   
+
    It is assumed that, for any entry (p, p'), neither p nor p'
    appears in any other entry. It may be the case that p = p', which
    is the only instance where some object-field pair may appear more
    than once.
-   
+
    This is implicitly assumed by other code which treats this list -
    and its symmetric closure - as an association list
    without duplicate keys. *)

ocaml/xapi/xapi_message.ml

@@ -730,8 +730,54 @@ let get_record ~__context ~self =
 
 let get_all_records ~__context = get_real message_dir (fun _ -> true) 0.0
 
-let get_all_records_where ~__context ~expr:_ =
-  get_real message_dir (fun _ -> true) 0.0
+let get_all_records_where ~__context ~expr =
+  let open Xapi_database in
+  let expr = Db_filter.expr_of_string expr in
+  let eval_val msg expr =
+    match expr with
+    | Db_filter_types.Literal x ->
+        x
+    | Db_filter_types.Field x -> (
+      match x with
+      | "name" ->
+          msg.API.message_name
+      | "uuid" ->
+          msg.API.message_uuid
+      | "priority" ->
+          Int64.to_string msg.API.message_priority
+      | "cls" ->
+          Record_util.cls_to_string msg.API.message_cls
+      | "obj_uuid" ->
+          msg.API.message_obj_uuid
+      | "timestamp" ->
+          Date.to_rfc3339 msg.API.message_timestamp
+      | "body" ->
+          msg.API.message_body
+      | any_other_key ->
+          raise (Db_exn.DBCache_NotFound ("missing field", any_other_key, ""))
+    )
+  in
+  let eval_expr (lookup_val : API.message_t -> Db_filter_types._val -> string)
+      (msg : API.message_t) =
+    let lookup_val = lookup_val msg in
+    let compare _a _b = lookup_val _a = lookup_val _b in
+    let rec f = function
+      | Db_filter_types.True ->
+          true
+      | Db_filter_types.False ->
+          false
+      | Db_filter_types.Not x ->
+          not (f x)
+      | Db_filter_types.And (a, b) ->
+          f a && f b
+      | Db_filter_types.Eq (_a, _b) ->
+          compare _a _b
+      | Db_filter_types.Or (a, b) ->
+          f a || f b
+    in
+    f expr
+  in
+  get_real message_dir (eval_expr eval_val) 0.0
 
 let repopulate_cache () =
   with_lock in_memory_cache_mutex (fun () ->