xapi_message: Fix incorrect slow path invocation (and its logs) (xapi…

…-project#6147)

`get_since_for_events` checks if the cache contains entries older than
the 'since' timepoint requested. If so, this means the cache contains
all entries strictly after (`>`) 'since'. If not, this means the cache
needs to be reconstructed, since some entries strictly after (`>`)
'since' might be just out of cache.

Previous code would reconstruct the cache when the oldest entry in the
cache was equal to the 'since' timepoint requested, which is incorrect
and caused a deluge of "x is older than x itself" in the logs.

So, change the check for 'entries older than the since timestamp' to be
less-or-equal (as the opposite of 'strictly after'), and clarify the
debug message.

ocaml/xapi/xapi_message.ml

@@ -647,7 +647,7 @@ let get_since_for_events ~__context since =
   let cached_result =
     with_lock in_memory_cache_mutex (fun () ->
         match List.rev !in_memory_cache with
-        | (last_in_memory, _, _) :: _ when last_in_memory < since ->
+        | (oldest_in_memory, _, _) :: _ when oldest_in_memory <= since ->
             Some
               (List.filter_map
                  (fun (gen, _ref, msg) ->
@@ -658,11 +658,11 @@ let get_since_for_events ~__context since =
                  )
                  !in_memory_cache
               )
-        | (last_in_memory, _, _) :: _ ->
+        | (oldest_in_memory, _, _) :: _ ->
             debug
-              "%s: cache (%Ld) is older than requested time (%Ld): Using slow \
-               message lookup"
-              __FUNCTION__ last_in_memory since ;
+              "%s: cache (%Ld) might not contain all messages since the \
+               requested time (%Ld): Using slow message lookup"
+              __FUNCTION__ oldest_in_memory since ;
             None
         | _ ->
             debug "%s: empty cache; Using slow message lookup" __FUNCTION__ ;