Reduce noise when OCaml exception tracking gets confused

We're not going to be able to address all the corner cases any time
soon, so in the meantime output exception tracking warnings at most once
per trace.

Signed-off-by: Tudor Brindus <[email protected]>

src/trace_writer.ml

@@ -144,6 +144,7 @@ type 'thread inner =
   ; trace : (module Trace with type thread = 'thread)
   ; annotate_inferred_start_times : bool
   ; mutable in_filtered_region : bool
+  ; suppressed_errors : Hash_set.M(Source_code_position).t
   }
 
 type t = T : 'thread inner -> t
@@ -154,6 +155,13 @@ let sexp_of_inner inner =
 
 let sexp_of_t (T inner) = sexp_of_inner inner
 
+let eprint_s_once t here sexp =
+  if not (Hash_set.mem t.suppressed_errors here)
+  then (
+    Hash_set.add t.suppressed_errors here;
+    eprint_s sexp)
+;;
+
 let allocate_pid (type thread) (t : thread inner) ~name : int =
   let module T = (val t.trace) in
   T.allocate_pid ~name
@@ -312,6 +320,7 @@ let create_expert
       ; trace
       ; annotate_inferred_start_times
       ; in_filtered_region = true
+      ; suppressed_errors = Hash_set.create (module Source_code_position)
       }
   in
   write_hits t hits;
@@ -831,12 +840,20 @@ end = struct
                   some data. *)
                if Callstack.depth thread_info.callstack <> 1
                then
-                 eprintf
-                   "Warning: expected callstack depth to be the same going into a [try] \
-                    block as when leaving it, but it wasn't (off by %d). Did Intel \
-                    Processor Trace drop some data? Will attempt to recover.\n\
-                    %!"
-                   (Callstack.depth thread_info.callstack - 1)
+                 (* Conditional on happening once, this is likely to happen again... don't
+                    spam the user's terminal. *)
+                 eprint_s_once
+                   t
+                   [%here]
+                   [%message
+                     "WARNING: expected callstack depth to be the same going into a \
+                      [try] block as when leaving it, but it wasn't. Did Intel Processor \
+                      Trace drop some data? Will attempt to recover. Further errors will \
+                      be suppressed.\n"
+                       ~depth:(Callstack.depth thread_info.callstack - 1 : int)
+                       (src : Event.Location.t)
+                       (dst : Event.Location.t)
+                       (last_known_instruction_pointer : Int64.Hex.t)]
                else (
                  (* Only pop the exception callstack if we're at the same callstack
                     depth as we were when we saw [Pushtrap]. This should let us recover