Be more robust to seeing a poptrap without a matching pushtrap

src/trace_writer.ml

@@ -826,18 +826,31 @@ end = struct
                thread_info.callstack <- Callstack.create ~create_time:time;
                Callstack.push thread_info.callstack top
              | Poptrap ->
-               (* We should only have the synthetic frame we created at this point. If we
-                  have more than that, we've gotten confused in our state tracking
-                  somewhere. *)
+               (* Assuming we didn't drop anything, we should only have the synthetic
+                  frame we created at this point. If we have more than that, we either got
+                  confused in our state tracking somewhere, or more likely, IPT dropped
+                  some data. *)
                if Callstack.depth thread_info.callstack <> 1
                then
-                 eprint_s
-                   [%message
-                     "BUG: expected callstack to be depth 1 at the time we encountered a \
-                      poptrap, but it wasn't"
-                       ~callstack:(thread_info.callstack : Callstack.t)];
-               ignore (Callstack.pop thread_info.callstack : _);
-               clear_trap_stack t thread_info ~time));
+                 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)
+               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
+                    from situations like:
+
+                    - Pushtrap 1
+                    - Pushtrap 2
+                    - Poptrap 2
+                    - Poptrap 1
+
+                    where "Pushtrap 2" gets dropped. *)
+                 ignore (Callstack.pop thread_info.callstack : _);
+                 clear_trap_stack t thread_info ~time)));
       last_known_instruction_pointer := Some dst.instruction_pointer
   ;;
 end
@@ -939,6 +952,15 @@ let write_event_and_callstack
       event_and_callstack
 ;;
 
+let warn_decode_error ~instruction_pointer ~message =
+  eprintf
+    "Warning: perf reported an error decoding the trace: %s\n!"
+    (match instruction_pointer with
+     | None -> [%string "'%{message}'"]
+     | Some instruction_pointer ->
+       [%string "'%{message}' @ IP %{instruction_pointer#Int64.Hex}."])
+;;
+
 (* Write perf_events into a file as a Fuschia trace (stack events). Events should be
    collected with --itrace=be or cre, and -F pid,tid,time,flags,addr,sym,symoff as per
    the constants defined above. *)
@@ -955,6 +977,7 @@ let write_event (T t) ?events_writer event =
   maybe_stop_filtered_region t ~should_write;
   match event with
   | Error { thread = _; instruction_pointer; message; time = _ } ->
+    warn_decode_error ~instruction_pointer ~message;
     let name = sprintf !"[decode error: %s]" message in
     write_duration_instant t ~thread ~name ~time ~args:[];
     let is_kernel_address =

test/decode_errors.ml

@@ -23,7 +23,9 @@ let%expect_test "decode error during memmove" =
     INPUT TRACE STREAM ENDED, any lines printed below this were deferred
     ->     21ns BEGIN itch_bbo::book::Book::add_order [inferred start time]
     ->    141ns END   __memmove_avx_unaligned_erms
-    ->    141ns END   itch_bbo::book::Book::add_order |}]
+    ->    141ns END   itch_bbo::book::Book::add_order
+    Warning: perf reported an error decoding the trace: 'Overflow packet' @ IP 0x7ffff7327730.
+    ! |}]
 ;;
 
 let%expect_test "decode error during rust B-tree rebalance" =
@@ -95,5 +97,8 @@ let%expect_test "decode error during rust B-tree rebalance" =
     ->    419ns END   _int_free
     ->    419ns END   merge_tracking_child_edge
     ->    420ns END   remove_leaf_kv
-    INPUT TRACE STREAM ENDED, any lines printed below this were deferred |}]
+    INPUT TRACE STREAM ENDED, any lines printed below this were deferred
+    Warning: perf reported an error decoding the trace: 'Overflow packet' @ IP 0x7ffff7327730.
+    !Warning: perf reported an error decoding the trace: 'Overflow packet' @ IP 0x7ffff7327730.
+    ! |}]
 ;;

test/page_fault.ml

@@ -643,5 +643,7 @@ let%expect_test "A page fault during demo.c" =
     ->    224ns BEGIN _dl_catch_exception [inferred start time]
     ->    224ns BEGIN _fini [inferred start time]
     ->  1.373us END   _fini
-    ->  1.373us END   _dl_catch_exception |}]
+    ->  1.373us END   _dl_catch_exception
+    Warning: perf reported an error decoding the trace: 'Overflow packet (foo.so)' @ IP 0x7f5948676e18.
+    ! |}]
 ;;