From c725c8ac36c731cbcbefeacf63bb03a90d0706c7 Mon Sep 17 00:00:00 2001
From: Bryan Garza <1396101+bryangarza@users.noreply.github.com>
Date: Fri, 29 Apr 2022 17:21:13 +0000
Subject: [PATCH] subscriber: if error occurs when formatting event, print to
 stderr

When `Format_event::format_event(...)` returns an error, we are
currently silently dropping that
Event. https://github.com/tokio-rs/valuable/issues/88 explains one
such case in which this was encountered (due to a bug in
valuable-serde). We want to be made aware whenever an Event is dropped.

This patch adds a single `eprintln` line to let the user know that we
were unable to format a specific event. We are not emitting an actual
tracing Event, to avoid the risk of a cycle (the new Event could
trigger the same formatting error again).

Resolves #1965.
---
 tracing-subscriber/src/fmt/fmt_layer.rs | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/tracing-subscriber/src/fmt/fmt_layer.rs b/tracing-subscriber/src/fmt/fmt_layer.rs
index 5e94fd0c71..df546b078f 100644
--- a/tracing-subscriber/src/fmt/fmt_layer.rs
+++ b/tracing-subscriber/src/fmt/fmt_layer.rs
@@ -896,6 +896,8 @@ where
             {
                 let mut writer = self.make_writer.make_writer_for(event.metadata());
                 let _ = io::Write::write_all(&mut writer, buf.as_bytes());
+            } else {
+                eprintln!("[tracing-subscriber] Unable to format the following event: {:?}", event);
             }
 
             buf.clear();