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fix: properly normalize erlang errors before emitting telemetry and logging crash_reason #455

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merged 2 commits into from
Jan 14, 2025
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fix: properly normalize erlang errors before emitting telemetry and logging crash_reason #455

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618c918
fix: logs normalized erlang error when unwrapping Plug.Conn.WrapperError
grzuy Jan 13, 2025
7453f5c
generalize exception normalization
grzuy Jan 14, 2025
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2 changes: 2 additions & 0 deletions lib/bandit/pipeline.ex
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Original file line number Diff line number Diff line change
Expand Up @@ -230,6 +230,8 @@ defmodule Bandit.Pipeline do
end

defp handle_error(kind, reason, stacktrace, transport, span, opts, metadata) do
reason = Exception.normalize(kind, reason, stacktrace)

Bandit.Telemetry.span_exception(span, kind, reason, stacktrace)
status = reason |> Plug.Exception.status() |> Plug.Conn.Status.code()

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18 changes: 12 additions & 6 deletions test/bandit/http1/plug_test.exs
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Expand Up @@ -46,19 +46,19 @@ defmodule HTTP1PlugTest do
assert SimpleHTTP1Client.connection_closed_for_reading?(client)

assert_receive {:log, %{level: :error, msg: {:string, msg}, meta: meta}}, 500
assert msg =~ "(RuntimeError) boom"
assert msg =~ "(ArithmeticError)"
assert msg =~ "lib/bandit/pipeline.ex:"

assert %{
domain: [:elixir, :bandit],
crash_reason: {%RuntimeError{message: "boom"}, [_ | _] = _stacktrace},
crash_reason: {%ArithmeticError{}, [_ | _] = _stacktrace},
conn: %Plug.Conn{},
plug: {__MODULE__, []}
} = meta
end

def unknown_crasher(_conn) do
raise "boom"
1 / 0
end

@tag :capture_log
Expand Down Expand Up @@ -112,7 +112,7 @@ defmodule HTTP1PlugTest do
get "/" do
# Quiet the compiler
_ = conn
raise "boom"
1 = 0
end
end

Expand All @@ -134,9 +134,15 @@ defmodule HTTP1PlugTest do
SimpleHTTP1Client.send(client, "GET", "/hello_world", ["host: banana"])
assert SimpleHTTP1Client.connection_closed_for_reading?(client)

assert_receive {:log, %{level: :error, msg: {:string, msg}}}, 500
assert_receive {:log, %{level: :error, msg: {:string, msg}, meta: meta}}, 500
refute msg =~ "(Plug.Conn.WrapperError)"
assert msg =~ "** (RuntimeError) boom"
assert msg =~ "** (MatchError)"
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What's the reason for this change? Is there an issue that comes up with MatchError that doesn't with a simple raise?

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@grzuy grzuy Jan 14, 2025 •
edited
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Is there an issue that comes up with MatchError that doesn't with a simple raise?

Yes, sorry for the lack of explanation.

Elixir's raise directly errors with an elixir's RuntimeError struct.

However, :one = :two or 1 / 0 will exit with {:badmatch, :two} and :badarith respectively, to name a few examples from https://www.erlang.org/doc/system/errors.html#exit-reasons.

Those need to be normalized with Exception.normalize somewhere, to become well-known elixir Exception structs (https://github.com/elixir-lang/elixir/blob/8deaaf4117fef24f516c12dc40989b9772a6d763/lib/elixir/lib/exception.ex#L2357-L2494).

The symptom in bandit right now is only that the non-normalized exit appears in crash_reason value.

The reason why is properly appearing normalized in the logged text message is because the existent Exception.format before Logger.error in

bandit/lib/bandit/pipeline.ex

Line 238 in 177b053

Logger.error(Exception.format(kind, reason, stacktrace), logger_metadata)
, which does the normalization behind the scenes: https://github.com/elixir-lang/elixir/blob/8deaaf4117fef24f516c12dc40989b9772a6d763/lib/elixir/lib/exception.ex#L134-L137.

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I see. I wonder if we should be doing that for all errors (ie continue to have the Plug.Conn.WrapperError pattern match be 'dumb' and just pass the value through unchanged) and normalize in the other handle_error clauses. Does that make sense, or is it only needed in the narrow case of wrapped errors?

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Yes, I think you're right.
That's regardless of whether it's wrapped or not.

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Updated.

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Also now realized this will fix/normalize the emitted telemetry exception.

Updated PR title.


assert %{
domain: [:elixir, :bandit],
crash_reason: {%MatchError{}, [_ | _] = _stacktrace},
conn: %Plug.Conn{}
} = meta
end
end

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