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Handle :EXIT messages in ingestion WriteBuffer #6454

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bca05de
Handle :EXIT messages in ingestion WriteBuffer to prevent crash
SAY-5 Jun 18, 2026
ef30baa
Merge branch 'master' into fix-writebuffer-exit-trap
SAY-5 Jun 20, 2026
01c9985
Merge 57ec8f28e50651b53e963361220e52cffb5f8d54 into fix-writebuffer-e…
SAY-5 Jun 26, 2026
9a06340
Merge branch 'master' into fix-writebuffer-exit-trap
SAY-5 Jul 2, 2026
da23d41
test: trigger real linked-process :EXIT in write buffer test
SAY-5 Jul 2, 2026
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7 changes: 7 additions & 0 deletions lib/plausible/ingestion/write_buffer.ex
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Original file line number Diff line number Diff line change
Expand Up @@ -66,6 +66,13 @@ defmodule Plausible.Ingestion.WriteBuffer do
{:noreply, %{state | buffer: [], buffer_size: 0, timer: timer}}
end

def handle_info({:EXIT, _from, _reason}, state) do
# We trap exits so terminate/2 can flush on shutdown. Linked ports and

@aerosol aerosol Jul 8, 2026 •
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This comment is unnecessary, let's log the reason instead (warning level is fine).

@aerosol aerosol Jul 8, 2026

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Bonus points for asserting the log entry in test 🤗

# processes (e.g. the ClickHouse connection socket) deliver :EXIT messages
# here on disconnect, which we ignore to keep the buffer alive.
{:noreply, state}
end

@impl true
def handle_call(:flush, _from, state) do
%{timer: timer, flush_interval_ms: flush_interval_ms} = state
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36 changes: 36 additions & 0 deletions test/plausible/ingestion/write_buffer_test.exs
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@@ -0,0 +1,36 @@
defmodule Plausible.Ingestion.WriteBufferTest do
use ExUnit.Case, async: true

alias Plausible.Ingestion.WriteBuffer

@opts WriteBuffer.compile_time_prepare(Plausible.ClickhouseEventV2)

test "keeps running when a linked process exits while trapping exits" do
opts =
@opts
|> Map.take([:header, :insert_sql, :insert_opts])
|> Map.to_list()
|> Keyword.put(:name, :"write_buffer_exit_#{System.unique_integer([:positive])}")
# large interval so the empty buffer never flushes to ClickHouse during the test
|> Keyword.put(:flush_interval_ms, 60_000)

@aerosol aerosol Jul 8, 2026

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What do you think about making this a bit more readable? Since compile_time_prepare result is a map, let's build a keyword list verbatim and only fetch necessary fields via the dot-notation access.

Something like:

test "...", %{test: test} do
opts = [
  header: @compile_time_opts.header,
  insert_sql: @compile_time_opts.insert_sql,
  insert_opts: @compile_time_opts.insert_opts,
  name: test, # unique atom is at our disposal already
  flush_interval_ms: 1_000_000_000 # no need to ever flush automatically on slow CI
]


{:ok, pid} = WriteBuffer.start_link(opts)

# The buffer traps exits, so a linked process exiting delivers a real
# {:EXIT, from, reason} message to handle_info/2. Linking a throwaway
# process to the buffer and killing it reproduces the crash in #6382
# without sending a synthetic message.
child =
spawn(fn ->
Process.link(pid)
Process.sleep(:infinity)
end)

ref = Process.monitor(child)
Process.exit(child, :shutdown)
assert_receive {:DOWN, ^ref, :process, ^child, _}, 1_000

assert Process.alive?(pid)
assert WriteBuffer.flush(opts[:name]) == :ok
end
end
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