Backport(v1.19): buffer: fix stage_size leak when a staged chunk is unstaged (#5440) - #5456
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**Which issue(s) this PR fixes**: Fixes #5439 **What this PR does / why we need it**: In `Buffer#write_step_by_step`, when a single record cannot be appended to an existing staged chunk without exceeding `chunk_limit_size`, the staged chunk is removed from `@stage`, marked unstaged and later enqueued via `enqueue_unstaged_chunk`. Unlike `enqueue_chunk`, `enqueue_unstaged_chunk` only adds to `@queue_size` and never subtracts from `@stage_size`, so the chunk's already-counted bytes remain in `@stage_size` forever. Over a long-lived process this makes `@stage_size` (and thus `fluentd_output_status_buffer_total_bytes` = `@stage_size` + `@queue_size`) drift upward, traffic-proportionally. Once it reaches `total_limit_size`, `storable?` returns `false` and every emit raises `Fluent::Plugin::Buffer::BufferOverflowError` even though the buffer is actually near-empty; input sources then reject incoming data. Subtract the chunk's original (already-counted) `bytesize` from `@stage_size` at the point of unstaging, so `total_queued_size` stays consistent with the real staged + queued bytes. **Docs Changes**: N/A **Release Note**: * buffer: fix `stage_byte_size` leak on staged→unstaged chunk demotion that could eventually raise spurious `BufferOverflowError`. Affects plugins implementing `#format`. Signed-off-by: Alexander Enrique Urieles Nieto <alexander@urieles.co> Signed-off-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>
Watson1978
approved these changes
Jul 27, 2026
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Which issue(s) this PR fixes:
Backport #5440
Fixes #5439
What this PR does / why we need it:
In
Buffer#write_step_by_step, when a single record cannot be appended to an existing staged chunk without exceedingchunk_limit_size, the staged chunk is removed from@stage, marked unstaged and later enqueued viaenqueue_unstaged_chunk. Unlikeenqueue_chunk,enqueue_unstaged_chunkonly adds to@queue_sizeand never subtracts from@stage_size, so the chunk's already-counted bytes remain in@stage_sizeforever.Over a long-lived process this makes
@stage_size(and thusfluentd_output_status_buffer_total_bytes=@stage_size+@queue_size) drift upward, traffic-proportionally. Once it reachestotal_limit_size,storable?returnsfalseand every emit raisesFluent::Plugin::Buffer::BufferOverflowErroreven though the buffer is actually near-empty; input sources then reject incoming data.Subtract the chunk's original (already-counted)
bytesizefrom@stage_sizeat the point of unstaging, sototal_queued_sizestays consistent with the real staged + queued bytes.Docs Changes:
N/A
Release Note:
stage_byte_sizeleak on staged→unstaged chunk demotion that couldeventually raise spurious
BufferOverflowError. Affects plugins implementing#format.