filelog receiver: Last line of compressed files without new line ending is not read

[ZoltanPeczoli]

Component(s)

receiver/filelog

What happened?

Description

When ingesting a compressed file (gzip) with filelog receiver, if the last line does not end in a newline character, the last line is never consumed.

If the same file (i.e. last line ending without newline character) is not compressed, the last line is consumed correctly.

Steps to Reproduce

echo -n '{"test": "uncompressed, no new line"}' > test_uncompressed.log
echo -n '{"test": "compressed, no new line"}' > test_compressed.log
gzip test_compressed.log

test.yaml:

receivers:
  filelog:
    include: [ "*log*" ]
    compression: auto
    start_at: beginning

exporters:
  debug:
    verbosity: normal # detailed

service:
  pipelines:
    logs:
      receivers: [filelog]
      exporters: [debug]

Expected Result

2026-01-22T06:57:47.033Z        info    fileconsumer/file.go:266        Started watching file   {"resource": {"service.instance.id": "35de927e-b634-4515-b6df-2fc2b446a6a8", "service.name": "otelcol-contrib", "service.version": "0.144.0"}, "otelcol.component.id": "filelog", "otelcol.component.kind": "receiver", "otelcol.signal": "logs", "component": "fileconsumer", "path": "test_compressed.log.gz"}
2026-01-22T06:57:47.034Z        info    fileconsumer/file.go:266        Started watching file   {"resource": {"service.instance.id": "35de927e-b634-4515-b6df-2fc2b446a6a8", "service.name": "otelcol-contrib", "service.version": "0.144.0"}, "otelcol.component.id": "filelog", "otelcol.component.kind": "receiver", "otelcol.signal": "logs", "component": "fileconsumer", "path": "test_uncompressed.log"}
2026-01-22T06:57:47.728Z        info    Logs    {"resource": {"service.instance.id": "35de927e-b634-4515-b6df-2fc2b446a6a8", "service.name": "otelcol-contrib", "service.version": "0.144.0"}, "otelcol.component.id": "debug", "otelcol.component.kind": "exporter", "otelcol.signal": "logs", "resource logs": 1, "log records": 1}
2026-01-22T06:57:47.728Z        info    ResourceLog #0
ScopeLog #0
{"test": "uncompressed, no new line"} log.file.name=test_uncompressed.log
{"test": "compressed, no new line"} log.file.name=test_compressed.log.gz

Actual Result

2026-01-22T06:57:47.033Z        info    fileconsumer/file.go:266        Started watching file   {"resource": {"service.instance.id": "35de927e-b634-4515-b6df-2fc2b446a6a8", "service.name": "otelcol-contrib", "service.version": "0.144.0"}, "otelcol.component.id": "filelog", "otelcol.component.kind": "receiver", "otelcol.signal": "logs", "component": "fileconsumer", "path": "test_compressed.log.gz"}
2026-01-22T06:57:47.034Z        info    fileconsumer/file.go:266        Started watching file   {"resource": {"service.instance.id": "35de927e-b634-4515-b6df-2fc2b446a6a8", "service.name": "otelcol-contrib", "service.version": "0.144.0"}, "otelcol.component.id": "filelog", "otelcol.component.kind": "receiver", "otelcol.signal": "logs", "component": "fileconsumer", "path": "test_uncompressed.log"}
2026-01-22T06:57:47.728Z        info    Logs    {"resource": {"service.instance.id": "35de927e-b634-4515-b6df-2fc2b446a6a8", "service.name": "otelcol-contrib", "service.version": "0.144.0"}, "otelcol.component.id": "debug", "otelcol.component.kind": "exporter", "otelcol.signal": "logs", "resource logs": 1, "log records": 1}
2026-01-22T06:57:47.728Z        info    ResourceLog #0
ScopeLog #0
{"test": "uncompressed, no new line"} log.file.name=test_uncompressed.log

Collector version

0.144.0

Environment information

OS: RHEL 9.5

OpenTelemetry Collector configuration

receivers:
  filelog:
    include: [ "*log*" ]
    compression: auto
    start_at: beginning

exporters:
  debug:
    verbosity: normal # detailed

service:
  pipelines:
    logs:
      receivers: [filelog]
      exporters: [debug]

Log output

2026-01-22T06:57:47.033Z        info    fileconsumer/file.go:266        Started watching file   {"resource": {"service.instance.id": "35de927e-b634-4515-b6df-2fc2b446a6a8", "service.name": "otelcol-contrib", "service.version": "0.144.0"}, "otelcol.component.id": "filelog", "otelcol.component.kind": "receiver", "otelcol.signal": "logs", "component": "fileconsumer", "path": "test_compressed.log.gz"}
2026-01-22T06:57:47.034Z        info    fileconsumer/file.go:266        Started watching file   {"resource": {"service.instance.id": "35de927e-b634-4515-b6df-2fc2b446a6a8", "service.name": "otelcol-contrib", "service.version": "0.144.0"}, "otelcol.component.id": "filelog", "otelcol.component.kind": "receiver", "otelcol.signal": "logs", "component": "fileconsumer", "path": "test_uncompressed.log"}
2026-01-22T06:57:47.728Z        info    Logs    {"resource": {"service.instance.id": "35de927e-b634-4515-b6df-2fc2b446a6a8", "service.name": "otelcol-contrib", "service.version": "0.144.0"}, "otelcol.component.id": "debug", "otelcol.component.kind": "exporter", "otelcol.signal": "logs", "resource logs": 1, "log records": 1}
2026-01-22T06:57:47.728Z        info    ResourceLog #0
ScopeLog #0
{"test": "uncompressed, no new line"} log.file.name=test_uncompressed.log

Additional context

No response

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[github-actions]

Pinging code owners:

• receiver/filelog: @andrzej-stencel

See Adding Labels via Comments if you do not have permissions to add labels yourself.

[paulojmdias]

Thanks for reporting this issue @ZoltanPeczoli.

I was able to reproduce it locally. I'll take a look at this and check what we can do to fix it.

[khushijain21]

@paulojmdias if you haven't already started working on it. I have a probable fix to it.

[paulojmdias]

@khushijain21 I was pushing it, but since you have opened a PR for it, feel free to follow up.

[khushijain21]

So I looked at the code again. And the reported behavior is expected. Only log lines ending with newline character are expected to be ingested. See code

I think it is rather a bug that logs from uncompressed file without newline ending are still being ingested. @andrzej-stencel can confirm

[ZoltanPeczoli]

Before opening this issue, I looked in the issue history for potentially related reports. In #26691 not reading the last line (without newline character) was considered a bug. To quote @djaglowski 's comment there:

A log is emitted if either 1) a complete log is read, as defined by the multiline configuration or lack thereof, or 2) the force_flush_period expires.

In that case, according to the comments, the flush period was not applied correctly, resulting in the last log line missed - and this was fixed.