Fluent Bit still buffers to the filesystem if the folder in storage.path exists even if Emitter_Storage.type is set to memory

[lpugoy]

Bug Report

Describe the bug
If a rewrite tag filter has Emitter_Storage.type set to memory, Fluent Bit will act as if Emitter_Storage.type is set to filesystem if storage.path is set in the [SERVICE] section and the filter's folder exists.

I can confirm this also happens with inputs with Storage.type set to memory.

To Reproduce

Fluent Bit config

[SERVICE]
    storage.path /fluent-bit/data

[INPUT]
    Name dummy
    Tag dummy

[FILTER]
    Name rewrite_tag
    Match dummy
    Rule $message dummy rewritten false
    Emitter_Name rewriter
    Emitter_Storage.type memory

[OUTPUT]
    Name stdout
    Match rewritten

• Steps to reproduce the problem:

1. mkdir -p "$(pwd)/data/emitter.2"
2. Watch for file system changes in the data directory: inotifywait in Linux or fswatch in Mac, for example.
3. Run Fluent Bit: docker run --name fluentbit --rm -v $(pwd)/data:/fluent-bit/data:rw -v $(pwd)/fluentbit.conf:/config/fluentbit.conf:ro cr.fluentbit.io/fluent/fluent-bit:2.2.0 -c /config/fluentbit.conf.

Expected behavior
No files will be created under the data/emitter.2 directory.

Your Environment

• Version used: 2.2.0
• Configuration: Fluent Bit config given above
• Environment name and version (e.g. Kubernetes? What version?): Docker
• Server type and version: n/a
• Operating System and version: n/a
• Filters and plugins: n/a

Additional context
This makes having some inputs/rewrite filters use filesystem buffering while others use memory impossible.

[nokute78]

@edsiper @leonardo-albertovich
cio_scan_streams from cio_load creates cio_stream and set storage_type to filesystem if storage.path is defined and sub directory is found.

mkdir -p "$(pwd)/data/emitter.2" means that fluent-bit create cio_stream for emitter.2 and its storage_type is filesystem before creating plugin instances.

We can't modify it since checking duplication.
https://github.com/fluent/fluent-bit/blob/v2.2.0/lib/chunkio/src/cio_stream.c#L145

[nokute78]

I sent a patch #8290 .