out_file: Support placeholders in symlink_path parameter

[repeatedly]

Since v1, path parameter supports placeholders and it means multiple buffer files exist at the same time.
So hard to refer latest buffer file via symlink_path. To resolve this problem, support placeholders in symlink_path.
The drawback is lots of symlink are created when there are various tags/field.

Signed-off-by: Masahiro Nakagawa repeatedly@gmail.com

[okkez]

The following configuration generates a path include placeholder like /tmp/logs/${tag}.
Is this behavior expected?

<source>
  @type dummy
  tag dummy1
</source>

<source>
  @type dummy
  tag dummy2
</source>

<match dummy*>
  @type file
  path /tmp/logs/${tag}
  symlink_path /tmp/logs/current-${tag}
  <buffer tag,time>
    @type file
    # path /tmp/logs/buffer
  </buffer>
</match>

$ tree /tmp/logs/
/tmp/logs/
├── ${tag}
│   ├── buffer.b57fb1dd96306dd0b308e094f7ec2228f.log
│   ├── buffer.b57fb1dd96306dd0b308e094f7ec2228f.log.meta
│   ├── buffer.b57fb1dd96339a870530991d4871cfe11.log
│   └── buffer.b57fb1dd96339a870530991d4871cfe11.log.meta
├── current-dummy1 -> /tmp/logs/${tag}/buffer.b57fb1dd96339a870530991d4871cfe11.log
└── current-dummy2 -> /tmp/logs/${tag}/buffer.b57fb1dd96306dd0b308e094f7ec2228f.log

[repeatedly]

I missed the reply.

Is this behavior expected?

Yes. This patch doesn't touch actual buffer file.

[repeatedly]

If no problem, I will merge the patch.

[okkez]

No problem, go ahead.