File input should validate the specified path

[untergeek]

And at least log a warning if the file does not exist.

(Migrated elastic/logstash#1441 here.)

[magnusbaeck]

What exact semantics do we have in mind here? Emit a warning (or whatever log level is visible without --debug or --verbose) for each supplied glob pattern that evaluates to zero files?

To avoid duplication with the actual globbing code in the filewatch library it seems we should put the warning code there instead of a single upfront warning by the file plugin. Emitting a warning every time _discover_file is called will spam the logs though. Periodic warnings or a single warning per pattern?

[jordansissel]

@magnusbaeck I think single warning may be sufficient. Something like "File not found, but I will keep watching for it [path: /some/specific/path]"

[TheVastyDeep]

I think this would be a huge help to people who use backslash in filenames on Windows -- it doesn't work. They specify a path that would work in any other Windows program and logstash just sits there waiting for them to create a file that they are certain already exists. A one-off at startup is fine.

(I wasn't tracking release-to-release changes for a few months there, but I think this backslash thing was a Javafication side-effect.)