Deprecate SINCEDB_PATH and allow sincedb_path to point to directory

[magnusbaeck]

A discussion in PR #102 led to the following proposal. Logstash's file input currently uses the following algorithm to select the sincedb path:

1. If the sincedb_path option is set, use that path and require that it points to a file.
2. If $SINCEDB_DIR is set, generate a sincedb filename and store it in $SINCEDB_DIR.
3. If $HOME is set, generate a sincedb filename and store it in $HOME.
4. Bail out with an error.

The SINCEDB_DIR environment variable support isn't documented, and in PR #102 which adds such documentation @jordansissel suggested that we instead deprecate that variable since the configuration interface is confusing.

So, to simplify the interface without making it less expressive I suggest that the algorithm instead becomes this:

1. If the sincedb_path option is set to a valid file, use that file.
2. If the sincedb_path option is set to a valid directory, generate a sincedb filename and store it in that directory.
3. If the sincedb_path option isn't explicitly set we fall back to its default value of $HOME (if set).
4. Bail out with an error.

This makes the sincedb_path option a one-stop shop for configuring the sincedb path.

[jordansissel]

Agreed! :)

On Thursday, March 3, 2016, Magnus Bäck notifications@github.com wrote:

A discussion in PR #102
#102 led to
the following proposal. Logstash's file input currently uses the following
algorithm to select the sincedb path:

1. If the sincedb_path option is set, use that path and require that
   it points to a file.
2. If $SINCEDB_DIR is set, generate a sincedb filename and store it in
   $SINCEDB_DIR.
3. If $HOME is set, generate a sincedb filename and store it in $HOME.
4. Bail out with an error.

The SINCEDB_DIR environment variable support isn't documented, and in PR
#102 #102
which adds such documentation @jordansissel
https://github.com/jordansissel suggested that we instead deprecate
that variable since the configuration interface is confusing.

So, to simplify the interface without making it less expressive I suggest
that the algorithm instead becomes this:

1. If the sincedb_path option is set to a valid file, use that file.
2. If the sincedb_path option is set to a valid directory, generate a
   sincedb filename and store it in that directory.
3. If the sincedb_path option isn't explicitly set we fall back to its
   default value of $HOME (if set).
4. Bail out with an error.

This makes the sincedb_path option a one-stop shop for configuring the
sincedb path.

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