On a file stat operation, return the largest byte that has been written

[JustinKyleJames]

• master
• 4-2-stable

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POSIX allows for a stat to be executed before a close has been called. The stat will return the file size up to that point.

In the S3 plugin, the stat translates to a HEAD request. However, the S3 object does not yet exist if the stat is done before the close.

To handle the itouch scenario (see irods/irods#6880), we return the file size of 0 if stat follows a create prior to a close. However, this does not cover a scenario like the following: create, write, stat, close.

To make it more POSIX-like, keep track of the maximum byte written and return this if a stat is called before a close. If the stat is called after a close, do a HEAD as normal.

This was found while testing istream which, unlike iput, does a stat before the final close.

[JustinKyleJames]

This has been changed slightly to handle istream --append.

On the stat, if the close operation has not been executed, look for a local cache file. (There should always be one if --append is used.) If it exists, do a stat() of it to get the object size.

If the cache file does not exist, then return maximum byte written.