If a file is opened for reading through the S3A connector
is not closed, then when garbage collection takes place
* An error message is reported at WARN, including the file name.
* A stack trace of where the stream was created is reported
at INFO.
* A best-effort attempt is made to release any active HTTPS
connection.
* The filesystem IOStatistic stream_leaks is incremented.
The intent is to make it easier to identify where streams
are being opened and not closed -as these consume resources
including often HTTPS connections from the connection pool
of limited size.
It MUST NOT be relied on as a way to clean up open
files/streams automatically; some of the normal actions of
the close() method are omitted.
Instead: view the warning messages and IOStatistics as a
sign of a problem, the stack trace as a way of identifying
what application code/library needs to be investigated.
Contributed by Steve Loughran
7999db55da