The attached patch does a rude-and-crude libeio pipe driven event loop to ramie.
The patch adds an 'N' (mnemonic: O_NONBLOCK) flag to Fopen() flags.
The libeio loop is lazily started, and atexit(3) is used to wait for all events
to be completed.
The operations of fdatasync/fadvise/fsync are pipelined (i.e. fdatasync/fsync
are each separate events).
Basically libeio is being used as a co-routine monitor to move the costly
operations off the main thread of operation in order to do a wall clock
benchmark:
```
$ sudo /usr/bin/time ./rpm -U --root=/var/tmp/xxx --nodeps --force
kernel-3.10.0-514.26.2.el7.x86_64.rpm
...
5.67user 0.88system 0:39.75elapsed 16%CPU (0avgtext+0avgdata 32420maxresident)k
0inputs+299992outputs (0major+13681minor)pagefuls 0swaps
```
So ~5x slower than no fsync(3) which is currently implemented in RPM.
Which is starting to get to a reasonable execution cost for the simultaneous
benefits of
* all file operations are durable (in the sense that all installed files are
known written to disk).
* all dirty pages in the buffer cache are invalidated (so there is no I/O cache
blowout from performing an RPM upgrade).
The underlying goal here was to assess whether a libeio event loop (which is
far more general than fdatasync/fsync operations here) has benefits if
implemented in RPM. The rude-and-crude implementation here is already 2x faster
than the synchronous rsync-on-close implementation, and is not much more
complex. JMHO, YMMV, everyone's does.
[rpm+fsync+libeio.patch.gz](https://github.com/rpm-software-management/rpm/files/1202849/rpm.fsync.libeio.patch.gz)
The next step(s) will be to do a proper RPM+LIBEIO implementation so that
libeio can be used everywhere as needed, not just on the peculiar
fire-and-forget fsync-on-close file write path used here.
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