partial file writes on power loss

[Gabriel Samfira]

While deploying a new unit, in the event of a power loss or forced reset, the uniter state file, and various charm files (actions.yaml, revision, etc) might get corrupted. This is 100% reproducible on Windows systems where file buffers and disk cache is enabled by default.

To get around this we have 2 options:

1) use directIO on all supported platforms. This approach should always be used when writing config files or state files. Well, I would extend this to any charm files to be hones. However, it involves aligning blocks in memory before sending them to Write()

2) Call Sync() on a file handle before explicitly calling Close(). I would also recommend we explicitly call Close() before returning in any function that opens a file for writing. Defering a file close might lead to leaked file handles. On Windows systems (unline Linux) it also prevents any other application/function from manipulating a file that still has an open handle.

A PR for this has been proposed at:

https://github.com/juju/utils/pull/279

[Anastasia (anastasia-macmood)]

Juju PR against develop (2.2): https://github.com/juju/juju/pull/7358