Changing `xcp.accessor` API to explicitly specify text or binary mode

[ydirson]

Context: commit "WIP xcp.repository: switch from ConfigParser to configparser" in #17 (current shortcut: ydirson@6667fef). When adding python3 compatibility, we see some Accessor implementations (e.g. FileAccessor) provide a text stream, while others (e.g. HTTPAccessor) provide a binary
stream. This is a problem for code written to be compatible with arbitrary Accessor implementations, as demonstrated in additional unit tests in commits WIP test_accessor: check for I/O on binary files and WIP test_accessor: write into copy file as binary.

To be in line with file-opening conventions, Accessor.openAddress() should be made to accept a mode argument so client code can get what it expects without a need for hacks such as illustrated above. To make sure not-yet-reviewed/converted client code is caught red-handed, I would rather add this parameter as mandatory.

Identified client code to be reviewed/adjusted in lockstep include:

• XAPI plugin prepare_host_upgrade (not in any public repo yet ?)
• xcp.repository (internal dependency, not a problem)

Additionally, https://github.com/xenserver/host-installer contains a forked version of this code in repository.py (itself an advanced fork of xcp.repository).

[edwintorok]

@kc284 would changing this API impact XenCenter's "RPU hotfix"? (the change to prepare_host_upgrade above)

[kc284]

If the script changes and the previous one is not working we will have to repackage the hotfix ISO shipped with XenCenter.

[rosslagerwall]

xcp.accessor is also used by host-installer/report.py.

Due to the limited number of uses and since it should be easy to safely update these clients with the suggested mode argument, I think it is fine to break backwards compatibility and update the clients in lockstep.

[bernhardkaindl]

In the end, this was the solution for it:

To support getting text where bytes-based IO (e.g. socket) needs to be wrapped, the only way to implement it properly is a @contextmanager that yields a io.TextIOWrapper to the caller and cleans up after the context is left.

It is merged with and tested with all relevant Accessor types in #76, here is a link to the implementation:

0d3d93b#diff-9491984f26a75ac2cde097f20f7072e360355d7fa7cad473909a276970aec00bR79

This was then the basis of fixing _getVersion() in xcp.repository.py properly:

a97be27 (Commit)

0d3d93b#diff-1f0bbacf5ade3c53537fd0bc3054283c5a134d8a4309d4ffe4f4d1a4cb56d7f2R176 (function in the PR code review)