ngclient: throws securesystemslib errors

If ngclient fails to write a target file to disk, we let securesystemslib.exceptions.StorageError propagate.

This seems like a bug: we should either handle this error, or possibly not use the securesystemslib.util.persist_temp_file() at all (it’s not super useful to us as we’re not using the storage abstraction and the rest is a few lines of code that actually hides the error we’d really like to get: OSError which we already use for metadata files in the same situation).

[joshuagl]

Filesystem abstraction was implemented for repository related code, I agree with the assertion that this seems unnecessary in the context of the client. Simpler use of standard file move methods seems like a reasonable decision.

Separately, it would be worth investigating whether the write/flush/fsync in securesystemslib is even necessary for the modern Python versions we support (it was copy/pasted from old code and written when we were supporting Python 2.7). The Python standard library is constantly evolving and, for example, since Python 3.8 a file copy may happen entirely within the OS kernel (avoiding user space buffers).

[jku]

in context of this issue and target persistence: I wanted to say that it seems like it’s not actually critical how target files are written to disk. A plain shutil.copyfileobj() instead of persist_temp_file() seems fine to me (and I’ll always prefer simplicity…).

If someone has a counter argument to that I’d be interested in hearing it.