process-file and Tramp's file cache (was: bug#1741: 23.0.60; vc fail to register file while logged as /su:: with tramp)

[Michael Albinus <Michael.Albinus@alcatel-lucent.de>]

Stefan Monnier <monnier@iro.umontreal.ca> writes:

> BTW, to improve the behavior in case of things like "git status", it
> might be worthwhile to add some way to indicate that a particular use of
> process-file is (supposed to be) free of side-effects.

I would like to introduce a new variable `process-file-modification-list'.
It's default value shall be `t', which means that any file on the remote
host could have been modified by the corresponding `process-file' call.
`nil' means that no file is modified during `process-file'. A list of
strings (remote file names) are indication, that exactly those files are
modified, when `process-file' runs.

This would allow Tramp to optimize its file cache. Currently, it assumes
after every `process-file' call, that all cached values are invalid.
This results in unnecessary check operations on the remote side.

A typical use case in `vc-registered' would look like this:

  (let (handler process-file-modification-list)
    (cond ...

Opinions?

>         Stefan

Best regards, Michael.