buffer.py: make files readonly only when possible

[kiwifb]

After #31306 we have doctest failure on sage-on-distros because files have been moved out of SAGE_EXTCODE. In sage/repl/rich_output/buffer.py files are made readonly with chmod so they are immutable during the process. Unfortunately chmod fails when trying to act on system installed files as a regular user. There is a safety for files in SAGE_EXTCODE, this is a useless complication and should be replaced by a try block.

CC: @mkoeppe

Component: distribution

Author: François Bissey

Branch/Commit: 2c3a144

Reviewer: Antonio Rojas

Issue created by migration from https://trac.sagemath.org/ticket/33256

[kiwifb]

comment:1

While we may want to avoid error on file that are not changeable by the user, just a try doesn't reproduce the original intent

        if filename.startswith(os.path.abspath(SAGE_EXTCODE)):
            # Do not change permissions on the sample rich output
            # files, as it will cause trouble when upgrading Sage

Should the original block be preserved but with SAGE_SRC instead of SAGE_EXTCODE?

[kiwifb]

New commits:

2c3a144

Make _chmod_readonly more robust. Extend sample protection to SAGE_SRC as samples have been moved out of SAGE_EXTCODE

[kiwifb]

Branch: u/fbissey/chmod_readonly

[kiwifb]

Author: François Bissey

[kiwifb]

Commit: 2c3a144

[mkoeppe]

comment:4

I don't know the details about this code there, but for the failures in #31306 I think the right fix is to change the doctests so that they don't attempt to make changes (including permissions) to the source tree.

[kiwifb]

comment:5

Replying to @mkoeppe:

I don't know the details about this code there, but for the failures in #31306 I think the right fix is to change the doctests so that they don't attempt to make changes (including permissions) to the source tree.

That's what I thought and why I switched the test to check for SAGE_SRC (it works as expected here).

What about the other part of the branch which basically is about not failing if you cannot change the permissions outside of the source tree? The code wants to make sure the file that it is dealing with cannot be modified by changing its permission, but will fail when it cannot do so - in which case you probably cannot modify the file already.

[antonio-rojas]

comment:7

Works fine here (also testing on installed package)

[antonio-rojas]

Reviewer: Antonio Rojas

[kiwifb]

comment:8

Thanks!

[vbraun]

Changed branch from u/fbissey/chmod_readonly to 2c3a144