cmd/snap-update-ns: detect and report read-only filesystems

[zyga]

This is based on #4163

This small patch adds detection of read-only filesystems to
secure-mkdir-all. This will be soon picked up by the hole-poking code.

Signed-off-by: Zygmunt Krynicki zygmunt.krynicki@canonical.com

[codecov-io]

Codecov Report

Merging #4166 into master will increase coverage by 0.05%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #4166      +/-   ##
==========================================
+ Coverage   75.52%   75.57%   +0.05%     
==========================================
  Files         436      436              
  Lines       37766    37814      +48     
==========================================
+ Hits        28521    28578      +57     
+ Misses       7250     7244       -6     
+ Partials     1995     1992       -3

Impacted Files	Coverage Δ
cmd/snap-update-ns/utils.go	100% <100%> (+14.06%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 533f5fa...c21a85a. Read the comment docs.

[stolowski]

Looks good, with two tiny remarks for you to consider.

[stolowski]

I'd initialize it to a sane and safe value (which is -1 in this code), just to avoid nasty bugs in the future.

[zyga]

This is a comment for the base PR. I'll address it there.

[zyga]

I did and later undid this. It is an ineffectual assignment as all code paths later overwrite the value before it has a chance of being used. This is an error according to our validation tools.

[stolowski]

Again, I'd initialize it to -1.

[zyga]

This is a comment for the base PR. I'll address it there.

[zyga]

Ditto.