interfaces: add common support for udev

[zyga]

Many interfaces will need to support udev-based device tagging. To
counter the explosion of custom interface types the common interface can
be grown to support per-app udev tags.

There's also some helper for testing, placed in export_test.go, since
common_test is not using builtin_test package. I'll clean that up
separately.

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

[codecov-io]

Codecov Report

Merging #3658 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3658      +/-   ##
==========================================
+ Coverage   75.16%   75.18%   +0.01%     
==========================================
  Files         388      388              
  Lines       33628    33637       +9     
==========================================
+ Hits        25278    25289      +11     
+ Misses       6534     6532       -2     
  Partials     1816     1816

Impacted Files	Coverage Δ
interfaces/builtin/common.go	62.29% <100%> (+6.52%)	⬆️
interfaces/sorting.go	100% <0%> (+1.28%)	⬆️
cmd/snap/cmd_aliases.go	95% <0%> (+1.66%)	⬆️

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[adglkh]

I prefer to use the following code for a few reasons here, instead of introducing tags and doing the string.Replace here.

1. less code for consistency with other functions in commonInterface
2. In favor of "format" here rather than "replace" for performance consideration (nitpicking, we have a large code snippet udev rules for udisk2, modemManager, maybe more in the future)

if iface.connectedPlugUdev != "" {
        for appName := range plug.Apps {
            tag := udevSnapSecurityName(plug.Snap.Name(), appName)
            spec.AddSnippet(fmt.Sprintf(iface.connectedPlugUdev, tag))
        } 
}

People could use %[1]s if there're multiple udev entries there or simply use "%s" if only one entry exists.

I'll leave the decision to you though.

[zyga]

I used format first but then changed this so that it is consistent with ###replaceme### markers we use elsewhere in the interface code. We can revisit that but then we should do it consistently in the whole stack (all of builtin/*.go)

[adglkh]

Okay. Sounds good!

[adglkh]

I prefer to use the following code for a few reasons here, instead of introducing tags and doing the string.Replace here.

1. less code for consistency with other functions in commonInterface
2. In favor of "format" here rather than "replace" for performance consideration (nitpicking, we have a large code snippet udev rules for udisk2, modemManager, maybe more in the future)

if iface.connectedPlugUdev != "" {
        for appName := range plug.Apps {
            tag := udevSnapSecurityName(plug.Snap.Name(), appName)
            spec.AddSnippet(fmt.Sprintf(iface.connectedPlugUdev, tag))
        } 
}

People could use %[1]s if there're multiple udev entries there or simply use "%s" if only one entry exists.

I'll leave the decision to you though.

[zyga]

I used format first but then changed this so that it is consistent with ###replaceme### markers we use elsewhere in the interface code. We can revisit that but then we should do it consistently in the whole stack (all of builtin/*.go)

[adglkh]

Okay. Sounds good!