Distinguish empty path from literal slash in Url.lastSegment

[xhon-pelushi]

Summary

• Make Url.lastSegment return null for URLs with no path segments instead of "/".
• Keep returning "/" when the last segment literally decodes to a slash (%2F).
• Display call sites continue to use ?: "/"; sync/InternalMemberState treat null as missing so a member named / cannot collide with the empty-path sentinel.

Fixes #2782

Test plan

• ./gradlew :core:testDebugUnitTest --tests 'at.bitfire.davdroid.util.DavUtilsTest'
• Spot-check collection titles still show / for empty-path homesets.

[CLAassistant]

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[rfc2822]

Hi,

Thanks for the efforts. Was the concept of this PR done by AI?

There are two additional helpers in dav4jvm:

• Response.hrefName
• HttpUtils.fileName

Should we use / unify / drop those?

I think at the end we should have only one method to get the file name from an Url, it it should be always the same.

[rfc2822]

I think a member should always have a file name. If it doesn't, it's not an (internal / direct) member.

So InternalMemberState should never be created without a proper file name.

Then all the require would fall away.

[xhon-pelushi]

Thanks for the feedback — pushed a follow-up that unifies this with dav4jvm:

• Url.lastSegment now delegates to HttpUtils.fileName (same helper behind Response.hrefName())
• Empty path → ""; literal / segment (e.g. %2F) → "/"
• Display sites use lastSegment.ifEmpty { "/" }
• InternalMemberState.fileName is non-null again: members without a file name are filtered out at construction (toInternalMemberStates / sync-collection classify), so the requires around downloads stay for defense only

On the design concept: the goal was specifically to stop conflating “no path” with a resource literally named /, which matches the distinction HttpUtils.fileName already provides. Happy to rename/drop lastSegment in favor of calling HttpUtils.fileName at call sites if you’d rather have only one name in this repo.

[xhon-pelushi]

Thanks @rfc2822 — follow-up commit unifies on HttpUtils.fileName (same as Response.hrefName):

• InternalMemberState now requires a non-empty file name at construction
• collection sync skips members without a file name instead of creating invalid state
• display/title call sites treat an empty last segment as /

I used tooling while exploring the helpers; happy to adjust further if you want lastSegment itself to go away in favor of only HttpUtils.fileName.