region: check if the query startkey and endkey specify an uncovered region

[JackL9u]

What problem does this PR solve?

handle a special case in GetRegion function

Issue Number: ref #6711

What is changed and how does it work?

The (item, index) returned by GetWithIndex(key) function has the following meaning:

1. if there exists an interval defined by [startkey, endkey) such that startkey == key, then item will be the interval object, and index will be the index of this interval in sorted order (sorted by startkey)
2. otherwise, item will be nil, and index will be the number of intervals whose startkey < key.

Since the return values have different meanings in different situations, I want to unify them. To do this, I want to make sure item object is the largest interval (sorted by startkey) whose startkey <= key, and index is the number of interval object whose startkey <= key. Line 1794 - 1805 did this.

To find out if a key lies in an interval, we need to

1. get the item, index defined above
2. make sure item is not nil (it will be nil if key is less then the smallest startkey)
3. and startkey <= key <= endkey
   Line 1807 - 1814 did this.

To find out if a region [k1, k2) does not overlap with any existing intervals, we need to

1. make sure both k1 and k2 do NOT lie in an existing interval, and
2. there are equal number of intervals before them
   The 2) check is necessary in case we have an existing interval [2, 4), and we are querying [1, 5). Without the 2) check, it would return true. Line 1824 - 1826 did this.

Line 1816 - 1823 is handling a special case

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Code changes

• Has the configuration change
• Has HTTP API interfaces changed (Don't forget to add the declarative for the new API)
• Has persistent data change

Side effects

• Possible performance regression
• Increased code complexity
• Breaking backward compatibility

Related changes

• PR to update pingcap/docs/pingcap/docs-cn:
• PR to update pingcap/tiup:
• Need to cherry-pick to the release branch

Release note

None.

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[ti-chi-bot]

Hi @JackL9u. Thanks for your PR.

I'm waiting for a tikv member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.64%. Comparing base (b27f021) to head (59f8fa0).
Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8790      +/-   ##
==========================================
- Coverage   75.71%   75.64%   -0.08%     
==========================================
  Files         461      461              
  Lines       72285    72329      +44     
==========================================
- Hits        54731    54712      -19     
- Misses      14045    14116      +71     
+ Partials     3509     3501       -8     

Flag	Coverage Δ
unittests	75.64% <100.00%> (-0.08%)	⬇️

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[rleungx]

/ok-to-test

[okJiang]

Can you add some description to explain your work? For example:

• How does it work?
• Why can it solve this issue?

This is very helpful to review your code

[JackL9u]

comments updated with explanations.

[okJiang]

If the startItem != nil (L1799), startInAnInterval is true?

[okJiang]

ditto

[okJiang]

why do you check endkey when it is empty?