fix CONSTRAINT syntax

[ichn-hu]

What problem does this PR solve?

Fix #413

What is changed and how it works?

Tests proposed in #413 reflect 2 different aspects of the incompatibility between our parser and MySQL's.

1. MySQL supports constraints in altering columns, therefore in addtition to modify parser.y to support the syntax, I also added a new attribute NewConstraints in parallel with already exsited NewColumns to AlterTableSpec, and also added the corresponding restoration and visit code. The modification is based on observation of how CreateTableStmt is implemented, because the syntax is similar in MySQL's sql_yacc.yy and our parser.y, and it also uses two different array Cols and Constraints to reflect the 2 different kinds of column definition.
2. The later 2 test cases in Fix CONSTRAINT syntax #413 is a different incompatibility case, it can be resolved easily as done in sql_yacc.yy

Check List

Tests

• Unit test
• Integration test

Code changes

• Has exported variable/fields change

[codecov]

Codecov Report

Merging #548 into master will decrease coverage by 0.01%.
The diff coverage is 63.63%.

@@            Coverage Diff             @@
##           master     #548      +/-   ##
==========================================
- Coverage   71.77%   71.76%   -0.02%     
==========================================
  Files          32       32              
  Lines        7802     7813      +11     
==========================================
+ Hits         5600     5607       +7     
- Misses       1674     1676       +2     
- Partials      528      530       +2

Impacted Files	Coverage Δ
parser.go	70.58% <ø> (ø)	⬆️
ast/ddl.go	80.45% <63.63%> (-0.12%)	⬇️

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[codecov]

Codecov Report

Merging #548 into master will decrease coverage by 0.01%.
The diff coverage is 63.63%.

@@            Coverage Diff             @@
##           master     #548      +/-   ##
==========================================
- Coverage   71.77%   71.76%   -0.02%     
==========================================
  Files          32       32              
  Lines        7802     7813      +11     
==========================================
+ Hits         5600     5607       +7     
- Misses       1674     1676       +2     
- Partials      528      530       +2

Impacted Files	Coverage Δ
parser.go	70.58% <ø> (ø)	⬆️
ast/ddl.go	80.45% <63.63%> (-0.12%)	⬇️

---

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 35a39aa...23a22c3. Read the comment docs.

[tangenta]

Rest LGTM

[ichn-hu]

@tangenta Hi, I've tried hard to increase the coverage, but it still drops, will this block the merge of this PR?

BTW, I'll be very appreciated if you could let me know how to increase the coverage. I figued out that it is because I added code in the visit of AlterTableSpec, however it won't be covered by tests in parser_test.go, since no visitor will be dispatched for parsing.

[zier-one]

@tangenta Hi, I've tried hard to increase the coverage, but it still drops, will this block the merge of this PR?

BTW, I'll be very appreciated if you could let me know how to increase the coverage. I figued out that it is because I added code in the visit of AlterTableSpec, however it won't be covered by tests in parser_test.go, since no visitor will be dispatched for parsing.

This won't block the merge.

To test Accept function, you can reference https://github.com/pingcap/parser/blob/master/ast/ddl_test.go#L26

[ichn-hu]

@tangenta @leoppro I've added two more test cases in https://github.com/pingcap/parser/blob/master/ast/ddl_test.go#L26, not sure if it will have effect on improving coverage.

But if it won't block the merge, please help review the code and merge it, I guess I won't continue to work on improving coverage.

[tangenta]

LGTM

[zier-one]

LGTM