*: fix cannot convert type: decimal.Decimal (#375)

[sre-bot]

cherry-pick #375 to release-1.0

---

What problem does this PR solve?

in golang v1.13, the Decimal decompose interface is introduced, see:

• proposal: database/sql: define a Decimal decompose interface for decimal packages golang/go#30870
• golang/go@683ffe0

in shopspring/decimal@75bb2cb, it implements the Decimal decompose interface.

when executing a statement with decimal type columns, a possible execution path is:

1. https://github.com/golang/go/blob/cc8838d645/src/database/sql/sql.go#L2275
2. https://github.com/golang/go/blob/cc8838d645/src/database/sql/sql.go#L1515
3. https://github.com/golang/go/blob/cc8838d645/src/database/sql/convert.go#L177
4. https://github.com/go-sql-driver/mysql/blob/v1.4.1/connection_go18.go#L196
5. https://github.com/go-sql-driver/mysql/blob/v1.4.1/statement.go#L141
6. https://github.com/golang/go/blob/cc8838d645/src/database/sql/driver/types.go#L184

the code block

case decimalDecompose:
	return true

will cause the skip of the value fetching (convert decimal.Decimal to string) in https://github.com/go-sql-driver/mysql/blob/v1.4.1/statement.go#L146.

then continue the execution flow:

1. https://github.com/golang/go/blob/cc8838d645/src/database/sql/sql.go#L1538
   • (triggered with https://github.com/go-sql-driver/mysql/blob/v1.4.1/connection.go#L296)
2. https://github.com/golang/go/blob/cc8838d645/src/database/sql/sql.go#L2435
3. https://github.com/golang/go/blob/cc8838d645/src/database/sql/ctxutil.go#L65
4. https://github.com/go-sql-driver/mysql/blob/v1.4.1/connection_go18.go#L142
5. https://github.com/go-sql-driver/mysql/blob/v1.4.1/statement.go#L53
6. https://github.com/go-sql-driver/mysql/blob/v1.4.1/packets.go#L1071

another useful link golang/go#34315.

What is changed and how it works?

some methods can be used to fix this problem:

1. chose a shopspring/decimal version without Decimal decompose interface implementation
2. downgrade golang to v1.12

I chose the first one, and update shopspring/decimal to shopspring/decimal@b054a8d. (revert Decimal decompose interface implementation)

go get -u github.com/shopspring/decimal@b054a8dfd10d5e651f48ab20c6c9bee309805368
go mod tidy

Check List

Tests

• Unit test
• Integration test

Related changes

• Need to cherry-pick to the release branch

[csuzhangxc]

/run-all-tests tidb=release-3.0

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (release-1.0@b99f0b7). Click here to learn what that means.
The diff coverage is n/a.

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[WangXiangUSTC]

LGTM

[amyangfei]

LGTM