Fix connection leak caused by rapid context cancellation

[mherr-google]

Fix connection leak caused by rapid context cancellation (#1023)

Description

This change fixes a connection leak when the context is canceled shortly after a net.Conn is created.

Checklist

• Code compiles correctly
• Created tests which fail without the change (if possible)
• All tests passing
• Extended the README / documentation, if necessary
• Added myself / the copyright holder to the AUTHORS file

[zjj]

I digged into the code,
When will the Connect's ctx.Err() return Error since I didn't find any cancel() to the Backgroud context ???

from context godoc

// If Done is not yet closed, Err returns nil.
// If Done is closed, Err returns a non-nil error explaining why:
// Canceled if the context was canceled
// or DeadlineExceeded if the context's deadline passed.
// After Err returns a non-nil error, successive calls to Err return the same error.
Err() error

[methane]

I can not understand your question. Please elaborate.

the Connect's ctx.Err() return Error

What? There is no ctx.Err() in this pull request.

I didn't find any cancel() to the Backgroud context ???

What is "the Background context"? I can not find background context in this pull request.

Do you know you can paste permalink of the code?

[zjj]

mysql/connection.go

Lines 595 to 598 in 15462c1

	// When ctx is already cancelled, don't watch it.
	if err := ctx.Err(); err != nil {
	return err
	}

above code, if the return value is an error, It must from L597
but the ctx here is from Connect, which is a BackgoudContext, when would the backgroudcontext's Err() is not nil ? since there's no any cancel() calls to a backgroud context.

mysql/connector.go

Lines 67 to 72 in 15462c1

	mc.startWatcher()
	if err := mc.watchCancel(ctx); err != nil {
	mc.cleanup()
	return nil, err
	}
	defer mc.finish()

mysql/driver.go

Lines 75 to 82 in 15462c1

	return nil, err
	}
	c := &connector{
	cfg: cfg,
	}
	return c.Connect(context.Background())
	}

maybe, I mistook something

[methane]

but the ctx here is from Connect, which is a BackgoudContext

Why do you think it is a BackgroundContext?

mysql/driver.go

Lines 75 to 82 in 15462c1

	return nil, err
	}
	c := &connector{
	cfg: cfg,
	}
	return c.Connect(context.Background())
	}

This is an legacy API which is designed before context era.
Connect() is called from database/sql in modern API.

https://github.com/golang/go/blob/39a9cb4b5dbf1e518b0c66fa3a7b4175f90226fc/src/database/sql/sql.go#L1078-L1083

[zjj]

@methane thx for your pantience. Finally, I found the gate...