Proposal: net: Add API to check connection Readability without blocking

[methane]

I'm a maintainer of go-sql-driver/mysql.
Since MySQL protocol is a request-response protocol, we don't have dedicated goroutine for Write() and Read(). We call them in the goroutine executing DB.Query() .

Sometimes, DB connection is closed from a server or other middlewares.
But we can not detect connection closed by peer until we call Read().

Since we send a query before reading result, we can not retry the query. Otherwise, the query may be executed twice. See also: https://golang.org/pkg/database/sql/driver/

To prevent duplicate operations, ErrBadConn should NOT be returned if there's a possibility that the database server might have performed the operation. Even if the server sends back an error, you shouldn't return ErrBadConn.

If we can detect connection closed by peer before sending a query, we can retry safely. But there is no easy way to do it now.

• Dedicated reader goroutine increase complexity. It will increase memory usage because we can not use same buffer for Read() and Write(). It will increase allocation because received data should be passed to the goroutine executing DB.Query().
• conn.SetReadDeadline(time.Now().Add(time.Millisecond)); conn.Read() introduces unnecessary sleep for normal case.
• Using SyscallConn is not easy, especially for cross platform.

So I want a easy way to check EOF. I have two ideas about it:

• c.Readable() bool -- Returns true when c.Read() will not block.
• c.ReadNonblock(buff []bytes) (int, error) -- Nonblocking variants of c.Read().

[agnivade]

Is the (*bufio).Peek method sufficient ? How do other sql drivers handle this scenario ?

/cc @kardianos

[kardianos]

The MS SQL driver spins up goroutines. That's what I recommen MySQL doing too.

Your proposal ends up racy or spinning I believe if you tried it. There was one proposal to expose the polling interface from the runtime, but I don't think it went anywhere.

[methane]

@kardianos Would you point out where go-mssqldb uses goroutine to check connection closed while it is idle (not waiting response)?

[rsc]

I'm confused about the proposal. The title says "check EOF" but the text seems to be about "check whether Read will proceed without blocking" (data available or EOF).

Assuming this is really about readability and not specifically EOF, this is a duplicate of #15735.

(Operating systems do provide us ways to watch general readability. I'm not sure how to watch EOF (without reading).) Closing as duplicate of #15735, since that's the one we could actually implement.

[methane]

@rsc It bit different. I want to check readable without blocking. I don't want to wait until readable. I don't want callbacks.
This is pseudo code:

func (mc mysqlConn) Query(query string, args... driver.Value) error {
    if mc.conn.Readable() {
        // Connection is closed from server.
        return mc.handleServerGone()
    }

    mc.sendQuery(query, args...)
    return mc.recvResult()
}

[ianlancetaylor]

@methane I suggest that you follow up on #15735. It sounds very similar.