client: make CLIENT_QUERY_ATTRIBUTES opt-out to restore proxy compatibility

[masahide]

Background — why proxies built with this SDK break

masahide/mysql8-audit-proxy#8

A common architecture for a MySQL audit / filter / router built on go-mysql looks like this:

[application] -- go-mysql/server  -->  [proxy]  -- go-mysql/client -->  [mysqld]

• Downstream side (go-mysql/server)
  Implements the old 5.7/8.0 protocol and does not recognise
  CLIENT_QUERY_ATTRIBUTES.
  When an application connects, the handshake therefore has the bit cleared.

• Upstream side (go-mysql/client ≥ v1.12.0)
  In writeAuthHandshake() the client automatically copies
  CLIENT_QUERY_ATTRIBUTES from the server greeting, claiming the proxy
  will send query attributes.

• Mismatch
  The proxy simply forwards the raw COM_QUERY it received from the
  application.
  Because the downstream never added the attributes section, the packet sent
  upstream is missing mandatory fields → MySQL returns
  ERROR 1835 (HY000) Malformed communication packet.

What the proxy author needs

A way to say “don’t advertise CLIENT_QUERY_ATTRIBUTES on the upstream
connection even if the server supports it, because my downstream side can’t
handle that mode.”

Currently that is impossible: UnsetCapability() removes the flag from
ccaps, but writeAuthHandshake() puts it back from the server’s flag set.

Patch overview

1. Add dcaps (“deny caps”) to client.Conn to track flags explicitly
   disabled by the caller.

2. Modify SetCapability / UnsetCapability:

   SetCapability(x): ccaps |= x ; dcaps &^= x
   UnsetCapability(x): ccaps &^= x ; dcaps |= x

3. In writeAuthHandshake():

   inherit := c.capability &^ c.dcaps          // server flags minus explicit denies
   // ...
   // Advertise QUERY_ATTRIBUTES only if:
   //   a) caller opted-in  OR
   //   b) server supports it AND caller did NOT opt-out

4. Store the final negotiated set back into c.capability for later use.

Resulting behaviour

Scenario	Downstream (server pkg)	Proxy call	Upstream handshake
Default	no support	none	bit set (auto)
Proxy wants legacy mode	no support	UnsetCapability(CLIENT_QUERY_ATTRIBUTES)	bit cleared
Force new mode anyway	anything	SetCapability(CLIENT_QUERY_ATTRIBUTES)	bit set

Example — disabling the flag in a proxy

backend, _ := client.Connect(
    upstreamAddr, user, pass, db,
    func(c *client.Conn) error {
        c.UnsetCapability(mysql.CLIENT_QUERY_ATTRIBUTES)
        return nil
    },
)

With this one-liner the proxy once again works with both modern MySQL servers
and the existing go-mysql/server front end.

Safety & compatibility

• No API break – existing code compiles; only behaviour changes when the
  caller explicitly opts out.
• Maintains the convenient “auto-enable” path for direct clients.
• Restores plug-and-play compatibility for every proxy built on
  go-mysql/server.

[Copilot]

Pull Request Overview

This pull request implements an opt-out mechanism for the CLIENT_QUERY_ATTRIBUTES capability to restore proxy compatibility with legacy MySQL servers. The key changes include:

• Adding a new field (dcaps) to track explicitly disabled capabilities.
• Updating the SetCapability and UnsetCapability functions to modify both ccaps and dcaps.
• Revising the capability negotiation logic in writeAuthHandshake to conditionally advertise CLIENT_QUERY_ATTRIBUTES.

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
client/conn.go	Introduced dcaps and updated SetCapability/UnsetCapability functions.
client/auth.go	Adjusted capability inheritance logic in writeAuthHandshake.

Comments suppressed due to low confidence (2)

client/conn.go:48

• [nitpick] Consider renaming 'dcaps' to 'disabledCaps' for improved clarity.

dcaps uint32 // disabled capabilities

client/auth.go:215

• [nitpick] Optionally, consider renaming 'inherit' to 'inheritedCaps' to more clearly convey its purpose.

inherit := c.capability & ^c.dcaps             // Server-side capabilities minus explicit denies

[lance6716]

lgtm at the first glance. If you are not urgent, I want to check if there are other potential problems before merging this PR.

[dveeden]

Seems fine to me on first glance

[dveeden]

maybe name this inheritedCaps or inherited?

[lance6716]

I see you followed the naming style of ccaps. Just a weak comment: capability, ccaps and dcaps are hard to understand at their usage functions. Maybe rename them to serverCaps, clientExplicitOnCaps, clientExplicitOffCaps or something.

[lance6716]

I think we don't need to specially process CLIENT_QUERY_ATTRIBUTES. There are 3 states of capability:

1. developer calls SetCapability, which means the ccaps bit is 1 and the dcaps bit is 0
2. developer calls UnsetCapability, which means ccaps bit is 0 and the dcaps bit is 1
3. No calls, which means both bits are 0

So here we turn on the bits by ccaps at around line 219, and have another round to turn off bits by dcaps. I suggest

capability |= c.ccaps&mysql.CLIENT_FOUND_ROWS | ...
capability &= c.dcaps... // ⬅️ here we add CLIENT_QUERY_ATTRIBUTES and other flags from above line to turn off