feat: Update client capability flags

Adjust client capability flags based on server support,
and account for explicitly disabled capabilities.

- client/auth.go: Update client capability flags
- client/conn.go: Add field to manage disabled capabilities

Author: masahide

client/auth.go

@@ -211,8 +211,15 @@ func (c *Conn) writeAuthHandshake() error {
 	capability := mysql.CLIENT_PROTOCOL_41 | mysql.CLIENT_SECURE_CONNECTION |
 		mysql.CLIENT_LONG_PASSWORD | mysql.CLIENT_TRANSACTIONS | mysql.CLIENT_PLUGIN_AUTH
 	// Adjust client capability flags based on server support
-	capability |= c.capability & mysql.CLIENT_LONG_FLAG
-	capability |= c.capability & mysql.CLIENT_QUERY_ATTRIBUTES
+	// ---- Inherit capabilities that the server has and the user has NOT explicitly denied ----
+	inherit := c.capability & ^c.dcaps             // Server-side capabilities minus explicit denies
+	capability |= inherit & mysql.CLIENT_LONG_FLAG // Existing
+	// ---- Handling of CLIENT_QUERY_ATTRIBUTES ----
+	if c.ccaps&mysql.CLIENT_QUERY_ATTRIBUTES != 0 { // Explicitly ON
+		capability |= mysql.CLIENT_QUERY_ATTRIBUTES
+	} else if inherit&mysql.CLIENT_QUERY_ATTRIBUTES != 0 { // Server has it and no denial
+		capability |= mysql.CLIENT_QUERY_ATTRIBUTES
+	}
 	// Adjust client capability flags on specific client requests
 	// Only flags that would make any sense setting and aren't handled elsewhere
 	// in the library are supported here
@@ -361,5 +368,7 @@ func (c *Conn) writeAuthHandshake() error {
 		data[pos] = 0x03
 	}
 
+	c.capability = capability // update capability to the one we sent
+
 	return c.WritePacket(data)
 }

client/conn.go

@@ -45,6 +45,7 @@ type Conn struct {
 	capability uint32
 	// client-set capabilities only
 	ccaps uint32
+	dcaps uint32 // disabled capabilities
 
 	attributes map[string]string
 
@@ -236,12 +237,14 @@ func (c *Conn) Ping() error {
 
 // SetCapability enables the use of a specific capability
 func (c *Conn) SetCapability(cap uint32) {
+	c.dcaps &^= cap
 	c.ccaps |= cap
 }
 
 // UnsetCapability disables the use of a specific capability
 func (c *Conn) UnsetCapability(cap uint32) {
-	c.ccaps &= ^cap
+	c.ccaps &^= cap
+	c.dcaps |= cap
 }
 
 // HasCapability returns true if the connection has the specific capability