DB driver implementation via RPC (#17779)

This PR builds up on the pass-through DB driver to a fully functioning DB driver implementation via our RPC layer.

To keep things separate from the plugin RPC API, and have the ability to move fast with changes, a separate field Driver is added to MattermostPlugin. Typically the field which is required to be compatible are the API and Helpers. It would be well-documented that Driver is purely for internal use by Mattermost plugins.

A new Driver interface was created which would have a client and server implementation. Every object (connection, statement, etc.) is created and added to a map on the server side. On the client side, the wrapper structs hold the object id, and communicate via the RPC API using this id.

When the server gets the object id, it picks up the appropriate object from its map and performs the operation, and sends back the data.

Some things that need to be handled are errors. Typical error types like pq.Error and mysql.MySQLError are registered with encoding/gob. But for error variables like sql.ErrNoRows, a special integer is encoded with the ErrorString struct. And on the cilent side, the integer is checked, and the appropriate error variable is returned.

Some pending things:

- Context support. This is tricky. Since context.Context is an interface, it's not possible to marshal it. We have to find a way to get the timeout value from the context and pass it.
- RowsColumnScanType(rowsID string, index int) reflect.Type API. Again, reflect.Type is an interface.
- Master/Replica API support.

shared/driver/conn.go

@@ -5,15 +5,16 @@ package driver
 
 import (
 	"context"
-	"database/sql"
 	"database/sql/driver"
+
+	"github.com/mattermost/mattermost-server/v5/plugin"
 )
 
 // Conn is a DB driver conn implementation
-// which will just pass-through all queries to its
-// underlying connection.
+// which executes queries using the Plugin DB API.
 type Conn struct {
-	conn *sql.Conn
+	id  string
+	api plugin.Driver
 }
 
 // driverConn is a super-interface combining the basic
@@ -33,66 +34,84 @@ var (
 )
 
 func (c *Conn) Begin() (tx driver.Tx, err error) {
-	err = c.conn.Raw(func(innerConn interface{}) error {
-		tx, err = innerConn.(driver.Conn).Begin() //nolint:staticcheck
-		return err
-	})
-	return tx, err
+	txID, err := c.api.Tx(c.id, driver.TxOptions{})
+	if err != nil {
+		return nil, err
+	}
+
+	t := &wrapperTx{
+		id:  txID,
+		api: c.api,
+	}
+	return t, nil
 }
 
-func (c *Conn) BeginTx(ctx context.Context, opts driver.TxOptions) (_ driver.Tx, err error) {
-	t := &wrapperTx{}
-	err = c.conn.Raw(func(innerConn interface{}) error {
-		t.Tx, err = innerConn.(driver.ConnBeginTx).BeginTx(ctx, opts)
-		return err
-	})
-	return t, err
+func (c *Conn) BeginTx(_ context.Context, opts driver.TxOptions) (driver.Tx, error) {
+	txID, err := c.api.Tx(c.id, opts)
+	if err != nil {
+		return nil, err
+	}
+
+	t := &wrapperTx{
+		id:  txID,
+		api: c.api,
+	}
+	return t, nil
 }
 
-func (c *Conn) Prepare(q string) (_ driver.Stmt, err error) {
-	st := &wrapperStmt{}
-	err = c.conn.Raw(func(innerConn interface{}) error {
-		st.Stmt, err = innerConn.(driver.Conn).Prepare(q)
-		return err
-	})
-	return st, err
+func (c *Conn) Prepare(q string) (driver.Stmt, error) {
+	stID, err := c.api.Stmt(c.id, q)
+	if err != nil {
+		return nil, err
+	}
+
+	st := &wrapperStmt{
+		id:  stID,
+		api: c.api,
+	}
+	return st, nil
 }
 
-func (c *Conn) PrepareContext(ctx context.Context, q string) (_ driver.Stmt, err error) {
-	st := &wrapperStmt{}
-	err = c.conn.Raw(func(innerConn interface{}) error {
-		st.Stmt, err = innerConn.(driver.ConnPrepareContext).PrepareContext(ctx, q)
-		return err
-	})
-	return st, err
+func (c *Conn) PrepareContext(_ context.Context, q string) (driver.Stmt, error) {
+	stID, err := c.api.Stmt(c.id, q)
+	if err != nil {
+		return nil, err
+	}
+	st := &wrapperStmt{
+		id:  stID,
+		api: c.api,
+	}
+	return st, nil
 }
 
-func (c *Conn) ExecContext(ctx context.Context, q string, args []driver.NamedValue) (_ driver.Result, err error) {
-	res := &wrapperResult{}
-	err = c.conn.Raw(func(innerConn interface{}) error {
-		res.Result, err = innerConn.(driver.ExecerContext).ExecContext(ctx, q, args)
-		return err
-	})
-	return res, err
+func (c *Conn) ExecContext(_ context.Context, q string, args []driver.NamedValue) (driver.Result, error) {
+	resultContainer, err := c.api.ConnExec(c.id, q, args)
+	if err != nil {
+		return nil, err
+	}
+	res := &wrapperResult{
+		res: resultContainer,
+	}
+	return res, nil
 }
 
-func (c *Conn) QueryContext(ctx context.Context, q string, args []driver.NamedValue) (_ driver.Rows, err error) {
-	rows := &wrapperRows{}
-	err = c.conn.Raw(func(innerConn interface{}) error {
-		rows.Rows, err = innerConn.(driver.QueryerContext).QueryContext(ctx, q, args)
-		return err
-	})
-	return rows, err
+func (c *Conn) QueryContext(_ context.Context, q string, args []driver.NamedValue) (driver.Rows, error) {
+	rowsID, err := c.api.ConnQuery(c.id, q, args)
+	if err != nil {
+		return nil, err
+	}
+
+	rows := &wrapperRows{
+		id:  rowsID,
+		api: c.api,
+	}
+	return rows, nil
 }
 
-func (c *Conn) Ping(ctx context.Context) error {
-	return c.conn.Raw(func(innerConn interface{}) error {
-		return innerConn.(driver.Pinger).Ping(ctx)
-	})
+func (c *Conn) Ping(_ context.Context) error {
+	return c.api.ConnPing(c.id)
 }
 
 func (c *Conn) Close() error {
-	return c.conn.Raw(func(innerConn interface{}) error {
-		return innerConn.(driver.Conn).Close()
-	})
+	return c.api.ConnClose(c.id)
 }