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.

plugin/helpers.go

@@ -4,6 +4,8 @@
 package plugin
 
 import (
+	"database/sql/driver"
+
 	"github.com/mattermost/mattermost-server/v5/model"
 )
 
@@ -95,3 +97,55 @@ type Helpers interface {
 type HelpersImpl struct {
 	API API
 }
+
+// ResultContainer contains the output from the LastInsertID
+// and RowsAffected methods for a given set of rows.
+// It is used to embed another round-trip to the server,
+// and helping to avoid tracking results on the server.
+type ResultContainer struct {
+	LastID            int64
+	LastIDError       error
+	RowsAffected      int64
+	RowsAffectedError error
+}
+
+type Driver interface {
+	// Connection
+	Conn() (string, error)
+	ConnPing(connID string) error
+	ConnClose(connID string) error
+	ConnQuery(connID, q string, args []driver.NamedValue) (string, error)         // rows
+	ConnExec(connID, q string, args []driver.NamedValue) (ResultContainer, error) // result
+
+	// Transaction
+	Tx(connID string, opts driver.TxOptions) (string, error)
+	TxCommit(txID string) error
+	TxRollback(txID string) error
+
+	// Statement
+	Stmt(connID, q string) (string, error)
+	StmtClose(stID string) error
+	StmtNumInput(stID string) int
+	StmtQuery(stID string, args []driver.NamedValue) (string, error)         // rows
+	StmtExec(stID string, args []driver.NamedValue) (ResultContainer, error) // result
+
+	// Rows
+	RowsColumns(rowsID string) []string
+	RowsClose(rowsID string) error
+	RowsNext(rowsID string, dest []driver.Value) error
+	RowsHasNextResultSet(rowsID string) bool
+	RowsNextResultSet(rowsID string) error
+	RowsColumnTypeDatabaseTypeName(rowsID string, index int) string
+	RowsColumnTypePrecisionScale(rowsID string, index int) (int64, int64, bool)
+
+	// TODO: add this
+	// RowsColumnScanType(rowsID string, index int) reflect.Type
+
+	// Note: the following cannot be implemented because either MySQL or PG
+	// does not support it. So this implementation has to be a common subset
+	// of both DB implementations.
+	// RowsColumnTypeLength(rowsID string, index int) (int64, bool)
+	// RowsColumnTypeNullable(rowsID string, index int) (bool, bool)
+	// ResetSession(ctx context.Context) error
+	// IsValid() bool
+}