Fixes inability to authenticate through the older cmdQuery mechanism

internal/handler/cmd_query.go

@@ -19,6 +19,7 @@ import (
 	"fmt"
 	"strings"
 
+	"github.com/FerretDB/FerretDB/internal/clientconn/conninfo"
 	"github.com/FerretDB/FerretDB/internal/handler/common"
 	"github.com/FerretDB/FerretDB/internal/handler/handlererrors"
 	"github.com/FerretDB/FerretDB/internal/types"
@@ -40,7 +41,14 @@ func (h *Handler) CmdQuery(ctx context.Context, query *wire.OpQuery) (*wire.OpRe
 
 	// database name typically is either "$external" or "admin"
 
-	if cmd == "saslStart" && strings.HasSuffix(collection, ".$cmd") {
+	if cmd == "saslStart" && strings.HasSuffix(collection, ".$cmd") && query.Query().Mechanism() == "PLAIN" {
+		username, password, err := saslStartPlain(query.Query())
+		if err != nil {
+			return nil, err
+		}
+
+		conninfo.Get(ctx).SetAuth(username, password)
+
 		var emptyPayload types.Binary
 		reply := wire.OpReply{
 			NumberReturned: 1,

internal/types/document.go

@@ -278,6 +278,26 @@ func (d *Document) Command() string {
 	return d.fields[0].key
 }
 
+// Command returns the mechanism key. This is used for authentication.
+// It returns an empty string if document is nil or empty, or if the key doesn't exist.
+func (d *Document) Mechanism() string {
+	if d == nil || len(d.fields) == 0 {
+		return ""
+	}
+
+	val, err := d.Get("mechanism")
+	if err != nil {
+		return ""
+	}
+
+	stringVal, ok := val.(string)
+	if ok != true {
+		return ""
+	}
+
+	return stringVal
+}
+
 // Has returns true if the given key is present in the document.
 func (d *Document) Has(key string) bool {
 	_, ok := d.keys[key]