PMM-6256 Updated ExplainCmd for MongoDB (#459)

* PMM-6256 Updated ExplainCmd for MongoDB

In order to make explain for MongoDB work, we need to remove the "$db"
field from the explain command generated in proto.system.profile since
it is a duplicated field that triggers a MongoDB error.

* PMM-6256 New test

* PMM-6256 Removed commented out code

Co-authored-by: Carlos <cfsalguero@gmail.com>

src/go/mongolib/proto/system.profile.go

@@ -178,7 +178,6 @@ func (self ExampleQuery) ExplainCmd() bson.D {
 					} else {
 						cmd = append(cmd[:i], cmd[i+1:]...)
 					}
-					break
 				}
 			}
 		}
@@ -250,6 +249,8 @@ func (self ExampleQuery) ExplainCmd() bson.D {
 			}
 		}
 	case "command":
+		cmd = sanitizeCommand(cmd)
+
 		if len(cmd) == 0 || cmd[0].Key != "group" {
 			break
 		}
@@ -284,3 +285,28 @@ func (self ExampleQuery) ExplainCmd() bson.D {
 		},
 	}
 }
+
+func sanitizeCommand(cmd bson.D) bson.D {
+	if len(cmd) < 1 {
+		return cmd
+	}
+
+	key := cmd[0].Key
+	if key != "count" && key != "distinct" {
+		return cmd
+	}
+
+	for i := range cmd {
+		// drop $db param as it is not supported in MongoDB 3.0
+		if cmd[i].Key == "$db" {
+			if len(cmd)-1 == i {
+				cmd = cmd[:i]
+			} else {
+				cmd = append(cmd[:i], cmd[i+1:]...)
+			}
+			break
+		}
+	}
+
+	return cmd
+}

src/go/mongolib/proto/system.profile_test.go

@@ -0,0 +1,44 @@
+package proto_test
+
+import (
+	"testing"
+
+	"github.com/percona/percona-toolkit/src/go/mongolib/proto"
+	"github.com/stretchr/testify/assert"
+	"go.mongodb.org/mongo-driver/bson"
+)
+
+func TestExplainCmd(t *testing.T) {
+	tests := []struct {
+		inDoc []byte
+		want  []byte
+	}{
+		{
+			inDoc: []byte(`{"ns":"sbtest.orders","op":"command","command":{"aggregate":"orders",` +
+				`"pipeline":[{"$match":{"status":"A"}},{"$group":{"_id":"$cust_id","total":{"$sum":"$amount"}}},` +
+				`{"$sort":{"total":-1}}],"cursor":{},"$db":"sbtest"}}`),
+			want: []byte(`{"explain":{"aggregate":"orders","pipeline":[{"$match":{"status":"A"}},` +
+				`{"$group":{"_id":"$cust_id","total":{"$sum":"$amount"}}},` +
+				`{"$sort":{"total":-1}}],"cursor":{},"$db":"sbtest"}}`),
+		},
+		{
+			inDoc: []byte(`{"ns":"sbtest.people","op":"command","command":` +
+				`{"count":"people","query":{},"fields":{},"$db":"sbtest"}}`),
+			want: []byte(`{"explain":{"count":"people","query":{},"fields":{}}}`),
+		},
+	}
+
+	for _, tc := range tests {
+		var want bson.D
+		err := bson.UnmarshalExtJSON(tc.want, false, &want)
+		assert.NoError(t, err)
+
+		var doc proto.SystemProfile
+		err = bson.UnmarshalExtJSON(tc.inDoc, false, &doc)
+		assert.NoError(t, err)
+
+		eq := proto.NewExampleQuery(doc)
+
+		assert.Equal(t, want, eq.ExplainCmd())
+	}
+}