[PBM-1179] Warn about unsupported MongoDB version

agent/agent.go

@@ -68,7 +68,12 @@ func (a *Agent) CanStart() error {
 		return errors.New("mongos is not supported")
 	}
 
-	return nil
+	ver, err := pbm.GetMongoVersion(context.Background(), a.pbm.Conn)
+	if err != nil {
+		return errors.WithMessage(err, "get mongo version")
+	}
+
+	return pbm.FeatureSupport(ver).PBMSupport()
 }
 
 // Start starts listening the commands stream.

cli/cli.go

@@ -5,6 +5,7 @@ import (
 	"context"
 	"encoding/json"
 	"fmt"
+	stdlog "log"
 	"os"
 	"strings"
 	"time"
@@ -352,6 +353,14 @@ func Main() {
 			exitErr(errors.Wrap(err, "connect to mongodb"), pbmOutF)
 		}
 		pbmClient.InitLogger("", "")
+
+		ver, err := pbm.GetMongoVersion(ctx, pbmClient.Conn)
+		if err != nil {
+			stdlog.Fatalf("get mongo version: %v", err)
+		}
+		if err := pbm.FeatureSupport(ver).PBMSupport(); err != nil {
+			stdlog.Fatal(err.Error())
+		}
 	}
 
 	switch cmd {

pbm/version.go

@@ -47,6 +47,20 @@ func GetMongoVersion(ctx context.Context, m *mongo.Client) (MongoVersion, error)
 
 type FeatureSupport MongoVersion
 
+func (f FeatureSupport) PBMSupport() error {
+	v := MongoVersion(f)
+
+	if v.Version[0] == 4 && v.Version[1] == 4 {
+		return nil
+	}
+
+	if (v.Version[0] == 5 || v.Version[0] == 6) && v.Version[1] == 0 {
+		return nil
+	}
+
+	return errors.New("Unsupported MongoDB version. PBM works with v4.4, v5.0, v6.0")
+}
+
 func (f FeatureSupport) FullPhysicalBackup() bool {
 	// PSMDB 4.2.15, 4.4.6
 	v := MongoVersion(f)