Default single-node connstrs to direct connection; improve docs

README.md

@@ -13,9 +13,9 @@ curl -sSL https://raw.githubusercontent.com/mongodb-labs/migration-verifier/refs
 
 Then start a local replica set to store verification metadata:
 ```
-docker run -it -p27017:27017 -v ./verifier_db:/data/db --entrypoint bash mongodb/mongodb-community-server -c 'mongod --bind_ip_all --replSet rs & mpid=$! && until mongosh --eval "rs.initiate()"; do sleep 1; done && wait $mpid'
+podman run -it --rm -p27017:27017 -v ./verifier_db:/data/db --entrypoint bash docker.io/mongodb/mongodb-community-server -c 'mongod --bind_ip_all --replSet rs & mpid=$! && until mongosh --eval "rs.initiate({_id: \"rs\", members: [{_id: 0, host: \"localhost:27017\"}]})"; do sleep 1; done && wait $mpid'
 ```
-(This will create a local `verifier_db` directory so that you can resume verification if needed.)
+(This will create a local `verifier_db` directory so that you can resume verification if needed. Omit `-v` with its argument to avoid that.)
 
 Finally, run verification:
 ```
@@ -40,7 +40,7 @@ The verifier can alternatively store its metadata on the destination cluster. Th
 
 # More Details
 
-To see all options: 
+To see all options:
 
 
 ```
@@ -52,7 +52,7 @@ To check all namespaces:
 
 
 ```
-./migration_verifier --srcURI mongodb://127.0.0.1:27002 --dstURI mongodb://127.0.0.1:27003 --metaURI mongodb://127.0.0.1:27001 --verifyAll
+./migration_verifier --srcURI mongodb://127.0.0.1:27002 --dstURI mongodb://127.0.0.1:27003 --verifyAll
 ```
 
 
@@ -133,9 +133,9 @@ The verifier will now check to completion to make sure that there are no inconsi
 | Flag                                    | Description                                                                                                                                                                                 |
 |-----------------------------------------|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
 | `--configFile <value>`                  | path to an optional YAML config file                                                                                                                                                        |
-| `--srcURI <URI>`                        | source Host URI for migration verification (default: "mongodb://localhost:27017")                                                                                                           |
-| `--dstURI <URI>`                        | destination Host URI for migration verification (default: "mongodb://localhost:27018")                                                                                                      |
-| `--metaURI <URI>`                       | host URI for storing migration verification metadata (default: "mongodb://localhost:27019")                                                                                                 |
+| `--srcURI <URI>`                        | source Host URI for migration verification (required)                                                                                                           |
+| `--dstURI <URI>`                        | destination Host URI for migration verification (required)                                                                                                      |
+| `--metaURI <URI>`                       | host URI for storing migration verification metadata (default: "mongodb://localhost")                                                                                                 |
 | `--serverPort <port>`                   | port for the control web server (default: 27020)                                                                                                                                            |
 | `--logPath <path>`                      | logging file path (default: "stdout")                                                                                                                                                       |
 | `--numWorkers <number>`                 | number of worker threads to use for verification (default: 10)                                                                                                                              |

main/migration_verifier.go

@@ -11,6 +11,7 @@ import (
 	"time"
 
 	"github.com/10gen/migration-verifier/internal/verifier"
+	"github.com/10gen/migration-verifier/mmongo"
 	"github.com/10gen/migration-verifier/mslices"
 	"github.com/pkg/errors"
 	"github.com/rs/zerolog"
@@ -275,18 +276,31 @@ func handleArgs(ctx context.Context, cCtx *cli.Context) (*verifier.Verifier, err
 		Int("processID", os.Getpid()).
 		Msg("migration-verifier started.")
 
-	err := v.SetSrcURI(ctx, cCtx.String(srcURI))
+	srcConnStr := cCtx.String(srcURI)
+	_, srcConnStr, err := mmongo.MaybeAddDirectConnection(srcConnStr)
+	if err != nil {
+		return nil, errors.Wrap(err, "parsing source connection string")
+	}
+	err = v.SetSrcURI(ctx, srcConnStr)
 	if err != nil {
 		return nil, err
 	}
 
 	dstConnStr := cCtx.String(dstURI)
+	_, dstConnStr, err = mmongo.MaybeAddDirectConnection(dstConnStr)
+	if err != nil {
+		return nil, errors.Wrap(err, "parsing destination connection string")
+	}
 	err = v.SetDstURI(ctx, dstConnStr)
 	if err != nil {
 		return nil, err
 	}
 
 	metaConnStr := cCtx.String(metaURI)
+	_, metaConnStr, err = mmongo.MaybeAddDirectConnection(metaConnStr)
+	if err != nil {
+		return nil, errors.Wrap(err, "parsing metadata connection string")
+	}
 	err = v.SetMetaURI(ctx, metaConnStr)
 	if err != nil {
 		return nil, err

mmongo/connstring.go

@@ -0,0 +1,47 @@
+package mmongo
+
+import (
+	"fmt"
+	"strings"
+
+	"go.mongodb.org/mongo-driver/x/mongo/driver/connstring"
+)
+
+// MaybeAddDirectConnection adds the `directConnection` parameter
+// to the connection string if:
+//   - There is only 1 host.
+//   - The connection string lacks parameters that contraindicate a
+//     direct connection.
+//
+// This logic mimics mongosh’s behavior. See:
+// https://github.com/mongodb-js/mongosh/blob/fea739edfa86edc2a60756d9a9d478f87d94ddda/packages/arg-parser/src/uri-generator.ts#L308
+func MaybeAddDirectConnection(in string) (bool, string, error) {
+	cs, err := connstring.ParseAndValidate(in)
+
+	if err != nil {
+		return false, "", fmt.Errorf("parsing connection string %#q: %w", in, err)
+	}
+
+	var added bool
+
+	switch len(cs.Hosts) {
+	case 0:
+		return false, "", fmt.Errorf("connection string has no hosts?? (%#q)", in)
+	case 1:
+		if cs.ReplicaSet == "" && !cs.DirectConnectionSet && !cs.LoadBalancedSet {
+			if !strings.Contains(in, "?") {
+				if cs.Database == "" {
+					in += "/"
+				}
+
+				in += "?"
+			}
+
+			in += "directConnection=true"
+
+			added = true
+		}
+	}
+
+	return added, in, nil
+}