Default single-node connstrs to direct connection; improve docs (#143)

mongosh [defaults single-node connections to a direct connection](https://github.com/mongodb-js/mongosh/blob/fea739edfa86edc2a60756d9a9d478f87d94ddda/packages/arg-parser/src/uri-generator.ts#L308). This changeset applies that logic to migration-verifier so that, if a connection string includes only a single node, the verifier will always connect to it.

This is useful, e.g., when using a single, containerized mongod for the metadata DB and the mongod’s replication is set up to reference its hostname. In that case a direct connection is necessary when using Podman or other container engines that don’t “inject” the container name into the host’s name resolution (as, e.g., Docker Desktop on macOS does).

At the same time, this changeset avoids the above problem by explicitly making the documentation’s container one-liner bind to `localhost`.

Author: FGasper

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
+}