GODRIVER-3361 Improve connection error message.

Author: qingyang-hu

x/mongo/driver/topology/connection.go

@@ -212,7 +212,7 @@ func (c *connection) connect(ctx context.Context) (err error) {
 	// Assign the result of DialContext to a temporary net.Conn to ensure that c.nc is not set in an error case.
 	tempNc, err := c.config.dialer.DialContext(ctx, c.addr.Network(), c.addr.String())
 	if err != nil {
-		return ConnectionError{Wrapped: err, init: true}
+		return ConnectionError{Wrapped: err, init: true, message: fmt.Sprintf("failed to connect to %s", c.addr)}
 	}
 	c.nc = tempNc
 
@@ -229,7 +229,7 @@ func (c *connection) connect(ctx context.Context) (err error) {
 		tlsNc, err := configureTLS(ctx, c.config.tlsConnectionSource, c.nc, c.addr, tlsConfig, ocspOpts)
 
 		if err != nil {
-			return ConnectionError{Wrapped: err, init: true}
+			return ConnectionError{Wrapped: err, init: true, message: fmt.Sprintf("failed to configure TLS for %s", c.addr)}
 		}
 		c.nc = tlsNc
 	}

x/mongo/driver/topology/errors.go

@@ -10,6 +10,10 @@ import (
 	"context"
 	"errors"
 	"fmt"
+	"io"
+	"net"
+	"os"
+	"strings"
 	"time"
 
 	"go.mongodb.org/mongo-driver/v2/x/mongo/driver/description"
@@ -28,21 +32,28 @@ type ConnectionError struct {
 
 // Error implements the error interface.
 func (e ConnectionError) Error() string {
-	message := e.message
+	var messages []string
 	if e.init {
-		fullMsg := "error occurred during connection handshake"
-		if message != "" {
-			fullMsg = fmt.Sprintf("%s: %s", fullMsg, message)
-		}
-		message = fullMsg
+		messages = append(messages, "error occurred during connection handshake")
 	}
-	if e.Wrapped != nil && message != "" {
-		return fmt.Sprintf("connection(%s) %s: %s", e.ConnectionID, message, e.Wrapped.Error())
+	if e.message != "" {
+		messages = append(messages, e.message)
 	}
 	if e.Wrapped != nil {
-		return fmt.Sprintf("connection(%s) %s", e.ConnectionID, e.Wrapped.Error())
+		if errors.Is(e.Wrapped, io.EOF) {
+			messages = append(messages, "connection closed unexpectedly by the other side")
+		}
+		if errors.Is(e.Wrapped, os.ErrDeadlineExceeded) {
+			messages = append(messages, "client timed out waiting for server response")
+		} else if err, ok := e.Wrapped.(net.Error); ok && err.Timeout() {
+			messages = append(messages, "client timed out waiting for server response")
+		}
+		messages = append(messages, e.Wrapped.Error())
+	}
+	if len(messages) > 0 {
+		return fmt.Sprintf("connection(%s) %s", e.ConnectionID, strings.Join(messages, ": "))
 	}
-	return fmt.Sprintf("connection(%s) %s", e.ConnectionID, message)
+	return fmt.Sprintf("connection(%s)", e.ConnectionID)
 }
 
 // Unwrap returns the underlying error.