[bug] regression from 5.02 - multi-network stacks broken on Docker API < 1.44 (regression in v5.1.0)

[jotka]

Bug Report

Summary

Docker Compose 5.1.0 broke multi-network stacks on Docker daemons older than API 1.44 (Docker Engine < 23.0), including Synology DSM 7.1 (API 1.41) and DSM 7.2 (API 1.43).

I maintain Dockhand, a Docker management application. Many of our users run Dockhand on Synology NAS devices (DSM 7.1/7.2), which ship with Docker 20.10 (API 1.41). This regression broke multi-network stack deployments for all of them when we updated docker-compose to 5.1.0. We had to cap docker-compose at 5.0.2 as a workaround while investigating the root cause.

Error Message

Container cannot be connected to network endpoints

This error comes from the Docker daemon when ContainerCreate receives multiple EndpointsConfig entries, which old daemons do not support.

Root Cause

Two changes in 5.1.0 combined to remove backward compatibility:

1. defaultNetworkSettings() refactor (commit fefdc9522)

In 5.0.2, extra networks were only added to EndpointsConfig for API >= 1.44:

// 5.0.2 — guarded by version check
if versions.GreaterThanOrEqualTo(version, APIVersion144) {
    for _, networkKey := range serviceNetworks {
        endpointsConfig[mobyNetworkName] = epSettings
    }
}

In 5.1.0, the guard was removed. Now ALL networks are unconditionally added to EndpointsConfig in ContainerCreate, even on old daemons:

// 5.1.0 — no guard, always adds all networks
for _, networkKey := range serviceNetworks {
    endpointsConfig[mobyNetworkName] = epSettings
}

2. The post-connect fallback was removed from convergence.go

In 5.0.2, createMobyContainer() had a fallback that connected extra networks one-by-one via NetworkConnect() for daemons older than API 1.44:

// 5.0.2 convergence.go — this was removed in 5.1.0
if versions.LessThan(apiVersion, APIVersion144) {
    for _, networkKey := range serviceNetworks {
        // skip primary (already in ContainerCreate)
        // NetworkConnect() for each extra network
    }
}

Affected Users

• Synology DSM 7.1 (Docker API 1.41 / Engine 20.10)
• Synology DSM 7.2 (Docker API 1.43 / Engine 23.0-pre)
• Any Docker Engine < 23.0 (API < 1.44)

Reproduction

# docker-compose.yml
services:
  web:
    image: nginx:alpine
    networks:
      - frontend
      - backend
networks:
  frontend:
  backend:

Run docker compose up -d against a Docker 20.10 daemon:

Error response from daemon: Container cannot be connected to network endpoints

Expected Behavior

Same behavior as 5.0.2: multi-network stacks work on all supported Docker daemon versions.

Fix

Restore the two behaviors that existed in 5.0.2:

1. In defaultNetworkSettings() (create.go): only include extra networks in EndpointsConfig when apiVersion >= 1.44
2. In createMobyContainer() (convergence.go): add back the post-connect fallback that calls NetworkConnect() for each extra network when apiVersion < 1.44

A fix is proposed in the linked PR.

Workaround

Downgrade to Docker Compose 5.0.2 or earlier.