API list networks returns "null" instead of empty array when used with no networks

[arctic-alpaca]

Is this a BUG REPORT or FEATURE REQUEST? (leave only one on its own line)

/kind bug

Description

The list volumes API endpoint returns [] when called with no volumes, list networks returns null when used with no networks. According to the API docs, the behavior should be similar.

Steps to reproduce the issue:

1. podman system service unix:///home/`whoami`/testing.sock --log-level=debug --time=500

2. curl --unix-socket /home/`whoami`/testing.sock http://d/v3.2.0/libpod/networks/json
   null
   

Describe the results you received:
null

Describe the results you expected:
[] - an emtpy array.

Additional information you deem important (e.g. issue happens only occasionally):

Output of podman version:

Version:      3.2.0-dev
API Version:  3.2.0-dev
Go Version:   go1.15.10
Git Commit:   6ff56ab50a10e2350013c4ff1bf4b8d5d7b5aa87-dirty
Built:        Tue Jan  1 01:00:00 1980
OS/Arch:      linux/amd64

Output of podman info --debug:

host:
  arch: amd64
  buildahVersion: 1.20.1-dev
  cgroupManager: cgroupfs
  cgroupVersion: v1
  conmon:
    package: 'conmon: /usr/libexec/podman/conmon'
    path: /usr/libexec/podman/conmon
    version: 'conmon version 2.0.27, commit: '
  cpus: 12
  distribution:
    distribution: debian
    version: "10"
  eventLogger: file
  hostname: DESKTOP
  idMappings:
    gidmap:
    - container_id: 0
      host_id: 1000
      size: 1
    - container_id: 1
      host_id: 100000
      size: 65536
    uidmap:
    - container_id: 0
      host_id: 1000
      size: 1
    - container_id: 1
      host_id: 100000
      size: 65536
  kernel: 5.4.72-microsoft-standard-WSL2
  linkmode: static
  memFree: 12503470080
  memTotal: 13346447360
  ociRuntime:
    name: crun
    package: 'crun: /usr/bin/crun'
    path: /usr/bin/crun
    version: |-
      crun version 0.18.1-7931a-dirty
      commit: 7931a1eab0590eff4041c1f74e2844b297c31cea
      spec: 1.0.0
      +SYSTEMD +SELINUX +APPARMOR +CAP +SECCOMP +EBPF +CRIU +YAJL
  os: linux
  remoteSocket:
    path: /tmp/podman-run-1000/podman/podman.sock
  security:
    apparmorEnabled: false
    capabilities: CAP_CHOWN,CAP_DAC_OVERRIDE,CAP_FOWNER,CAP_FSETID,CAP_KILL,CAP_NET_BIND_SERVICE,CAP_SETFCAP,CAP_SETGID,CAP_SETPCAP,CAP_SETUID,CAP_SYS_CHROOT
    rootless: true
    seccompEnabled: true
    selinuxEnabled: false
  slirp4netns:
    executable: /usr/bin/slirp4netns
    package: 'slirp4netns: /usr/bin/slirp4netns'
    version: |-
      slirp4netns version 1.1.8
      commit: unknown
      libslirp: 4.3.1-git
      SLIRP_CONFIG_VERSION_MAX: 3
      libseccomp: 2.4.4
  swapFree: 4294967296
  swapTotal: 4294967296
  uptime: 51m 52.44s
registries:
  search:
  - docker.io
  - quay.io
store:
  configFile: /home/alpaca/.config/containers/storage.conf
  containerStore:
    number: 4
    paused: 0
    running: 0
    stopped: 4
  graphDriverName: overlay
  graphOptions:
    overlay.mount_program:
      Executable: /usr/bin/fuse-overlayfs
      Package: 'fuse-overlayfs: /usr/bin/fuse-overlayfs'
      Version: |-
        fusermount3 version: 3.4.1
        fuse-overlayfs: version 1.4
        FUSE library version 3.4.1
        using FUSE kernel interface version 7.27
  graphRoot: /home/alpaca/.local/share/containers/storage
  graphStatus:
    Backing Filesystem: extfs
    Native Overlay Diff: "false"
    Supports d_type: "true"
    Using metacopy: "false"
  imageStore:
    number: 15
  runRoot: /tmp/podman-run-1000/containers
  volumePath: /home/alpaca/.local/share/containers/storage/volumes
version:
  APIVersion: 3.2.0-dev
  Built: 315532800
  BuiltTime: Tue Jan  1 01:00:00 1980
  GitCommit: 6ff56ab50a10e2350013c4ff1bf4b8d5d7b5aa87-dirty
  GoVersion: go1.15.10
  OsArch: linux/amd64
  Version: 3.2.0-dev

Package info (e.g. output of rpm -q podman or apt list podman):

Static build from here.

Have you tested with the latest version of Podman and have you checked the Podman Troubleshooting Guide? (https://github.com/containers/podman/blob/master/troubleshooting.md)

Checked troubleshooting guide, didn't find any static builds from CI besides the one from the 3.1.2 release I downloaded earlier.

Additional environment details (AWS, VirtualBox, physical, etc.):
Windows 10, 64bit, WLS2, Debian 10

[vrothberg]

Thanks for reaching out!

@cdoern, are you interested in tackling the issue?

[cdoern]

@vrothberg yeah sure! I've been working on another issue with API endpoints for the past few days so I should be able to pick this one up.

[vrothberg]

Thanks, @cdoern! Feel free to ping me any time if you have a question.

[therealdjryan]

I am having what appears to be this issue while starting containers with podman-compose. Is there a workaround I can use until this fix is released? I apologize if the answer tis obvious, I am a novice with containers.

[zhangguanzhang]

I am having what appears to be this issue while starting containers with podman-compose. Is there a workaround I can use until this fix is released? I apologize if the answer tis obvious, I am a novice with containers.

https://github.com/containers/podman/pull/10658/files#diff-68d24fa81558aae3d8c59e2aa57a4fa719ea3b04d7fa14beff45f16f00858f50R21
could use the latest version v3.2.1