高版本 containerd 下载镜像失败

问题描述

在 containerd 运行时的 kubernetes 线上环境中,出现了镜像无法下载的情况,具体报错如下:

Failed to pull image `  `"ccr.ccs.tencentyun.com/tkeimages/tke-hpc-controller:v1.0.0"`  `: rpc error: code = NotFound desc = failed to pull and unpack image `  `"ccr.ccs.tencentyun.com/tkeimages/tke-hpc-controller:v1.0.0"`  `: failed to unpack image on snapshotter overlayfs: failed to extract layer sha256:d72a74c56330b347f7d18b64d2effd93edd695fde25dc301d52c37efbcf4844e: failed to get reader from content store: content digest sha256:2bf487c4beaa6fa7ea6e46ec1ff50029024ebf59f628c065432a16a940792b58: not found

containerd 的日志中也有相关日志:

containerd[136]: time="2020-11-19T16:11:56.975489200Z" level=info msg="PullImage \"redis:2.8.23\""
containerd[136]: time="2020-11-19T16:12:00.140053300Z" level=warning msg="reference for unknown type: application/octet-stream" digest="sha256:481995377a044d40ca3358e4203fe95eca1d58b98a1d4c2d9cec51c0c4569613" mediatype=application/octet-stream size=5946

尝试复现

分析环境信息:

  • container v1.4.3 运行时。
  • 基于 1.10 版本的 docker 制作的镜像(比如 dockerhub 镜像仓库中的 redis:2.8.23)。

然后根据以上版本信息构造相同环境,通过如下命令拉取镜像:

$ crictl pull docker.io/libraryredis:2.8.23
FATA[0001] pulling image failed: rpc error: code = NotFound desc = failed to pull and unpack image "docker.io/library/redis:2.8.23": failed to unpack image on snapshotter overlayfs: failed to extract layer sha256:4dcab49015d47e8f300ec33400a02cebc7b54cadd09c37e49eccbc655279da90: failed to get reader from content store: content digest sha256:51f5c6a04d83efd2d45c5fd59537218924bc46705e3de6ffc8bc07b51481610b: not found

问题复现,基本确认跟 containerd 版本与打包镜像的 docker 版本有关。

分析镜像下载的过程

在 containerd 运行时环境中,完整拉取一个镜像,主要会经历以下几步,如图所示:

接下来以 centos:latest 镜像的拉取过程为例。

  1. 将镜像名解析成 oci 规范里 descriptor

主要是 HEAD 请求,并且记录下返回中的 Content-TypeDocker-Content-Digest:

$ curl -v -X HEAD -H "Accept: application/vnd.docker.distribution.manifest.v2+json, application/vnd.docker.distribution.manifest.list.v2+json, application/vnd.oci.image.manifest.v1+json, application/vnd.oci.image.index.v1+json, */*" https://mirror.ccs.tencentyun.com/v2/library/centos/manifests/latest?ns=docker.io
...
< HTTP/1.1 200 OK
< Date: Mon, 17 May 2021 11:53:29 GMT
< Content-Type: application/vnd.docker.distribution.manifest.list.v2+json
< Content-Length: 762
< Connection: keep-alive
< Docker-Content-Digest: sha256:5528e8b1b1719d34604c87e11dcd1c0a20bedf46e83b5632cdeac91b8c04efc1
  1. 获取镜像的 list 列表:
$ curl -X GET -H "Accept: application/vnd.docker.distribution.manifest.list.v2+json"  https://mirror.ccs.tencentyun.com/v2/library/centos/manifests/sha256:5528e8b1b1719d34604c87e11dcd1c0a20bedf46e83b5632cdeac91b8c04efc1
{
    "manifests":[
        {
            "digest":"sha256:dbbacecc49b088458781c16f3775f2a2ec7521079034a7ba499c8b0bb7f86875",
            "mediaType":"application\/vnd.docker.distribution.manifest.v2+json",
            "platform":{
                "architecture":"amd64",
                "os":"linux"
            },
            "size":529
        },
        {
            "digest":"sha256:7723d6b5d15b1c64d0a82ee6298c66cf8c27179e1c8a458e719041ffd08cd091",
            "mediaType":"application\/vnd.docker.distribution.manifest.v2+json",
            "platform":{
                "architecture":"arm64",
                "os":"linux",
                "variant":"v8"
            },
            "size":529
        },
        ...
    "mediaType":"application\/vnd.docker.distribution.manifest.list.v2+json",
    "schemaVersion":2
}
  1. 获取特定操作系统上的镜像 manifest。由于宿主机的环境是 linux,所以 containerd 会选择适合该平台的镜像进行拉取:
$ curl -X GET -H "Accept: application/vnd.docker.distribution.manifest.v2+json"  https://mirror.ccs.tencentyun.com/v2/library/centos/manifests/sha256:dbbacecc49b08458781c16f3775f2a2ec7521079034a7ba499c8b0bb7f86875
{
   "schemaVersion": 2,
   "mediaType": "application/vnd.docker.distribution.manifest.v2+json",
   "config": {
      "mediaType": "application/vnd.docker.container.image.v1+json",
      "size": 2143,
      "digest": "sha256:300e315adb2f96afe5f0b2780b87f28ae95231fe3bdd1e16b9ba606307728f55"
   },
   "layers": [
      {
         "mediaType": "application/vnd.docker.image.rootfs.diff.tar.gzip",
         "size": 75181999,
         "digest": "sha256:7a0437f04f83f084b7ed68ad9c4a4947e12fc4e1b006b38129bac89114ec3621"
      }
   ]
}
  1. 拉取镜像的 config 和 layers。最后一步就是解析第三步中获取的 manifest,分别再下载镜像的 config 和 layers 就可以。

关于 mediaType:application/octet-stream

mediaType:application/octet-stream 是 docker 较早(docker v1.10 之前)支持的 docker/oci 标准,现在已经不支持了,而社区也任务该 mediaType 也太老了,所以 containerd 后续也就不再支持了 (详情可以参考 PR #5497) 。

定位根因

接下来以 redis:2.8.23 镜像的拉取过程为例说明一下拉取失败的原因。

  1. 将镜像名解析成 OCI 规范里 descriptor。这里还是 HEAD 请求,但返回中的 Content-Type 已经不是 list 类型了,而是 application/vnd.docker.distribution.manifest.v2+json:
$ curl -v -X HEAD -H "Accept: application/vnd.docker.distribution.manifest.v2+json, application/vnd.docker.distribution.manifest.list.v2+json, application/vnd.oci.image.manifest.v1+json, application/vnd.oci.image.index.v1+json, */*" https://mirror.ccs.tencentyun.com/v2/library/redis/manifests/2.8.23?ns=docker.io
...
< HTTP/1.1 200 OK
< Date: Thu, 20 May 2021 02:25:08 GMT
< Content-Type: application/vnd.docker.distribution.manifest.v2+json 
< Content-Length: 1968
< Connection: keep-alive
< Docker-Content-Digest: sha256:e507029ca6a11b85f8628ff16d7ff73ae54582f16fd757e64431f5ca6d27a13c
  1. 直接解析 manifest。因为 HEAD 请求中返回的是 manifest 类型,而不是 list 类型,所以这里会直接解析,解析出的 config 的 mediaType 是 application/octet-stream:
$ curl -X GET -H "Accept: application/vnd.docker.distribution.manifest.v2+json"  https://mirror.ccs.tencentyun.com/v2/library/redis/manifests/sha256:e507029ca6a11b85f8628ff16d7ff73ae54582f16fd757e64431f5ca6d27a13c
{
   "schemaVersion": 2,
   "mediaType": "application/vnd.docker.distribution.manifest.v2+json",
   "config": {
      "mediaType": "application/octet-stream", # 这里的 mediaType 是 application/octet-stream
      "size": 5946,
      "digest": "sha256:481995377a044d40ca3358e4203fe95eca1d58b98a1d4c2d9cec51c0c4569613"
   },
   "layers": [
      {
         "mediaType": "application/vnd.docker.image.rootfs.diff.tar.gzip",
         "size": 51356334,
         "digest": "sha256:51f5c6a04d83efd2d45c5fd59537218924bc46705e3de6ffc8bc07b51481610b"
      },
      ...
   ]
  1. containerd 中已经不支持 mediaType: application/octet-stream。 在 unpacker 逻辑中, containerd 已经不再支持 mediaType: application/octet-stream,导致了不会再下载 layers,故而出错。具体代码在 containerd/unpacker.go 中:
    case images.MediaTypeDockerSchema2Config, ocispec.MediaTypeImageConfig:

解决方案

如果遇到该问题,应基于新的 dockerd 运行时(>= docker v1.11)来重新构建镜像,并推送到镜像仓库中。

疑问:为什么 containerd v1.3.4 版本支持,而新版 v1.4.3 版本却不支持 ?

在 containerd v1.3.4 的版本中,合进了 PR #2814:bugfix: support application/octet-stream during pull ,支持了 mediaType:application/octet-stream 镜像格式的下载。

而在 v1.4.3 中,包含 PR #3870 ,又去掉了对 mediaType:application/octet-stream 镜像格式的支持,导致了 v1.3.4 和 v1.4.3 版本的行为不一致。

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