Failed to load docker image

php Editor Strawberry may encounter a common problem when using Docker, namely "failed to load Docker image". This problem may prevent us from using Docker to build and run containers normally. But don’t worry, there are usually multiple solutions to this problem. This article will introduce you to some common solutions to help you successfully load the Docker image and solve this annoying problem. Whether you are a beginner or an experienced Docker user, I hope this article will be helpful to you.

Question content

I am using golang, docker client to load the docker image in the .tar format.

func loadimagefromtar(cli *client.client, tarfilepath string) (string, error) {
    // read tar file
    tarfile, err := os.open(tarfilepath)
    if err != nil {
        return "", fmt.errorf("failed to open tar file: %w", err)
    }
    defer tarfile.close()

    // create a pipe to stream data between tar reader and docker client
    pr, pw := io.pipe()

    // set up a waitgroup for synchronization
    var wg sync.waitgroup
    wg.add(2)

    // load the docker image in a separate goroutine
    var imageloadresponse types.imageloadresponse
    go func() {
        defer wg.done()
        imageloadresponse, err = cli.imageload(context.background(), pr, false)
        if err != nil {
            err = fmt.errorf("failed to load docker image: %w", err)
        }
    }()

    // read tar file metadata and copy the tar file to the pipe writer in a separate goroutine
    var repotag string
    go func() {
        defer wg.done()
        defer pw.close()

        tarreader := tar.newreader(tarfile)

        for {
            header, err := tarreader.next()
            if err == io.eof {
                break
            }
            if err != nil {
                err = fmt.errorf("failed to read tar header: %w", err)
                fmt.printf("error: %v", err)
                return
            }

            // extract the repository and tag from the manifest file
            if header.name == "manifest.json" {
                data, err := io.readall(tarreader)
                if err != nil {
                    err = fmt.errorf("failed to read manifest file: %w", err)
                    fmt.printf("error: %v", err)
                    return
                }

                var manifest []map[string]interface{}
                err = json.unmarshal(data, &manifest)
                if err != nil {
                    err = fmt.errorf("failed to unmarshal manifest: %w", err)
                    fmt.printf("error: %v", err)
                    return
                }

                repotag = manifest[0]["repotags"].([]interface{})[0].(string)
            }

            // copy the tar file data to the pipe writer
            _, err = io.copy(pw, tarreader)
            if err != nil {
                err = fmt.errorf("failed to copy tar data: %w", err)
                fmt.printf("error: %v", err)
                return
            }
        }
    }()

    // wait for both goroutines to finish
    wg.wait()

    // check if any error occurred in the goroutines
    if err != nil {
        return "", err
    }

    // close the image load response body
    defer imageloadresponse.body.close()

    // get the image id
    imageid, err := getimageidbyrepotag(cli, repotag)
    if err != nil {
        return "", fmt.errorf("failed to get image id: %w", err)
    }

    return imageid, nil
}

// Function: getimageidbyrepotag

func getImageIDByRepoTag(cli *client.Client, repoTag string) (string, error) {
    images, err := cli.ImageList(context.Background(), types.ImageListOptions{})
    if err != nil {
        return "", fmt.Errorf("failed to list images: %w", err)
    }

    for _, image := range images {
        for _, tag := range image.RepoTags {
            if tag == repoTag {
                return image.ID, nil
            }
        }
    }

    return "", fmt.Errorf("image ID not found for repo tag: %s", repoTag)
}

getimageidbyrepotag Always returns fmt.errorf("Repository tagged image id: %s", repotag not found). Also, when I run docker images I don't see the images being loaded. It looks like the image loading hasn't finished yet.

In my other code, although the docker client cli.imageload returns immediately, the docker image load usually takes some time. I usually add about 30 seconds of wait time before checking getimageidbyrepotag. Adding a waiting time doesn't help in this case either.

Thank you

Solution

There are several questions:

• These two goroutines share err so some error handling may be lost
  • You should use a unique error variable for each goroutine here and check for both errors after wg.wait()
• Main problem: You are reading from a tar reader to find the manifest file and extract the tag information - which is fine - but once found, you copy the rest of the byte stream to a pipe middle. Therefore, you will lose a chunk of the byte stream that never reaches the docker client

To avoid reading the tar byte stream twice, you can use io.teereader. This allows you to read the tar archive - scanning the manifest file - but also write this stream entirely elsewhere (i.e. to the docker client).

Create teereader:

tr := io.teereader(tarfile, pw)  // reading `tr` will read the tarfile - but simultaneously write to `pw`

Image loading will now read from here (instead of a pipe):

//imageloadresponse, err = cli.imageload(context.background(), pr, false)
imageloadresponse, err = cli.imageload(context.background(), tr, false)

Then change your archive/tar reader to read from the pipe:

//tarreader := tar.newreader(tarfile) // direct from file
tarreader := tar.newreader(pr) // read from pipe (indirectly from the file)

You can then delete the io.copy block:

// no longer needed:
//
// _, err = io.copy(pw, tarreader)
//

Because the tar-inspection code will read the entire stream into eof.

Side note You may want to reset io.eof to nil to avoid thinking eof is one when checking for any potential errors from either goroutine More serious errors:

header, err = tarReader.Next()
if err == io.EOF {
    err = nil  //  EOF is a non-fatal error here
    break
}

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