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go-getter

CircleCI Go Documentation

go-getter is a library for Go (golang) for downloading files or directories from various sources using a URL as the primary form of input.

The power of this library is being flexible in being able to download from a number of different sources (file paths, Git, HTTP, Mercurial, etc.) using a single string as input. This removes the burden of knowing how to download from a variety of sources from the implementer.

The concept of a detector automatically turns invalid URLs into proper URLs. For example: "github.com/hashicorp/go-getter" would turn into a Git URL. Or "./foo" would turn into a file URL. These are extensible.

This library is used by Terraform for downloading modules and Nomad for downloading binaries.

Installation and Usage

Package documentation can be found on GoDoc.

Installation can be done with a normal go get:

$ go get github.com/hashicorp/go-getter

go-getter also has a command you can use to test URL strings:

$ go install github.com/hashicorp/go-getter/cmd/go-getter
...

$ go-getter github.com/foo/bar ./foo
...

The command is useful for verifying URL structures.

Security

Fetching resources from user-supplied URLs is an inherently dangerous operation and may leave your application vulnerable to server side request forgery, path traversal, denial of service or other security flaws.

go-getter contains mitigations for some of these security issues, but should still be used with caution in security-critical contexts. See the available security options that can be configured to mitigate some of these risks.

go-getter may return values that contain caller-provided query parameters that can contain sensitive data. Context around what parameters are and are not sensitive is known only by the caller of go-getter, and specific to each use case. We recommend the caller ensure that go-getter's return values (e.g., error messages) are properly handled and sanitized to ensure sensitive data is not persisted to logs.

URL Format

go-getter uses a single string URL as input to download from a variety of protocols. go-getter has various "tricks" with this URL to do certain things. This section documents the URL format.

Supported Protocols and Detectors

Protocols are used to download files/directories using a specific mechanism. Example protocols are Git and HTTP.

Detectors are used to transform a valid or invalid URL into another URL if it matches a certain pattern. Example: "github.com/user/repo" is automatically transformed into a fully valid Git URL. This allows go-getter to be very user friendly.

go-getter out of the box supports the following protocols. Additional protocols can be augmented at runtime by implementing the Getter interface.

In addition to the above protocols, go-getter has what are called "detectors." These take a URL and attempt to automatically choose the best protocol for it, which might involve even changing the protocol. The following detection is built-in by default:

Forced Protocol

In some cases, the protocol to use is ambiguous depending on the source URL. For example, "http://github.com/mitchellh/vagrant.git" could reference an HTTP URL or a Git URL. Forced protocol syntax is used to disambiguate this URL.

Forced protocol can be done by prefixing the URL with the protocol followed by double colons. For example: git::http://github.com/mitchellh/vagrant.git would download the given HTTP URL using the Git protocol.

Forced protocols will also override any detectors.

In the absence of a forced protocol, detectors may be run on the URL, transforming the protocol anyways. The above example would've used the Git protocol either way since the Git detector would've detected it was a GitHub URL.

Protocol-Specific Options

Each protocol can support protocol-specific options to configure that protocol. For example, the git protocol supports specifying a ref query parameter that tells it what ref to checkout for that Git repository.

The options are specified as query parameters on the URL (or URL-like string) given to go-getter. Using the Git example above, the URL below is a valid input to go-getter:

github.com/hashicorp/go-getter?ref=abcd1234

The protocol-specific options are documented below the URL format section. But because they are part of the URL, we point it out here so you know they exist.

Subdirectories

If you want to download only a specific subdirectory from a downloaded directory, you can specify a subdirectory after a double-slash //. go-getter will first download the URL specified before the double-slash (as if you didn't specify a double-slash), but will then copy the path after the double slash into the target directory.

For example, if you're downloading this GitHub repository, but you only want to download the testdata directory, you can do the following:

https://github.com/hashicorp/go-getter.git//testdata

If you downloaded this to the /tmp directory, then the file /tmp/archive.gz would exist. Notice that this file is in the testdata directory in this repository, but because we specified a subdirectory, go-getter automatically copied only that directory contents.

Subdirectory paths may also use filesystem glob patterns. The path must match exactly one entry or go-getter will return an error. This is useful if you're not sure the exact directory name but it follows a predictable naming structure.

For example, the following URL would also work:

https://github.com/hashicorp/go-getter.git//test-*

Checksumming

For file downloads of any protocol, go-getter can automatically verify a checksum for you. Note that checksumming only works for downloading files, not directories, but checksumming will work for any protocol.

To checksum a file, append a checksum query parameter to the URL. go-getter will parse out this query parameter automatically and use it to verify the checksum. The parameter value can be in the format of type:value or just value, where type is "md5", "sha1", "sha256", "sha512" or "file" . The "value" should be the actual checksum value or download URL for "file". When type part is omitted, type will be guessed based on the length of the checksum string. Examples:

./foo.txt?checksum=md5:b7d96c89d09d9e204f5fedc4d5d55b21

./foo.txt?checksum=b7d96c89d09d9e204f5fedc4d5d55b21

./foo.txt?checksum=file:./foo.txt.sha256sum

When checksumming from a file - ex: with checksum=file:url - go-getter will get the file linked in the URL after file: using the same configuration. For example, in file:http://releases.ubuntu.com/cosmic/MD5SUMS go-getter will download a checksum file under the aforementioned url using the http protocol. All protocols supported by go-getter can be used. The checksum file will be downloaded in a temporary file then parsed. The destination of the temporary file can be changed by setting system specific environment variables: TMPDIR for unix; TMP, TEMP or USERPROFILE on windows. Read godoc of os.TempDir for more information on the temporary directory selection. Content of files are expected to be BSD or GNU style. Once go-getter is done with the checksum file; it is deleted.

The checksum query parameter is never sent to the backend protocol implementation. It is used at a higher level by go-getter itself.

If the destination file exists and the checksums match: download will be skipped.

Unarchiving

go-getter will automatically unarchive files into a file or directory based on the extension of the file being requested (over any protocol). This works for both file and directory downloads.

go-getter looks for an archive query parameter to specify the format of the archive. If this isn't specified, go-getter will use the extension of the path to see if it appears archived. Unarchiving can be explicitly disabled by setting the archive query parameter to false.

The following archive formats are supported:

For example, an example URL is shown below:

./foo.zip

This will automatically be inferred to be a ZIP file and will be extracted. You can also be explicit about the archive type:

./some/other/path?archive=zip

And finally, you can disable archiving completely:

./some/path?archive=false

You can combine unarchiving with the other features of go-getter such as checksumming. The special archive query parameter will be removed from the URL before going to the final protocol downloader.

Protocol-Specific Options

This section documents the protocol-specific options that can be specified for go-getter. These options should be appended to the input as normal query parameters (HTTP headers are an exception to this, however). Depending on the usage of go-getter, applications may provide alternate ways of inputting options. For example, Nomad provides a nice options block for specifying options rather than in the URL.

General (All Protocols)

The options below are available to all protocols:

Local Files (file)

None

Git (git)

The git getter accepts both URL-style SSH addresses like git::ssh://git@example.com/foo/bar, and "scp-style" addresses like git::git@example.com/foo/bar. In the latter case, omitting the git:: force prefix is allowed if the username prefix is exactly git@.

The "scp-style" addresses cannot be used in conjunction with the ssh:// scheme prefix, because in that case the colon is used to mark an optional port number to connect on, rather than to delimit the path from the host.

Mercurial (hg)

HTTP (http)

Basic Authentication

To use HTTP basic authentication with go-getter, simply prepend username:password@ to the hostname in the URL such as https://Aladdin:OpenSesame@www.example.com/index.html. All special characters, including the username and password, must be URL encoded.

Headers

Optional request headers can be added by supplying them in a custom HttpGetter (not as query parameters like most other options). These headers will be sent out on every request the getter in question makes.

S3 (s3)

S3 takes various access configurations in the URL. Note that it will also read these from standard AWS environment variables if they're set. S3 compliant servers like Minio are also supported. If the query parameters are present, these take priority.

Using IAM Instance Profiles with S3

If you use go-getter and want to use an EC2 IAM Instance Profile to avoid using credentials, then just omit these and the profile, if available will be used automatically.

Using S3 with Minio

If you use go-gitter for Minio support, you must consider the following:

S3 Bucket Examples

S3 has several addressing schemes used to reference your bucket. These are listed here: https://docs.aws.amazon.com/AmazonS3/latest/userguide/access-bucket-intro.html

Some examples for these addressing schemes:

GCS (gcs)

GCS Authentication

In order to access to GCS, authentication credentials should be provided. More information can be found here

GCS Bucket Examples

GCS Testing

The tests for get_gcs.go require you to have GCP credentials set in your environment. These credentials can have any level of permissions to any project, they just need to exist. This means setting GOOGLE_APPLICATION_CREDENTIALS="~/path/to/credentials.json" or GOOGLE_CREDENTIALS="{stringified-credentials-json}". Due to this configuration, get_gcs_test.go will fail for external contributors in CircleCI.

Security Options

Disable Symlinks

In your getter client config, we recommend using the DisableSymlinks option, which prevents writing through or copying from symlinks (which may point outside the directory).

client := getter.Client{
    // This will prevent copying or writing files through symlinks
    DisableSymlinks: true,
}

Disable or Limit X-Terraform-Get

Go-Getter supports arbitrary redirects via the X-Terraform-Get header. This functionality exists to support Terraform use cases, but is likely not needed in most applications.

For code that uses the HttpGetter, add the following configuration options:

var httpGetter = &getter.HttpGetter{
    // Most clients should disable X-Terraform-Get
    // See the note below
    XTerraformGetDisabled: true,
    // Your software probably doesn’t rely on X-Terraform-Get, but
    // if it does, you should set the above field to false, plus
    // set XTerraformGet Limit to prevent endless redirects
    // XTerraformGetLimit: 10,
}

Enforce Timeouts

The HttpGetter supports timeouts and other resource-constraining configuration options. The GitGetter and HgGetter only support timeouts.

Configuration for the HttpGetter:

var httpGetter = &getter.HttpGetter{
    // Disable pre-fetch HEAD requests
    DoNotCheckHeadFirst: true,
    
    // As an alternative to the above setting, you can
    // set a reasonable timeout for HEAD requests
    // HeadFirstTimeout: 10 * time.Second,

    // Read timeout for HTTP operations
    ReadTimeout: 30 * time.Second,

    // Set the maximum number of bytes
    // that can be read by the getter
    MaxBytes: 500000000, // 500 MB
}

For code that uses the GitGetter or HgGetter, set the Timeout option:

var gitGetter = &getter.GitGetter{
    // Set a reasonable timeout for git operations
    Timeout: 5 * time.Minute,
}

var hgGetter = &getter.HgGetter{
    // Set a reasonable timeout for hg operations
    Timeout: 5 * time.Minute,
}