feat: support terraform_version_file

README.md

@@ -31,6 +31,15 @@ steps:
     terraform_version: "1.1.7"
 ```
 
+A specific version of Terraform CLI can be installed using a version file:
+
+```yaml
+steps:
+- uses: hashicorp/setup-terraform@v3
+  with:
+    terraform_version_file: ".tool-versions"
+```
+
 Credentials for HCP Terraform ([app.terraform.io](https://app.terraform.io/)) can be configured:
 
 ```yaml
@@ -251,10 +260,24 @@ The action supports the following inputs:
    for available range specifications). Examples are: `"<1.2.0"`, `"~1.1.0"`, `"1.1.7"` (all three installing
    the latest available `1.1` version). Prerelease versions can be specified and a range will stay within the
    given tag such as `beta` or `rc`. If no version is given, it will default to `latest`.
+- `terraform_version_file` - (optional) The path to a file containing terraform version. Supported file types are `.tool-versions` or `.terraform-version`. See more details in [about version-file](#Terraform-version-file).
 - `terraform_wrapper` - (optional) Whether to install a wrapper to wrap subsequent calls of
    the `terraform` binary and expose its STDOUT, STDERR, and exit code as outputs
    named `stdout`, `stderr`, and `exitcode` respectively. Defaults to `true`.
 
+### Terraform version file
+
+If the `terraform_version_file` input is specified, the action will extract the version from the file and install it.
+
+Supported files names are `.tool-versions` or `.terraform-version`.
+In `.tool-versions` file, terraform version should be preceded by the terraform keyword (e.g., `terraform 1.13.0`).
+The `.tool-versions` file supports version specifications in accordance with Semantic Versioning ([semver](https://semver.org/)) and [Semver Ranges](https://www.npmjs.com/package/semver#ranges).
+The `.terraform-version` file supports version specifications as explained in the `terraform_version` input.
+
+If both `terraform_version` and `terraform_version_file` inputs are provided, the `terraform_version` input will be used.
+
+If the file contains multiple versions, only the first one will be recognized.
+
 ## Outputs
 
 This action does not configure any outputs directly. However, when you set the `terraform_wrapper` input

action.yml

@@ -11,7 +11,9 @@ inputs:
     required: false
   terraform_version:
     description: 'The version of Terraform CLI to install. Instead of full version string you can also specify constraint string starting with "<" (for example `<1.13.0`) to install the latest version satisfying the constraint. A value of `latest` will install the latest version of Terraform CLI. Defaults to `latest`.'
-    default: 'latest'
+    required: false
+  terraform_version_file:
+    description: 'The path to the `.terraform-version` file. See examples of supported syntax in README file'
     required: false
   terraform_wrapper:
     description: 'Whether or not to install a wrapper to wrap subsequent calls of the `terraform` binary and expose its STDOUT, STDERR, and exit code as outputs named `stdout`, `stderr`, and `exitcode` respectively. Defaults to `true`.'

lib/setup-terraform.js

@@ -121,14 +121,70 @@ credentials "${credentialsHostname}" {
   await fs.writeFile(credsFile, creds);
 }
 
+async function getVersionFromFileContent (versionFile) {
+  if (!versionFile) {
+    return;
+  }
+
+  let versionRegExp;
+  const versionFileName = path.basename(versionFile);
+  if (versionFileName === '.tool-versions') {
+    versionRegExp = /^(terraform\s+)(?<version>[^\s]+)$/m;
+  } else if (versionFileName === '.terraform-version') {
+    versionRegExp = /(?<version>[^\s]+)/;
+  } else {
+    return;
+  }
+
+  try {
+    const content = fs.readFileSync(versionFile).toString().trim();
+    let fileContent = '';
+    if (content.match(versionRegExp)?.groups?.version) {
+      fileContent = content.match(versionRegExp)?.groups?.version;
+    }
+    if (!fileContent) {
+      return;
+    }
+    core.debug(`Version from file '${fileContent}'`);
+    return fileContent;
+  } catch (error) {
+    if (error.code === 'ENOENT') {
+      return;
+    }
+    throw error;
+  }
+}
+
+// get the Terraform version from the action inputs
+async function getTerraformVersion (versionInput, versionFile) {
+  const DEFAULT_VERSION = 'latest';
+
+  let version = versionInput;
+  if (!versionInput && !versionFile) {
+    core.info(`Set default value for version to ${DEFAULT_VERSION}`);
+    version = DEFAULT_VERSION;
+  }
+
+  if (!version && versionFile) {
+    version = await getVersionFromFileContent(versionFile);
+    if (!version) {
+      throw new Error(`No supported version was found in file ${versionFile}`);
+    }
+  }
+  return version;
+}
+
 async function run () {
   try {
     // Gather GitHub Actions inputs
-    const version = core.getInput('terraform_version');
+    const versionInput = core.getInput('terraform_version', { required: false });
+    const versionFile = core.getInput('terraform_version_file', { required: false });
     const credentialsHostname = core.getInput('cli_config_credentials_hostname');
     const credentialsToken = core.getInput('cli_config_credentials_token');
     const wrapper = core.getInput('terraform_wrapper') === 'true';
 
+    const version = await getTerraformVersion(versionInput, versionFile);
+
     // Gather OS details
     const osPlatform = os.platform();
     const osArch = os.arch();