pkg/terraform: use JSON output while checking version

As it is more idiomatic than using Sscanf. '-json' parameter is a new
option added in Terraform 0.13.

Signed-off-by: Mateusz Gozdek <mateusz@kinvolk.io>

Author: invidian

pkg/terraform/executor.go

@@ -518,25 +518,22 @@ func (ex *Executor) logPath(id int) string {
 }
 
 func (ex *Executor) checkVersion() error {
-	vOutput, err := ex.executeSync("--version")
+	versionOutputRaw, err := ex.executeSync("version", "-json")
 	if err != nil {
-		return fmt.Errorf("executing 'terraform --version': %w", err)
+		return fmt.Errorf("executing 'terraform version -json': %w", err)
 	}
 
-	format := "Terraform v%s\n"
-	var vStr string
-	n, err := fmt.Sscanf(string(vOutput), format, &vStr)
-	if err != nil {
-		return fmt.Errorf("output %q does not match format %q: %w", string(vOutput), format, err)
-	}
+	versionOutput := struct {
+		TerraformVersion string `json:"terraform_version"`
+	}{}
 
-	if n != 1 {
-		return fmt.Errorf("version not found in 'terraform --version' output")
+	if err := json.Unmarshal(versionOutputRaw, &versionOutput); err != nil {
+		return fmt.Errorf("unmarshaling version command output %q: %w", string(versionOutputRaw), err)
 	}
 
-	v, err := version.NewVersion(vStr)
+	v, err := version.NewVersion(versionOutput.TerraformVersion)
 	if err != nil {
-		return fmt.Errorf("parsing Terraform version %q: %w", vStr, err)
+		return fmt.Errorf("parsing Terraform version %q: %w", versionOutput.TerraformVersion, err)
 	}
 
 	// requiredVersion is const, so we test it in unit tests.

pkg/terraform/executor_test.go

@@ -31,21 +31,50 @@ func TestVersionConstraint(t *testing.T) {
 		expectError bool
 	}{
 		"valid": {
-			output: "Terraform v0.13.10",
+			output: `{
+  "terraform_version": "0.13.10",
+  "terraform_revision": "",
+  "provider_selections": {},
+  "terraform_outdated": false
+}
+`,
 		},
 		"outdated": {
-			output:      "Terraform v0.11.0",
+			output: `{
+  "terraform_version": "0.11.0",
+  "terraform_revision": "",
+  "provider_selections": {},
+  "terraform_outdated": false
+}
+`,
 			expectError: true,
 		},
 		"unsupported": {
-			output:      "Terraform v0.14.5",
+			output: `{
+  "terraform_version": "0.14.5",
+  "terraform_revision": "",
+  "provider_selections": {},
+  "terraform_outdated": false
+}
+`,
 			expectError: true,
 		},
-		"with extra test": {
+		"not JSON": {
 			output: `Terraform v0.13.11
 
 Your version of Terraform is out of date! The latest version
 is 0.13.3. You can update by downloading from https://www.terraform.io/downloads.html`,
+			expectError: true,
+		},
+		"empty version field": {
+			output: `{
+  "terraform_version": "",
+  "terraform_revision": "",
+  "provider_selections": {},
+  "terraform_outdated": false
+}
+`,
+			expectError: true,
 		},
 	}