init: warn if no requirements specified

When users call `packer init' on a template that does not specify a
`required_plugin' block, the command succeeds but does nothing, which is
not helpful for users that may expect their plugins to install.

To remedy that problem, we now output a message like what `packer
plugins required' does on templates without such a block, so that users
have an idea of what to change in order to get the command to work.

command/init.go

@@ -66,6 +66,14 @@ func (c *InitCommand) RunContext(buildCtx context.Context, cla *InitArgs) int {
 		return ret
 	}
 
+	if len(reqs) == 0 {
+		c.Ui.Message(`
+No plugins requirement found, make sure you reference a Packer config
+containing a packer.required_plugins block. See
+https://www.packer.io/docs/templates/hcl_templates/blocks/packer
+for more info.`)
+	}
+
 	opts := plugingetter.ListInstallationsOptions{
 		FromFolders: c.Meta.CoreConfig.Components.PluginConfig.KnownPluginFolders,
 		BinaryInstallationOptions: plugingetter.BinaryInstallationOptions{

command/init_test.go

@@ -13,6 +13,7 @@ import (
 	"path/filepath"
 	"runtime"
 	"sort"
+	"strings"
 	"testing"
 
 	"github.com/google/go-cmp/cmp"
@@ -414,3 +415,55 @@ func (opts initTestGoGetPlugin) fn(t *testing.T, _ testCaseInit) {
 		t.Fatalf("get: %v", err)
 	}
 }
+
+// TestInitCmd aims to test the init command, with output validation
+func TestInitCmd(t *testing.T) {
+	tests := []struct {
+		name         string
+		args         []string
+		expectedCode int
+		outputCheck  func(string, string) error
+	}{
+		{
+			name: "Ensure init warns on template without required_plugin blocks",
+			args: []string{
+				testFixture("hcl", "build-var-in-pp.pkr.hcl"),
+			},
+			expectedCode: 0,
+			outputCheck: func(stdout, stderr string) error {
+				if !strings.Contains(stdout, "No plugins requirement found") {
+					return fmt.Errorf("command should warn about plugin requirements not found, but did not")
+				}
+				return nil
+			},
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			c := &InitCommand{
+				Meta: TestMetaFile(t),
+			}
+
+			exitCode := c.Run(tt.args)
+			if exitCode != tt.expectedCode {
+				t.Errorf("process exit code mismatch: expected %d, got %d",
+					tt.expectedCode,
+					exitCode)
+			}
+
+			out, stderr := GetStdoutAndErrFromTestMeta(t, c.Meta)
+			err := tt.outputCheck(out, stderr)
+			if err != nil {
+				if len(out) != 0 {
+					t.Logf("command stdout: %q", out)
+				}
+
+				if len(stderr) != 0 {
+					t.Logf("command stderr: %q", stderr)
+				}
+				t.Error(err.Error())
+			}
+		})
+	}
+}