go(feat): Add the dry-run flag to gomplate (#2301)

config.go

@@ -76,6 +76,7 @@ type Config struct {
 
 	ExecPipe     bool `yaml:"execPipe,omitempty"`
 	Experimental bool `yaml:"experimental,omitempty"`
+	DryRun       bool `yaml:"dryRun,omitempty"`
 }
 
 // TODO: remove when we remove the deprecated array format for templates
@@ -107,6 +108,7 @@ type rawConfig struct {
 
 	ExecPipe     bool `yaml:"execPipe,omitempty"`
 	Experimental bool `yaml:"experimental,omitempty"`
+	DryRun       bool `yaml:"dryRun,omitempty"`
 }
 
 // TODO: remove when we remove the deprecated array format for templates
@@ -140,6 +142,7 @@ func (c *Config) UnmarshalYAML(value *yaml.Node) error {
 		PluginTimeout:         r.PluginTimeout,
 		ExecPipe:              r.ExecPipe,
 		Experimental:          r.Experimental,
+		DryRun:                r.DryRun,
 	}
 
 	return nil
@@ -170,6 +173,7 @@ func (c Config) MarshalYAML() (interface{}, error) {
 		PluginTimeout:         c.PluginTimeout,
 		ExecPipe:              c.ExecPipe,
 		Experimental:          c.Experimental,
+		DryRun:                c.DryRun,
 	}
 
 	return aux, nil
@@ -274,6 +278,10 @@ func (c *Config) MergeFrom(o *Config) *Config {
 		}
 	}
 
+	if !isZero(o.DryRun) {
+		c.DryRun = true
+	}
+
 	if !isZero(o.OutputMap) {
 		c.OutputDir = ""
 		c.OutputFiles = nil

gomplate.go

@@ -71,6 +71,17 @@ func Run(ctx context.Context, cfg *Config) error {
 	}
 	Metrics.TemplatesGathered = len(tmpl)
 
+	// Check for dry-run flag
+	slog.InfoContext(ctx, "dry-run", "value", cfg.DryRun)
+	if cfg.DryRun {
+		err = tr.RenderTemplatesToBuffer(ctx, cfg, namer, tmpl)
+		if err != nil {
+			return fmt.Errorf("failed to render templates: %w", err)
+		}
+
+		return nil
+	}
+
 	err = tr.RenderTemplates(ctx, tmpl)
 	if err != nil {
 		return err
@@ -133,6 +144,7 @@ func mappingNamer(outMap string, tr *renderer) outputNamer {
 			return "", fmt.Errorf("failed to render outputMap with ctx %+v and inPath %s: %w", tctx, inPath, err)
 		}
 
+		slog.Info("outputMap rendered", "in", inPath, "out", out.String())
 		return filepath.Clean(strings.TrimSpace(out.String())), nil
 	})
 }

internal/cmd/config.go

@@ -215,6 +215,12 @@ func cobraConfig(cmd *cobra.Command, args []string) (cfg *gomplate.Config, err e
 	if err != nil {
 		return nil, err
 	}
+
+	cfg.DryRun, err = getBool(cmd, "dry-run")
+	if err != nil {
+		return nil, err
+	}
+
 	return cfg, nil
 }
 

internal/cmd/main.go

@@ -161,6 +161,8 @@ func InitFlags(command *cobra.Command) {
 	command.Flags().BoolP("verbose", "V", false, "output extra information about what gomplate is doing")
 
 	command.Flags().String("config", defaultConfigFile, "config file (overridden by commandline flags)")
+
+	command.Flags().Bool("dry-run", false, "render the templates without writing the output files")
 }
 
 // Main -

render.go

@@ -1,6 +1,7 @@
 package gomplate
 
 import (
+	"bytes"
 	"context"
 	"fmt"
 	"io"
@@ -173,13 +174,7 @@ type Template struct {
 }
 
 func (r *renderer) RenderTemplates(ctx context.Context, templates []Template) error {
-	if datafs.FSProviderFromContext(ctx) == nil {
-		ctx = datafs.ContextWithFSProvider(ctx, DefaultFSProvider)
-	}
-
-	// configure the template context with the refreshed Data value
-	// only done here because the data context may have changed
-	tmplctx, err := createTmplContext(ctx, r.tctxAliases, r.sr)
+	ctx, tmplctx, err := r.initializeTemplateContext(ctx)
 	if err != nil {
 		return err
 	}
@@ -188,28 +183,33 @@ func (r *renderer) RenderTemplates(ctx context.Context, templates []Template) er
 }
 
 func (r *renderer) renderTemplatesWithData(ctx context.Context, templates []Template, tmplctx interface{}) error {
-	// update funcs with the current context
-	// only done here to ensure the context is properly set in func namespaces
-	f := CreateFuncs(ctx)
-
-	// add datasource funcs here because they need to share the source reader
-	addToMap(f, funcs.CreateDataSourceFuncs(ctx, r.sr))
-
-	// add user-defined funcs last so they override the built-in funcs
-	addToMap(f, r.funcs)
+	f := r.initializeTemplateFuncs(ctx)
 
 	// track some metrics for debug output
 	start := time.Now()
 	defer func() { Metrics.TotalRenderDuration = time.Since(start) }()
-	for _, template := range templates {
-		err := r.renderTemplate(ctx, template, f, tmplctx)
+	for _, t := range templates {
+		err := r.renderTemplate(ctx, t, f, tmplctx)
 		if err != nil {
 			return fmt.Errorf("renderTemplate: %w", err)
 		}
 	}
 	return nil
 }
 
+func (r *renderer) initializeTemplateFuncs(ctx context.Context) template.FuncMap {
+	// update funcs with the current context
+	// only done here to ensure the context is properly set in func namespaces
+	f := CreateFuncs(ctx)
+
+	// add datasource funcs here because they need to share the source reader
+	addToMap(f, funcs.CreateDataSourceFuncs(ctx, r.sr))
+
+	// add user-defined funcs last so they override the built-in funcs
+	addToMap(f, r.funcs)
+	return f
+}
+
 func (r *renderer) renderTemplate(ctx context.Context, template Template, f template.FuncMap, tmplctx interface{}) error {
 	if template.Writer != nil {
 		if wr, ok := template.Writer.(io.Closer); ok {
@@ -335,6 +335,58 @@ func (r *renderer) parseNestedTemplates(ctx context.Context, tmpl *template.Temp
 	return nil
 }
 
+func (r *renderer) RenderTemplatesToBuffer(ctx context.Context, cfg *Config, namer outputNamer, tmpl []Template) error {
+	ctx, tmplctx, err := r.initializeTemplateContext(ctx)
+	if err != nil {
+		return err
+	}
+
+	f := r.initializeTemplateFuncs(ctx)
+
+	// track some metrics for debug output
+	start := time.Now()
+	defer func() { Metrics.TotalRenderDuration = time.Since(start) }()
+	for _, t := range tmpl {
+		inFile := t.Name
+		if cfg.InputDir != "" {
+			inFile = strings.Replace(inFile, cfg.InputDir, "", 1)
+		}
+
+		outFile, err := namer.Name(ctx, inFile)
+		fmt.Println("---")
+		fmt.Println("##::gomplate::output::file", outFile)
+		fmt.Println("##::gomplate::template::file", t.Name)
+		fmt.Println("---")
+		var buf bytes.Buffer
+		// Set the template's writer to the buffer
+		t.Writer = &buf
+		err = r.renderTemplate(ctx, t, f, tmplctx)
+		if err != nil {
+			return fmt.Errorf("renderTemplate: %w", err)
+		}
+
+		// Print the rendered content to the console
+		fmt.Println(buf.String() + "\n")
+	}
+
+	return nil
+}
+
+func (r *renderer) initializeTemplateContext(ctx context.Context) (context.Context, interface{}, error) {
+	if datafs.FSProviderFromContext(ctx) == nil {
+		ctx = datafs.ContextWithFSProvider(ctx, DefaultFSProvider)
+	}
+
+	// configure the template context with the refreshed Data value
+	// only done here because the data context may have changed
+	tmplctx, err := createTmplContext(ctx, r.tctxAliases, r.sr)
+	if err != nil {
+		return nil, nil, err
+	}
+
+	return ctx, tmplctx, nil
+}
+
 func parseNestedTemplateDir(ctx context.Context, fsys fs.FS, alias, fname string, tmpl *template.Template) error {
 	files, err := fs.ReadDir(fsys, fname)
 	if err != nil {