Only open output files when necessary

Signed-off-by: Dave Henderson <dhenderson@gmail.com>

1 files changed, 16 insertions, 11 deletions

diff --git a/template.go b/template.go
index b500ce73..448eff84 100644
--- a/template.go
+++ b/template.go
@@ -9,7 +9,7 @@ import (
 	"text/template"
 
 	"github.com/hairyhenderson/gomplate/v3/internal/config"
-	"github.com/hairyhenderson/gomplate/v3/internal/writers"
+	"github.com/hairyhenderson/gomplate/v3/internal/iohelpers"
 	"github.com/hairyhenderson/gomplate/v3/tmpl"
 
 	"github.com/spf13/afero"
@@ -101,7 +101,7 @@ func gatherTemplates(cfg *config.Config, outFileNamer func(string) (string, erro
 
 	// --exec-pipe redirects standard out to the out pipe
 	if cfg.OutWriter != nil {
-		Stdout = &writers.NopCloser{Writer: cfg.OutWriter}
+		Stdout = &iohelpers.NopCloser{Writer: cfg.OutWriter}
 	}
 
 	switch {
@@ -227,7 +227,7 @@ func fileToTemplates(inFile, outFile string, mode os.FileMode, modeOverride bool
 
 func openOutFile(cfg *config.Config, filename string, mode os.FileMode, modeOverride bool) (out io.WriteCloser, err error) {
 	if cfg.SuppressEmpty {
-		out = writers.NewEmptySkipper(func() (io.WriteCloser, error) {
+		out = iohelpers.NewEmptySkipper(func() (io.WriteCloser, error) {
 			if filename == "-" {
 				return Stdout, nil
 			}
@@ -260,14 +260,19 @@ func createOutFile(filename string, mode os.FileMode, modeOverride bool) (out io
 	}
 
 	// if the output file already exists, we'll use a SameSkipper
-	f, err := fs.OpenFile(filename, os.O_RDONLY, mode.Perm())
+	fi, err := fs.Stat(filename)
 	if err != nil {
-		// likely means the file just doesn't exist - open's error will be more useful
-		return open()
+		// likely means the file just doesn't exist - further errors will be more useful
+		return iohelpers.LazyWriteCloser(open), nil
 	}
-	out = writers.SameSkipper(f, func() (io.WriteCloser, error) {
-		return open()
-	})
+	if fi.IsDir() {
+		// error because this is a directory
+		return nil, isDirError(fi.Name())
+	}
+
+	out = iohelpers.SameSkipper(iohelpers.LazyReadCloser(func() (io.ReadCloser, error) {
+		return fs.OpenFile(filename, os.O_RDONLY, mode.Perm())
+	}), open)
 
 	return out, err
 }
@@ -280,14 +285,14 @@ func readInput(filename string) (string, error) {
 	} else {
 		inFile, err = fs.OpenFile(filename, os.O_RDONLY, 0)
 		if err != nil {
-			return "", fmt.Errorf("failed to open %s\n%v", filename, err)
+			return "", fmt.Errorf("failed to open %s: %w", filename, err)
 		}
 		// nolint: errcheck
 		defer inFile.Close()
 	}
 	bytes, err := ioutil.ReadAll(inFile)
 	if err != nil {
-		err = fmt.Errorf("read failed for %s\n%v", filename, err)
+		err = fmt.Errorf("read failed for %s: %w", filename, err)
 		return "", err
 	}
 	return string(bytes), nil