html/template, text/template: add ParseFS

Now templates can be parsed not just from operating system files
but from arbitrary file systems, including zip files.

For #41190.

Change-Id: I2172001388ddb1f13defa6c5e644e8ec8703ee80
Reviewed-on: https://go-review.googlesource.com/c/go/+/243938
Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>

2 files changed, 83 insertions, 6 deletions

diff --git a/src/text/template/helper.go b/src/text/template/helper.go
index c9e890078c..8269fa28c5 100644
--- a/src/text/template/helper.go
+++ b/src/text/template/helper.go
@@ -8,7 +8,9 @@ package template
 
 import (
 	"fmt"
+	"io/fs"
 	"io/ioutil"
+	"path"
 	"path/filepath"
 )
 
@@ -35,7 +37,7 @@ func Must(t *Template, err error) *Template {
 // For instance, ParseFiles("a/foo", "b/foo") stores "b/foo" as the template
 // named "foo", while "a/foo" is unavailable.
 func ParseFiles(filenames ...string) (*Template, error) {
-	return parseFiles(nil, filenames...)
+	return parseFiles(nil, readFileOS, filenames...)
 }
 
 // ParseFiles parses the named files and associates the resulting templates with
@@ -51,23 +53,22 @@ func ParseFiles(filenames ...string) (*Template, error) {
 // the last one mentioned will be the one that results.
 func (t *Template) ParseFiles(filenames ...string) (*Template, error) {
 	t.init()
-	return parseFiles(t, filenames...)
+	return parseFiles(t, readFileOS, filenames...)
 }
 
 // parseFiles is the helper for the method and function. If the argument
 // template is nil, it is created from the first file.
-func parseFiles(t *Template, filenames ...string) (*Template, error) {
+func parseFiles(t *Template, readFile func(string) (string, []byte, error), filenames ...string) (*Template, error) {
 	if len(filenames) == 0 {
 		// Not really a problem, but be consistent.
 		return nil, fmt.Errorf("template: no files named in call to ParseFiles")
 	}
 	for _, filename := range filenames {
-		b, err := ioutil.ReadFile(filename)
+		name, b, err := readFile(filename)
 		if err != nil {
 			return nil, err
 		}
 		s := string(b)
-		name := filepath.Base(filename)
 		// First template becomes return value if not already defined,
 		// and we use that one for subsequent New calls to associate
 		// all the templates together. Also, if this file has the same name
@@ -126,5 +127,51 @@ func parseGlob(t *Template, pattern string) (*Template, error) {
 	if len(filenames) == 0 {
 		return nil, fmt.Errorf("template: pattern matches no files: %#q", pattern)
 	}
-	return parseFiles(t, filenames...)
+	return parseFiles(t, readFileOS, filenames...)
+}
+
+// ParseFS is like ParseFiles or ParseGlob but reads from the file system fsys
+// instead of the host operating system's file system.
+// It accepts a list of glob patterns.
+// (Note that most file names serve as glob patterns matching only themselves.)
+func ParseFS(fsys fs.FS, patterns ...string) (*Template, error) {
+	return parseFS(nil, fsys, patterns)
+}
+
+// ParseFS is like ParseFiles or ParseGlob but reads from the file system fsys
+// instead of the host operating system's file system.
+// It accepts a list of glob patterns.
+// (Note that most file names serve as glob patterns matching only themselves.)
+func (t *Template) ParseFS(fsys fs.FS, patterns ...string) (*Template, error) {
+	t.init()
+	return parseFS(t, fsys, patterns)
+}
+
+func parseFS(t *Template, fsys fs.FS, patterns []string) (*Template, error) {
+	var filenames []string
+	for _, pattern := range patterns {
+		list, err := fs.Glob(fsys, pattern)
+		if err != nil {
+			return nil, err
+		}
+		if len(list) == 0 {
+			return nil, fmt.Errorf("template: pattern matches no files: %#q", pattern)
+		}
+		filenames = append(filenames, list...)
+	}
+	return parseFiles(t, readFileFS(fsys), filenames...)
+}
+
+func readFileOS(file string) (name string, b []byte, err error) {
+	name = filepath.Base(file)
+	b, err = ioutil.ReadFile(file)
+	return
+}
+
+func readFileFS(fsys fs.FS) func(string) (string, []byte, error) {
+	return func(file string) (name string, b []byte, err error) {
+		name = path.Base(file)
+		b, err = fs.ReadFile(fsys, file)
+		return
+	}
 }
diff --git a/src/text/template/multi_test.go b/src/text/template/multi_test.go
index 34d2378e38..b543ab5c47 100644
--- a/src/text/template/multi_test.go
+++ b/src/text/template/multi_test.go
@@ -9,6 +9,7 @@ package template
 import (
 	"bytes"
 	"fmt"
+	"os"
 	"testing"
 	"text/template/parse"
 )
@@ -153,6 +154,35 @@ func TestParseGlob(t *testing.T) {
 	testExecute(multiExecTests, template, t)
 }
 
+func TestParseFS(t *testing.T) {
+	fs := os.DirFS("testdata")
+
+	{
+		_, err := ParseFS(fs, "DOES NOT EXIST")
+		if err == nil {
+			t.Error("expected error for non-existent file; got none")
+		}
+	}
+
+	{
+		template := New("root")
+		_, err := template.ParseFS(fs, "file1.tmpl", "file2.tmpl")
+		if err != nil {
+			t.Fatalf("error parsing files: %v", err)
+		}
+		testExecute(multiExecTests, template, t)
+	}
+
+	{
+		template := New("root")
+		_, err := template.ParseFS(fs, "file*.tmpl")
+		if err != nil {
+			t.Fatalf("error parsing files: %v", err)
+		}
+		testExecute(multiExecTests, template, t)
+	}
+}
+
 // In these tests, actual content (not just template definitions) comes from the parsed files.
 
 var templateFileExecTests = []execTest{