Add writeToFile function

docs/templating-language.md

@@ -79,6 +79,7 @@ provides the following functions:
   - [toUpper](#toupper)
   - [toYAML](#toyaml)
   - [sockaddr](#sockaddr)
+  - [writeToFile](#writeToFile)
 - [Math Functions](#math-functions)
   - [add](#add)
   - [subtract](#subtract)
@@ -575,7 +576,7 @@ To iterate and list over every secret in the generic secret backend in Vault:
 {{ with secret (printf "secret/%s" .) }}{{ range $k, $v := .Data }}
 {{ $k }}: {{ $v }}
 {{ end }}{{ end }}{{ end }}
-``` 
+```
 
 `.Data` should be replaced with `.Data.data` for KV-V2 secrets engines.
 
@@ -1573,6 +1574,19 @@ Takes a quote-escaped template string as an argument and passes it on to
 See [hashicorp/go-sockaddr documentation](https://godoc.org/github.com/hashicorp/go-sockaddr)
 for more information.
 
+### `writeToFile`
+
+Writes the content to a file with username, group name, permissions. There are optional flags to
+select appending mode or add a newline.
+
+For example:
+
+```golang
+{{ key "my/key/path" | writeToFile "/my/file/path.txt" "my-user" "my-group" "0644" }}
+{{ key "my/key/path" | writeToFile "/my/file/path.txt" "my-user" "my-group" "0644" "append" }}
+{{ key "my/key/path" | writeToFile "/my/file/path.txt" "my-user" "my-group" "0644" "append,newline" }}
+```
+
 ---
 
 ## Math Functions
@@ -1704,7 +1718,7 @@ or an error.
 renders
 
 ```golang
-> 
+>
 (map[string]interface {}) (len=1) {
  (string) (len=3) "foo": (map[string]interface {}) (len=3) {
   (string) (len=3) "bar": (bool) true,
@@ -1727,7 +1741,7 @@ Creates a string containing the values with full newlines, indentation, type, an
 renders
 
 ```golang
-> 
+>
 (map[string]interface {}) (len=1) {
  (string) (len=3) "foo": (map[string]interface {}) (len=3) {
   (string) (len=3) "bar": (bool) true,
@@ -1760,19 +1774,19 @@ Given this template fragment,
 {{- $OBJ := parseJSON $JSON -}}
 ```
 
-#### using `%v` 
+#### using `%v`
 
 ```golang
 {{- spew_printf "%v\n" $OBJ }}
 ```
 
-outputs 
+outputs
 
 ```golang
 map[foo:map[bar:true baz:string theAnswer:42]]
 ```
 
-#### using `%+v` 
+#### using `%+v`
 
 
 ```golang

template/funcs.go

@@ -11,6 +11,7 @@ import (
 	"io/ioutil"
 	"os"
 	"os/exec"
+	"os/user"
 	"path/filepath"
 	"reflect"
 	"regexp"
@@ -1596,6 +1597,77 @@ func md5sum(item string) (string, error) {
 	return fmt.Sprintf("%x", md5.Sum([]byte(item))), nil
 }
 
+// writeToFile writes the content to a file with username, group name, permissions and optional
+// flags to select appending mode or add a newline.
+//
+// For example:
+//   key "my/key/path" | writeToFile "/my/file/path.txt" "my-user" "my-group" "0644"
+//   key "my/key/path" | writeToFile "/my/file/path.txt" "my-user" "my-group" "0644" "append"
+//   key "my/key/path" | writeToFile "/my/file/path.txt" "my-user" "my-group" "0644" "append,newline"
+//
+func writeToFile(path, username, groupName, permissions string, args ...string) (string, error) {
+	// Parse arguments
+	flags := ""
+	if len(args) == 2 {
+		flags = args[0]
+	}
+	content := args[len(args)-1]
+
+	p_u, err := strconv.ParseUint(permissions, 8, 32)
+	if err != nil {
+		return "", err
+	}
+	perm := os.FileMode(p_u)
+
+	// Write to file
+	var f *os.File
+	shouldAppend := strings.Contains(flags, "append")
+	if shouldAppend {
+		f, err = os.OpenFile(path, os.O_APPEND|os.O_WRONLY|os.O_CREATE, perm)
+		if err != nil {
+			return "", err
+		}
+	} else {
+		f, err = os.Create(path)
+		if err != nil {
+			return "", err
+		}
+	}
+	defer f.Close()
+
+	writingContent := []byte(content)
+	shouldAddNewLine := strings.Contains(flags, "newline")
+	if shouldAddNewLine {
+		writingContent = append(writingContent, []byte("\n")...)
+	}
+	if _, err = f.Write(writingContent); err != nil {
+		return "", err
+	}
+
+	// Change ownership and permissions
+	u, err := user.Lookup(username)
+	if err != nil {
+		return "", err
+	}
+	g, err := user.LookupGroup(groupName)
+	if err != nil {
+		return "", err
+	}
+	uid, _ := strconv.Atoi(u.Uid)
+	gid, _ := strconv.Atoi(g.Gid)
+	err = os.Chown(path, uid, gid)
+	if err != nil {
+		return "", err
+	}
+
+	err = os.Chmod(path, perm)
+	if err != nil {
+		return "", err
+	}
+
+	return "", nil
+}
+
 func spewSdump(args ...interface{}) (string, error) {
 	return spewLib.Sdump(args...), nil
 }

template/template.go

@@ -294,7 +294,8 @@ func funcMap(i *funcMapInput) template.FuncMap {
 		"split":                 split,
 		"byMeta":                byMeta,
 		"sockaddr":              sockaddr,
-
+		"writeToFile":           writeToFile,
+
 		// Math functions
 		"add":      add,
 		"subtract": subtract,

template/template_test.go

@@ -5,7 +5,9 @@ import (
 	"fmt"
 	"io/ioutil"
 	"os"
+	"os/user"
 	"reflect"
+	"strconv"
 	"testing"
 	"time"
 
@@ -1876,3 +1878,126 @@ func TestTemplate_Execute(t *testing.T) {
 		})
 	}
 }
+
+func Test_writeToFile(t *testing.T) {
+	cases := []struct {
+		name        string
+		content     string
+		permissions string
+		flags       string
+		expectation string
+		wantErr     bool
+	}{
+		{
+			"writeToFile_without_flags",
+			"after",
+			"0644",
+			"",
+			"after",
+			false,
+		},
+		{
+			"writeToFile_with_different_file_permissions",
+			"after",
+			"0666",
+			"",
+			"after",
+			false,
+		},
+		{
+			"writeToFile_with_append",
+			"after",
+			"0644",
+			`"append"`,
+			"beforeafter",
+			false,
+		},
+		{
+			"writeToFile_with_newline",
+			"after",
+			"0644",
+			`"newline"`,
+			"after\n",
+			false,
+		},
+		{
+			"writeToFile_with_append_and_newline",
+			"after",
+			"0644",
+			`"append,newline"`,
+			"beforeafter\n",
+			false,
+		},
+	}
+
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			outDir, err := ioutil.TempDir("", "")
+			if err != nil {
+				t.Fatal(err)
+			}
+			defer os.RemoveAll(outDir)
+			outputFile, err := ioutil.TempFile(outDir, "")
+			if err != nil {
+				t.Fatal(err)
+			}
+			outputFile.WriteString("before")
+
+			// Use current user and its primary group for input
+			currentUser, err := user.Current()
+			if err != nil {
+				t.Fatal(err)
+			}
+			currentUsername := currentUser.Username
+			currentGroup, err := user.LookupGroupId(currentUser.Gid)
+			if err != nil {
+				t.Fatal(err)
+			}
+			currentGroupName := currentGroup.Name
+
+			templateContent := fmt.Sprintf(
+				"{{ \"%s\" | writeToFile \"%s\" \"%s\" \"%s\" \"%s\"  %s}}",
+				tc.content, outputFile.Name(), currentUsername, currentGroupName, tc.permissions, tc.flags)
+			ti := &NewTemplateInput{
+				Contents: templateContent,
+			}
+			tpl, err := NewTemplate(ti)
+			if err != nil {
+				t.Fatal(err)
+			}
+
+			a, err := tpl.Execute(nil)
+			if (err != nil) != tc.wantErr {
+				t.Errorf("writeToFile() error = %v, wantErr %v", err, tc.wantErr)
+				return
+			}
+
+			// Compare generated file content with the expectation.
+			// The function should generate an empty string to the output.
+			_generatedFileContent, err := ioutil.ReadFile(outputFile.Name())
+			generatedFileContent := string(_generatedFileContent)
+			if err != nil {
+				t.Fatal(err)
+			}
+			if a != nil && !bytes.Equal([]byte(""), a.Output) {
+				t.Errorf("writeToFile() template = %v, want empty string", a.Output)
+			}
+			if generatedFileContent != tc.expectation {
+				t.Errorf("writeToFile() got = %v, want %v", generatedFileContent, tc.expectation)
+			}
+			// Assert output file permissions
+			sts, err := outputFile.Stat()
+			if err != nil {
+				t.Fatal(err)
+			}
+			p_u, err := strconv.ParseUint(tc.permissions, 8, 32)
+			if err != nil {
+				t.Fatal(err)
+			}
+			perm := os.FileMode(p_u)
+			if sts.Mode() != perm {
+				t.Errorf("writeToFile() wrong permissions got = %v, want %v", perm, tc.permissions)
+			}
+		})
+	}
+}