Append path arguments to diff command if not provided by user

docs/REFERENCE.md

@@ -1026,8 +1026,9 @@ If `diff.command` is set then it will be invoked to show individual file
 differences with `diff.args` passed as arguments. Each element of `diff.args` is
 interpreted as a template with the variables `.Destination` and `.Target`
 available corresponding to the path of the file in the source and target state
-respectively. The default value of `diff.args` is `["{{ .Destination }}",
-"{{ .Target }}"]`.
+respectively. The default value of `diff.args` is `["{{ .Destination }}", "{{
+.Target }}"]`. If `diff.args` does not contain any template arguments then `{{
+.Destination }}` and `{{ .Target }}` will be appended automatically.
 
 #### `--pager` *pager*
 

internal/chezmoi/externaldiffsystem.go

@@ -178,17 +178,41 @@ func (s *ExternalDiffSystem) runDiffCommand(destAbsPath, targetAbsPath AbsPath)
 		Destination: string(destAbsPath),
 		Target:      string(targetAbsPath),
 	}
+
 	args := make([]string, 0, len(s.args))
+	// Work around a regression introduced in 2.1.5
+	// (https://github.com/twpayne/chezmoi/pull/1328) in a user-friendly way.
+	//
+	// Prior to #1328, the diff.args config option was prepended to the default
+	// order of files to the diff command. Post #1328, the diff.args config
+	// option replaced all arguments to the diff command.
+	//
+	// Work around this by looking for any templates in diff.args. An arg is
+	// considered a template if, after execution as as template, it is not equal
+	// to the original arg.
+	anyTemplateArgs := false
 	for i, arg := range s.args {
 		tmpl, err := template.New("diff.args[" + strconv.Itoa(i) + "]").Parse(arg)
 		if err != nil {
 			return err
 		}
+
 		var sb strings.Builder
 		if err := tmpl.Execute(&sb, templateData); err != nil {
 			return err
 		}
 		args = append(args, sb.String())
+
+		// Detect template arguments.
+		if arg != sb.String() {
+			anyTemplateArgs = true
+		}
+	}
+
+	// If there are no template arguments, then append the destination and
+	// target paths as prior to #1328.
+	if !anyTemplateArgs {
+		args = append(args, templateData.Destination, templateData.Target)
 	}
 
 	//nolint:gosec

internal/cmd/config.go

@@ -265,10 +265,6 @@ func newConfig(options ...configOption) (*Config, error) {
 			recursive: true,
 		},
 		Diff: diffCmdConfig{
-			Args: []string{
-				"{{ .Destination }}",
-				"{{ .Target }}",
-			},
 			Exclude: chezmoi.NewEntryTypeSet(chezmoi.EntryTypesNone),
 			include: chezmoi.NewEntryTypeSet(chezmoi.EntryTypesAll),
 		},

internal/cmd/testdata/scripts/diff_unix.txt

@@ -1,19 +1,18 @@
 [windows] skip 'UNIX only'
 
-chmod 755 bin/vimdiff
-
 # test that chezmoi diff invokes diff.command when configured
 chezmoi diff
-stdout 'vimdiff .*/home/user/\.file .*/chezmoi-diff.*/\.file' # FIXME should be able to use ${HOME@R} in this regexp
+stdout ^${HOME@R}/\.file\s+${WORK@R}/.*/\.file$
 
 # test that chezmoi diff --use-builtin-diff uses the builtin diff even if diff.command is configured
 chezmoi diff --use-builtin-diff
 cmp stdout golden/diff
 
--- bin/vimdiff --
-#!/bin/sh
+# test that chezmoi diff appends the destination and target paths if diff.args does not contain any templates
+chhome home2/user
+chezmoi diff
+stdout ^arg\s+${HOME@R}/\.file\s+${WORK@R}/.*/\.file$
 
-echo vimdiff "$*"
 -- golden/diff --
 diff --git a/.file b/.file
 index bd729e8ee3cc005444c67dc77eed60016886b5e0..b508963510528ab709627ec448026a10a64c72ef 100644
@@ -24,8 +23,14 @@ index bd729e8ee3cc005444c67dc77eed60016886b5e0..b508963510528ab709627ec448026a10
 +# target contents of .file
 -- home/user/.config/chezmoi/chezmoi.toml --
 [diff]
-    command = "vimdiff"
+    command = "echo"
 -- home/user/.file --
 # destination contents of .file
 -- home/user/.local/share/chezmoi/dot_file --
 # target contents of .file
+-- home2/user/.config/chezmoi/chezmoi.toml --
+[diff]
+    command = "echo"
+    args = ["arg"]
+-- home2/user/.local/share/chezmoi/dot_file --
+# source