cli: Implement d2 fmt --check

ci/release/changelogs/next.md

@@ -4,6 +4,7 @@
 - Connections now support `link` [#1955](https://github.com/terrastruct/d2/pull/1955)
 - Vars: vars in markdown blocks are substituted [#2218](https://github.com/terrastruct/d2/pull/2218)
 - Markdown: Github-flavored tables work in `md` blocks [#2221](https://github.com/terrastruct/d2/pull/2221)
+- `d2 fmt` now supports a `--check` flag [#2253](https://github.com/terrastruct/d2/pull/2253)
 
 #### Improvements 🧹
 

ci/release/template/man/d2.1

@@ -125,6 +125,9 @@ In watch mode, images used in icons are cached for subsequent compilations. This
 .It Fl -timeout Ar 120
 The maximum number of seconds that D2 runs for before timing out and exiting. When rendering a large diagram, it is recommended to increase this value
 .Ns .
+.It Fl -check Ar false
+Check that the specified files are formatted correctly
+.Ns .
 .It Fl h , -help
 Print usage information and exit
 .Ns .
@@ -180,6 +183,8 @@ See --font-semibold flag.
 See --animate-interval flag.
 .It Ev Sy D2_TIMEOUT
 See --timeout flag.
+.It Ev Sy D2_CHECK
+See --check flag.
 .El
 .Bl -tag -width Ds
 .It Ev Sy DEBUG

d2cli/fmt.go

@@ -12,16 +12,19 @@ import (
 
 	"oss.terrastruct.com/d2/d2format"
 	"oss.terrastruct.com/d2/d2parser"
+	"oss.terrastruct.com/d2/lib/log"
 )
 
-func fmtCmd(ctx context.Context, ms *xmain.State) (err error) {
+func fmtCmd(ctx context.Context, ms *xmain.State, check bool) (err error) {
 	defer xdefer.Errorf(&err, "failed to fmt")
 
 	ms.Opts = xmain.NewOpts(ms.Env, ms.Opts.Flags.Args()[1:])
 	if len(ms.Opts.Args) == 0 {
 		return xmain.UsageErrorf("fmt must be passed at least one file to be formatted")
 	}
 
+	unformattedCount := 0
+
 	for _, inputPath := range ms.Opts.Args {
 		if inputPath != "-" {
 			inputPath = ms.AbsPath(inputPath)
@@ -43,10 +46,25 @@ func fmtCmd(ctx context.Context, ms *xmain.State) (err error) {
 
 		output := []byte(d2format.Format(m))
 		if !bytes.Equal(output, input) {
-			if err := ms.WritePath(inputPath, output); err != nil {
-				return err
+			if check {
+				unformattedCount += 1
+				log.Warn(ctx, inputPath)
+			} else {
+				if err := ms.WritePath(inputPath, output); err != nil {
+					return err
+				}
 			}
 		}
 	}
+
+	if unformattedCount > 0 {
+		pluralFiles := "file"
+		if unformattedCount > 1 {
+			pluralFiles = "files"
+		}
+
+		return xmain.ExitErrorf(1, "found %d unformatted %s. Run d2 fmt to fix.", unformattedCount, pluralFiles)
+	}
+
 	return nil
 }

d2cli/main.go

@@ -119,6 +119,11 @@ func Run(ctx context.Context, ms *xmain.State) (err error) {
 	fontBoldFlag := ms.Opts.String("D2_FONT_BOLD", "font-bold", "", "", "path to .ttf file to use for the bold font. If none provided, Source Sans Pro Bold is used.")
 	fontSemiboldFlag := ms.Opts.String("D2_FONT_SEMIBOLD", "font-semibold", "", "", "path to .ttf file to use for the semibold font. If none provided, Source Sans Pro Semibold is used.")
 
+	checkFlag, err := ms.Opts.Bool("D2_CHECK", "check", "", false, "check that the specified files are formatted correctly.")
+	if err != nil {
+		return err
+	}
+
 	plugins, err := d2plugin.ListPlugins(ctx)
 	if err != nil {
 		return err
@@ -153,7 +158,7 @@ func Run(ctx context.Context, ms *xmain.State) (err error) {
 			themesCmd(ctx, ms)
 			return nil
 		case "fmt":
-			return fmtCmd(ctx, ms)
+			return fmtCmd(ctx, ms, *checkFlag)
 		case "version":
 			if len(ms.Opts.Flags.Args()) > 1 {
 				return xmain.UsageErrorf("version subcommand accepts no arguments")

e2etests-cli/main_test.go

@@ -1005,6 +1005,29 @@ layers: {
 				assert.Equal(t, "x -> y\n", string(gotBar))
 			},
 		},
+		{
+			name: "fmt-check-unformatted",
+			run: func(t *testing.T, ctx context.Context, dir string, env *xos.Env) {
+				writeFile(t, dir, "foo.d2", `a ---> b`)
+				writeFile(t, dir, "bar.d2", `x ---> y`)
+				writeFile(t, dir, "baz.d2", "a -> z\n")
+				err := runTestMainPersist(t, ctx, dir, env, "fmt", "--check", "foo.d2", "bar.d2", "baz.d2")
+				assert.ErrorString(t, err, "failed to wait xmain test: e2etests-cli/d2: failed to fmt: exiting with code 1: found 2 unformatted files. Run d2 fmt to fix.")
+				gotFoo := readFile(t, dir, "foo.d2")
+				gotBar := readFile(t, dir, "bar.d2")
+				assert.Equal(t, "a ---> b", string(gotFoo))
+				assert.Equal(t, "x ---> y", string(gotBar))
+			},
+		},
+		{
+			name: "fmt-check-formatted",
+			run: func(t *testing.T, ctx context.Context, dir string, env *xos.Env) {
+				writeFile(t, dir, "foo.d2", "a -> b\n")
+				writeFile(t, dir, "bar.d2", "x -> y\n")
+				err := runTestMainPersist(t, ctx, dir, env, "fmt", "--check", "foo.d2", "bar.d2")
+				assert.Success(t, err)
+			},
+		},
 		{
 			name:   "watch-regular",
 			serial: true,