cmd/cue: allow "fmt" to show unformatted files

Adds a --check flag that will cause cue to fail with exit code 1
in case any files require formatting. A list of non formatted files
will be displayed, line by line, to stdout.

Also, a typo is fixed in cue/ast/ast.go.

resolves #363.

Change-Id: I27c8e9b18bb01f981cd061a23ee3323bc403c9a9
Signed-off-by: Noam Dolovich <noam.tzvi.dolovich@gmail.com>

cmd/cue/cmd/fmt.go

@@ -15,7 +15,10 @@
 package cmd
 
 import (
+	"bytes"
+	"fmt"
 	"github.com/spf13/cobra"
+	"os"
 
 	"cuelang.org/go/cue/ast"
 	"cuelang.org/go/cue/build"
@@ -56,6 +59,9 @@ func newFmtCmd(c *Command) *cobra.Command {
 			cfg.Format = opts
 			cfg.Force = true
 
+			check := flagCheck.Bool(cmd)
+			var modifiedFiles []string
+
 			for _, inst := range builds {
 				if inst.Err != nil {
 					switch {
@@ -67,7 +73,7 @@ func newFmtCmd(c *Command) *cobra.Command {
 					}
 				}
 				for _, file := range inst.BuildFiles {
-					files := []*ast.File{}
+					var files []*ast.File
 					d := encoding.NewDecoder(file, &cfg)
 					for ; !d.Done(); d.Next() {
 						f := d.File()
@@ -86,20 +92,54 @@ func newFmtCmd(c *Command) *cobra.Command {
 						exitOnErr(cmd, err, true)
 					}
 
+					var formatted bytes.Buffer
+					if check {
+						cfg.Out = &formatted
+					}
+
 					e, err := encoding.NewEncoder(file, &cfg)
 					exitOnErr(cmd, err, true)
 
 					for _, f := range files {
 						err := e.EncodeFile(f)
 						exitOnErr(cmd, err, false)
 					}
+
+					if check {
+						var originalBytes []byte
+						if file.Filename != "-" {
+							originalBytes, err = os.ReadFile(file.Filename)
+							exitOnErr(cmd, err, true)
+						} else {
+							originalBytes = file.Source.([]byte)
+						}
+
+						if !bytes.Equal(formatted.Bytes(), originalBytes) {
+							modifiedFiles = append(modifiedFiles, file.Filename)
+						}
+					}
+
 					if err := e.Close(); err != nil {
 						exitOnErr(cmd, err, true)
 					}
 				}
 			}
+
+			if check && len(modifiedFiles) > 0 {
+				stdout := cmd.OutOrStdout()
+				for _, f := range modifiedFiles {
+					if f != "-" {
+						fmt.Fprintln(stdout, f)
+					}
+				}
+				os.Exit(1)
+			}
+
 			return nil
 		}),
 	}
+
+	cmd.Flags().Bool(string(flagCheck), false, "exits with non-zero status if any files are not formatted")
+
 	return cmd
 }

cmd/cue/cmd/testdata/script/fmt_check.txtar

@@ -0,0 +1,15 @@
+# succeeds when file is formatted
+exec cue fmt --check formatted.cue
+
+# files are not modified
+exec stat -f %c formatted.cue
+cp stdout formatted_time_before
+exec cue fmt formatted.cue
+exec stat -f %c formatted.cue
+cmp stdout formatted_time_before
+
+stdin formatted.cue
+exec cue fmt --check -
+
+-- formatted.cue --
+foo: "bar"

cmd/cue/cmd/testdata/script/fmt_check_fail.txtar

@@ -0,0 +1,20 @@
+# fails and displays non formatted files
+! exec cue fmt --check not_formatted.cue another_not_formatted.cue
+cp stdout actual-stdout
+grep not_formatted.cue actual-stdout
+grep another_not_formatted.cue actual-stdout
+! grep correct.cue actual-stdout
+
+# stdin fails with no output
+stdin not_formatted.cue
+! exec cue fmt --check -
+cmp stdout expect-empty-stdout
+
+-- correct.cue --
+foo: "bar"
+-- not_formatted.cue --
+foo:      "bar"
+-- another_not_formatted.cue --
+bar:   "baz"
+x:   1
+-- expect-empty-stdout --

cue/ast/ast.go

@@ -951,7 +951,7 @@ func (d *EmbedDecl) End() token.Pos { return d.Expr.End() }
 // ----------------------------------------------------------------------------
 // Files and packages
 
-// A File node represents a Go source file.
+// A File node represents a Cue source file.
 //
 // The Comments list contains all comments in the source file in order of
 // appearance, including the comments that are pointed to from other nodes