cue/cmd: always write stdin to stdout in cue fmt

A recent CL (https://cuelang.org/cl/1193934) modified cue fmt to only
write to files in case they were not well formatted.

A bug was introduced by this change, in which `cue fmt -` would only
write to stdout in case the standard input was not formatted. This is
not correct, as users would expect there to always be output in this
case.

This is fixed by always writing the provided stdin to stdout,
regardless of whether it was well formatted or not.

Fixes #3067.

Signed-off-by: Noam Dolovich <noam.tzvi.dolovich@gmail.com>
Change-Id: Id69a50c454c3871ec56526de2cd622cdd6ded730
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1193969
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>
Reviewed-by: Paul Jolly <paul@myitcv.io>
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>

cmd/cue/cmd/fmt.go

@@ -17,6 +17,7 @@ package cmd
 import (
 	"bytes"
 	"fmt"
+	"io"
 	"os"
 	"path/filepath"
 
@@ -98,6 +99,10 @@ func newFmtCmd(c *Command) *cobra.Command {
 						file.Source = original
 					}
 					cfg.Out = &formatted
+					if file.Filename == "-" && !doDiff && !check {
+						// Always write to stdout if the file is read from stdin.
+						cfg.Out = io.MultiWriter(cfg.Out, stdout)
+					}
 
 					var files []*ast.File
 					d := encoding.NewDecoder(cmd.ctx, file, &cfg)
@@ -150,9 +155,7 @@ func newFmtCmd(c *Command) *cobra.Command {
 					case check:
 						fmt.Fprintln(stdout, path)
 					case file.Filename == "-":
-						if _, err := fmt.Fprint(stdout, formatted.String()); err != nil {
-							exitOnErr(cmd, err, false)
-						}
+						// already written to stdout during encoding
 					default:
 						if err := os.WriteFile(file.Filename, formatted.Bytes(), 0644); err != nil {
 							exitOnErr(cmd, err, false)

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

@@ -1,10 +1,12 @@
 cd standalone
 
-# succeeds when file is formatted
+# succeeds with no output when file is formatted
 exec cue fmt --check formatted.cue
+! stdout .
 
 stdin formatted.cue
 exec cue fmt --check -
+! stdout .
 
 # fails and displays non formatted files
 ! exec cue fmt --check not_formatted.cue another/not_formatted.cue

cmd/cue/cmd/testdata/script/fmt_stdin.txtar

@@ -1,11 +1,29 @@
-stdin feed
+# Verify that `cue fmt -` always writes valid CUE provided
+# in stdin to stdout, even if it is well formatted.
+
+stdin not-formatted
+exec cue fmt -
+cmp stdout formatted
+
+stdin formatted
 exec cue fmt -
-cmp stdout expect-stdout
+cmp stdout formatted
+
+# Verify that `cue fmt -` does not write invalid CUE to stdout.
+stdin syntax-error
+! exec cue fmt -
+! stdout .
+cmp stderr syntax-error.stderr-golden
 
--- feed --
+-- syntax-error --
+a: {
+-- syntax-error.stderr-golden --
+expected '}', found 'EOF':
+    -:1:6
+-- not-formatted --
 foo : 2
 a: {b: 3} // a comment
--- expect-stdout --
+-- formatted --
 foo: 2
 a: {b: 3} // a comment
 -- fmt/cue.mod/module.cue --