From d945600d060e7a0b7c5e72ac606a017d105a17f3 Mon Sep 17 00:00:00 2001
From: Robbie McMichael <robbie.mcmichael.git@gmail.com>
Date: Mon, 6 Oct 2025 12:47:34 +0000
Subject: [PATCH] cmd/gofmt: change -d to exit 1 if diffs exist

When using the -d flag, set the exit code to 1 if there is a diff.

Fixes #46289

Change-Id: I802e8ccd1798ed7f4448696bec4bc82835ec71a2
GitHub-Last-Rev: db2207fba9a8f7a2f50138ec1f086ac6a74e1b10
GitHub-Pull-Request: golang/go#75649
Reviewed-on: https://go-review.googlesource.com/c/go/+/707635
Reviewed-by: Carlos Amedee <carlos@golang.org>
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Sean Liao <sean@liao.dev>
Auto-Submit: Sean Liao <sean@liao.dev>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
---
 src/cmd/gofmt/gofmt.go                 | 12 +++++-
 src/cmd/gofmt/gofmt_test.go            | 55 ++++++++++++++++++++++++--
 src/cmd/gofmt/testdata/exitcode.golden |  1 +
 src/cmd/gofmt/testdata/exitcode.input  |  1 +
 4 files changed, 64 insertions(+), 5 deletions(-)
 create mode 100644 src/cmd/gofmt/testdata/exitcode.golden
 create mode 100644 src/cmd/gofmt/testdata/exitcode.input

diff --git a/src/cmd/gofmt/gofmt.go b/src/cmd/gofmt/gofmt.go
index bbb8b4fd15..ad6ad63652 100644
--- a/src/cmd/gofmt/gofmt.go
+++ b/src/cmd/gofmt/gofmt.go
@@ -41,6 +41,9 @@ var (
 
 	// debugging
 	cpuprofile = flag.String("cpuprofile", "", "write cpu profile to this file")
+
+	// errors
+	errFormattingDiffers = fmt.Errorf("formatting differs from gofmt's")
 )
 
 // Keep these in sync with go/format/format.go.
@@ -218,8 +221,12 @@ func (r *reporter) Report(err error) {
 		panic("Report with nil error")
 	}
 	st := r.getState()
-	scanner.PrintError(st.err, err)
-	st.exitCode = 2
+	if err == errFormattingDiffers {
+		st.exitCode = 1
+	} else {
+		scanner.PrintError(st.err, err)
+		st.exitCode = 2
+	}
 }
 
 func (r *reporter) ExitCode() int {
@@ -281,6 +288,7 @@ func processFile(filename string, info fs.FileInfo, in io.Reader, r *reporter) e
 			newName := filepath.ToSlash(filename)
 			oldName := newName + ".orig"
 			r.Write(diff.Diff(oldName, src, newName, res))
+			return errFormattingDiffers
 		}
 	}
 
diff --git a/src/cmd/gofmt/gofmt_test.go b/src/cmd/gofmt/gofmt_test.go
index 6b80673af1..2aba0f03ff 100644
--- a/src/cmd/gofmt/gofmt_test.go
+++ b/src/cmd/gofmt/gofmt_test.go
@@ -53,10 +53,19 @@ func gofmtFlags(filename string, maxLines int) string {
 	return ""
 }
 
-func runTest(t *testing.T, in, out string) {
-	// process flags
-	*simplifyAST = false
+// Reset global variables for all flags to their default value.
+func resetFlags() {
+	*list = false
+	*write = false
 	*rewriteRule = ""
+	*simplifyAST = false
+	*doDiff = false
+	*allErrors = false
+	*cpuprofile = ""
+}
+
+func runTest(t *testing.T, in, out string) {
+	resetFlags()
 	info, err := os.Lstat(in)
 	if err != nil {
 		t.Error(err)
@@ -159,6 +168,46 @@ func TestRewrite(t *testing.T) {
 	}
 }
 
+// TestDiff runs gofmt with the -d flag on the input files and checks that the
+// expected exit code is set.
+func TestDiff(t *testing.T) {
+	tests := []struct {
+		in       string
+		exitCode int
+	}{
+		{in: "testdata/exitcode.input", exitCode: 1},
+		{in: "testdata/exitcode.golden", exitCode: 0},
+	}
+
+	for _, tt := range tests {
+		resetFlags()
+		*doDiff = true
+
+		initParserMode()
+		initRewrite()
+
+		info, err := os.Lstat(tt.in)
+		if err != nil {
+			t.Error(err)
+			return
+		}
+
+		const maxWeight = 2 << 20
+		var buf, errBuf bytes.Buffer
+		s := newSequencer(maxWeight, &buf, &errBuf)
+		s.Add(fileWeight(tt.in, info), func(r *reporter) error {
+			return processFile(tt.in, info, nil, r)
+		})
+		if errBuf.Len() > 0 {
+			t.Logf("%q", errBuf.Bytes())
+		}
+
+		if s.GetExitCode() != tt.exitCode {
+			t.Errorf("%s: expected exit code %d, got %d", tt.in, tt.exitCode, s.GetExitCode())
+		}
+	}
+}
+
 // Test case for issue 3961.
 func TestCRLF(t *testing.T) {
 	const input = "testdata/crlf.input"   // must contain CR/LF's
diff --git a/src/cmd/gofmt/testdata/exitcode.golden b/src/cmd/gofmt/testdata/exitcode.golden
new file mode 100644
index 0000000000..06ab7d0f9a
--- /dev/null
+++ b/src/cmd/gofmt/testdata/exitcode.golden
@@ -0,0 +1 @@
+package main
diff --git a/src/cmd/gofmt/testdata/exitcode.input b/src/cmd/gofmt/testdata/exitcode.input
new file mode 100644
index 0000000000..4f2f092ce5
--- /dev/null
+++ b/src/cmd/gofmt/testdata/exitcode.input
@@ -0,0 +1 @@
+	package main
-- 
2.52.0