butane: use friendly filename in stdin read error

butane/internal/main.go

@@ -40,6 +40,26 @@ func isCharDevice(f *os.File) bool {
 	return stat.Mode()&os.ModeCharDevice != 0
 }
 
+func readInput(input string) ([]byte, string, error) {
+	infile := os.Stdin
+	filename := "<stdin>"
+	if input != "" {
+		f, err := os.Open(input)
+		if err != nil {
+			return nil, input, fmt.Errorf("failed to open %s: %w", input, err)
+		}
+		defer func() { _ = f.Close() }()
+		infile = f
+		filename = input
+	}
+
+	data, err := io.ReadAll(infile)
+	if err != nil {
+		return nil, filename, fmt.Errorf("failed to read %s: %w", filename, err)
+	}
+	return data, filename, nil
+}
+
 func main() {
 	var (
 		input       string
@@ -111,21 +131,9 @@ func main() {
 		os.Exit(0)
 	}
 
-	infile := os.Stdin
-	filename := "<stdin>"
-	if input != "" {
-		var err error
-		infile, err = os.Open(input)
-		if err != nil {
-			fail("failed to open %s: %v\n", input, err)
-		}
-		defer func() { _ = infile.Close() }()
-		filename = input
-	}
-
-	dataIn, err := io.ReadAll(infile)
+	dataIn, filename, err := readInput(input)
 	if err != nil {
-		fail("failed to read %s: %v\n", infile.Name(), err)
+		fail("%v\n", err)
 	}
 
 	dataOut, r, err := config.TranslateBytes(dataIn, options)

butane/internal/main_test.go

@@ -0,0 +1,167 @@
+// Copyright 2019 Red Hat, Inc
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package main
+
+import (
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+func TestReadInput(t *testing.T) {
+	tests := []struct {
+		name       string
+		setup      func(t *testing.T) (input string, cleanup func())
+		wantData   []byte
+		wantErr    bool
+		wantErrMsg string
+	}{
+		{
+			name: "stdin",
+			setup: func(t *testing.T) (string, func()) {
+				content := []byte("hello from stdin")
+				orig := os.Stdin
+				tmp, err := os.CreateTemp("", "butane-stdin")
+				if err != nil {
+					t.Fatalf("failed to create temp file: %v", err)
+				}
+				if _, err := tmp.Write(content); err != nil {
+					t.Fatalf("failed to write temp file: %v", err)
+				}
+				if _, err := tmp.Seek(0, 0); err != nil {
+					t.Fatalf("failed to seek temp file: %v", err)
+				}
+				os.Stdin = tmp
+				return "", func() {
+					os.Stdin = orig
+					if err := tmp.Close(); err != nil {
+						t.Errorf("failed to close temp file: %v", err)
+					}
+					if err := os.Remove(tmp.Name()); err != nil {
+						t.Errorf("failed to remove temp file: %v", err)
+					}
+				}
+			},
+			wantData: []byte("hello from stdin"),
+			wantErr:  false,
+		},
+		{
+			name: "empty stdin",
+			setup: func(t *testing.T) (string, func()) {
+				orig := os.Stdin
+				tmp, err := os.CreateTemp("", "butane-stdin-empty")
+				if err != nil {
+					t.Fatalf("failed to create temp file: %v", err)
+				}
+				if _, err := tmp.Seek(0, 0); err != nil {
+					t.Fatalf("failed to seek temp file: %v", err)
+				}
+				os.Stdin = tmp
+				return "", func() {
+					os.Stdin = orig
+					if err := tmp.Close(); err != nil {
+						t.Errorf("failed to close temp file: %v", err)
+					}
+					if err := os.Remove(tmp.Name()); err != nil {
+						t.Errorf("failed to remove temp file: %v", err)
+					}
+				}
+			},
+			wantData: []byte{},
+			wantErr:  false,
+		},
+		{
+			name: "file",
+			setup: func(t *testing.T) (string, func()) {
+				dir := t.TempDir()
+				path := filepath.Join(dir, "input.bu")
+				content := []byte("variant: fcos")
+				if err := os.WriteFile(path, content, 0644); err != nil {
+					t.Fatalf("failed to write file: %v", err)
+				}
+				return path, func() {}
+			},
+			wantData: []byte("variant: fcos"),
+			wantErr:  false,
+		},
+		{
+			name: "missing file",
+			setup: func(t *testing.T) (string, func()) {
+				missing := filepath.Join(t.TempDir(), "does-not-exist")
+				return missing, func() {}
+			},
+			wantData: nil,
+			wantErr:  true,
+		},
+		{
+			name: "stdin read error",
+			setup: func(t *testing.T) (string, func()) {
+				orig := os.Stdin
+				closed, err := os.CreateTemp("", "butane-closed-stdin")
+				if err != nil {
+					t.Fatalf("failed to create temp file: %v", err)
+				}
+				name := closed.Name()
+				if err := closed.Close(); err != nil {
+					t.Fatalf("failed to close temp file: %v", err)
+				}
+				os.Stdin = closed
+				return "", func() {
+					os.Stdin = orig
+					if err := os.Remove(name); err != nil {
+						t.Errorf("failed to remove temp file: %v", err)
+					}
+				}
+			},
+			wantData:   nil,
+			wantErr:    true,
+			wantErrMsg: "failed to read <stdin>:",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			input, cleanup := tt.setup(t)
+			defer cleanup()
+
+			data, filename, err := readInput(input)
+			if tt.wantErr {
+				if err == nil {
+					t.Fatalf("expected error, got nil")
+				}
+				if tt.wantErrMsg != "" {
+					if !strings.Contains(err.Error(), tt.wantErrMsg) {
+						t.Errorf("expected error containing %q, got %q", tt.wantErrMsg, err.Error())
+					}
+				}
+			} else if err != nil {
+				t.Fatalf("unexpected error: %v", err)
+			}
+
+			if string(data) != string(tt.wantData) {
+				t.Errorf("expected data %q, got %q", tt.wantData, data)
+			}
+
+			wantName := "<stdin>"
+			if input != "" {
+				wantName = input
+			}
+			if filename != wantName {
+				t.Errorf("expected filename %q, got %q", wantName, filename)
+			}
+		})
+	}
+}

docs/release-notes.md

@@ -28,6 +28,7 @@ nav_order: 9
 - Fix the Butane root partition constraint check to only examine subsequent partitions ([#2304](https://github.com/coreos/ignition/pull/2304))
 - Close `/proc/mounts` after checking whether block devices are mounted, preventing file descriptor leaks during disk setup ([#2289](https://github.com/coreos/ignition/pull/2289))
 - Drop supplementary groups when dropping privileges to write files as a user ([#2242](https://github.com/coreos/ignition/issues/2242))
+- butane: report the friendly input name (`<stdin>` instead of `/dev/stdin`) in stdin read errors
 
 ## Ignition 2.27.0 (2026-08-26)