cmd/age: detect output/input file reuse when possible

Fixes #491

cmd/age/age.go

@@ -11,6 +11,7 @@ import (
 	"fmt"
 	"io"
 	"os"
+	"path/filepath"
 	"regexp"
 	"runtime/debug"
 	"strings"
@@ -223,9 +224,21 @@ func main() {
 		}
 	}
 
+	var inUseFiles []string
+	for _, i := range identityFlags {
+		if i.Type != "i" {
+			continue
+		}
+		inUseFiles = append(inUseFiles, absPath(i.Value))
+	}
+	for _, f := range recipientsFileFlags {
+		inUseFiles = append(inUseFiles, absPath(f))
+	}
+
 	var in io.Reader = os.Stdin
 	var out io.Writer = os.Stdout
 	if name := flag.Arg(0); name != "" && name != "-" {
+		inUseFiles = append(inUseFiles, absPath(name))
 		f, err := os.Open(name)
 		if err != nil {
 			errorf("failed to open input file %q: %v", name, err)
@@ -246,6 +259,11 @@ func main() {
 		}
 	}
 	if name := outFlag; name != "" && name != "-" {
+		for _, f := range inUseFiles {
+			if f == absPath(name) {
+				errorf("input and output file are the same: %q", name)
+			}
+		}
 		f := newLazyOpener(name)
 		defer func() {
 			if err := f.Close(); err != nil {
@@ -532,3 +550,10 @@ func (l *lazyOpener) Close() error {
 	}
 	return nil
 }
+
+func absPath(name string) string {
+	if abs, err := filepath.Abs(name); err == nil {
+		return abs
+	}
+	return name
+}

cmd/age/testdata/output_file.txt

@@ -25,9 +25,32 @@ age -d -i key.txt -o new empty.age
 ! stderr .
 cmp new empty
 
+# https://github.com/FiloSottile/age/issues/491
+cp input inputcopy
+! age -r age1xmwwc06ly3ee5rytxm9mflaz2u56jjj36s0mypdrwsvlul66mv4q47ryef -o inputcopy inputcopy
+stderr 'input and output file are the same'
+cmp inputcopy input
+! age -r age1xmwwc06ly3ee5rytxm9mflaz2u56jjj36s0mypdrwsvlul66mv4q47ryef -o ./inputcopy inputcopy
+stderr 'input and output file are the same'
+cmp inputcopy input
+mkdir foo
+! age -r age1xmwwc06ly3ee5rytxm9mflaz2u56jjj36s0mypdrwsvlul66mv4q47ryef -o inputcopy foo/../inputcopy
+stderr 'input and output file are the same'
+cmp inputcopy input
+cp key.txt keycopy
+age -e -i keycopy -o test.age input
+! age -d -i keycopy -o keycopy test.age
+stderr 'input and output file are the same'
+cmp key.txt keycopy
+
 [!linux] [!darwin] skip # no pty support
 [darwin] [go1.20] skip # https://go.dev/issue/61779
 
+ttyin terminal
+! age -p -o inputcopy inputcopy
+stderr 'input and output file are the same'
+cmp inputcopy input
+
 # https://github.com/FiloSottile/age/issues/159
 ttyin terminal
 age -p -a -o test.age input