fix: return a safe file handle in dry-run mode

e2e/run_test.go

@@ -2,6 +2,7 @@ package e2e
 
 import (
 	"fmt"
+	"os"
 	"strings"
 	"testing"
 	"time"
@@ -320,6 +321,46 @@ func TestRunDryRun(t *testing.T) {
 	assert.Assert(t, ensureS3Object(s3client, bucket, files[2], "content"))
 }
 
+// --dry-run run (cp s3://bucket/file dir/ from stdin)
+func TestRunDryRunDownloadFromStdin(t *testing.T) {
+	t.Parallel()
+
+	s3client, s5cmd := setup(t)
+
+	bucket := s3BucketFromTestName(t)
+	createBucket(t, s3client, bucket)
+
+	// Feed more than one read buffer's worth of commands so stdin is still
+	// being read while the first downloads complete. A dry-run download must
+	// hand out a file that is safe to close, i.e. one that does not alias
+	// stdin.
+	const numFiles = 200
+
+	var lines []string
+	expected := make(map[int]compareFunc)
+	for i := 0; i < numFiles; i++ {
+		filename := fmt.Sprintf("file%03d.txt", i)
+		putFile(t, s3client, bucket, filename, "content")
+
+		lines = append(lines, fmt.Sprintf("cp s3://%v/%s dir/", bucket, filename))
+		expected[i] = equals("cp s3://%v/%s dir/%s", bucket, filename, filename)
+	}
+
+	input := strings.NewReader(strings.Join(lines, "\n"))
+	cmd := s5cmd("--dry-run", "run")
+	result := icmd.RunCmd(cmd, icmd.WithStdin(input))
+
+	result.Assert(t, icmd.Success)
+
+	assertLines(t, result.Stdout(), expected, sortInput(true))
+
+	assertLines(t, result.Stderr(), map[int]compareFunc{})
+
+	// not even outermost directory should be created
+	_, err := os.Stat(cmd.Dir + "/dir")
+	assert.Assert(t, os.IsNotExist(err))
+}
+
 func TestRunFixDataRace_Issue301(t *testing.T) {
 	t.Parallel()
 

storage/fs.go

@@ -218,7 +218,7 @@ func (f *Filesystem) MkdirAll(path string) error {
 // Create creates a new os.File.
 func (f *Filesystem) Create(path string) (*os.File, error) {
 	if f.dryRun {
-		return &os.File{}, nil
+		return os.OpenFile(os.DevNull, os.O_WRONLY, 0)
 	}
 
 	return os.Create(path)
@@ -237,7 +237,7 @@ func (f *Filesystem) Open(path string) (*os.File, error) {
 // CreateTemp creates a new temporary file
 func (f *Filesystem) CreateTemp(dir, pattern string) (*os.File, error) {
 	if f.dryRun {
-		return &os.File{}, nil
+		return os.OpenFile(os.DevNull, os.O_WRONLY, 0)
 	}
 
 	file, err := os.CreateTemp(dir, pattern)

storage/fs_test.go

@@ -1,10 +1,82 @@
 package storage
 
-import "testing"
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
 
 func TestFilesystemImplementsStorageInterface(t *testing.T) {
 	var i interface{} = new(Filesystem)
 	if _, ok := i.(Storage); !ok {
 		t.Errorf("expected %t to implement Storage interface", i)
 	}
 }
+
+func TestFilesystemCreateDryRun(t *testing.T) {
+	t.Parallel()
+
+	fs := &Filesystem{dryRun: true}
+	tmpDir := t.TempDir()
+
+	testcases := []struct {
+		name   string
+		create func() (*os.File, error)
+	}{
+		{
+			name: "Create",
+			create: func() (*os.File, error) {
+				return fs.Create(filepath.Join(tmpDir, "should-not-exist.txt"))
+			},
+		},
+		{
+			name: "CreateTemp",
+			create: func() (*os.File, error) {
+				return fs.CreateTemp(tmpDir, "dryrun-*.txt")
+			},
+		},
+	}
+
+	for _, tc := range testcases {
+		t.Run(tc.name, func(t *testing.T) {
+			f, err := tc.create()
+			if err != nil {
+				t.Fatalf("%s() returned error: %v", tc.name, err)
+			}
+			if f == nil {
+				t.Fatalf("%s() returned nil file", tc.name)
+			}
+
+			if f.Name() != os.DevNull {
+				t.Fatalf("expected file name %q, got %q", os.DevNull, f.Name())
+			}
+
+			// the handle must not alias stdin, otherwise closing it would
+			// close stdin as well.
+			if f.Fd() == 0 {
+				t.Fatal("expected file descriptor other than stdin")
+			}
+
+			if _, err := f.Write([]byte("content")); err != nil {
+				t.Fatalf("Write() returned error: %v", err)
+			}
+
+			if err := f.Close(); err != nil {
+				t.Fatalf("Close() returned error: %v", err)
+			}
+
+			if _, err := os.Stdin.Stat(); err != nil {
+				t.Fatalf("stdin is not usable after Close(): %v", err)
+			}
+		})
+	}
+
+	// no file should be created on disk during dry run
+	entries, err := os.ReadDir(tmpDir)
+	if err != nil {
+		t.Fatalf("ReadDir failed: %v", err)
+	}
+	if len(entries) != 0 {
+		t.Fatalf("expected no files in temp dir during dry run, got %d", len(entries))
+	}
+}