move "link or copy" functionality to moxio

and add a bit more logging for unexpected failures when closing files.
and make tests pass with a TMPDIR on a different filesystem than the testdata directory.

moxio/linkcopy.go

@@ -0,0 +1,71 @@
+package moxio
+
+import (
+	"fmt"
+	"io"
+	"os"
+
+	"github.com/mjl-/mox/mlog"
+)
+
+// LinkOrCopy attempts to make a hardlink dst. If that fails, it will try to do a
+// regular file copy. If srcReaderOpt is not nil, it will be used for reading. If
+// sync is true and the file is copied, Sync is called on the file after writing to
+// ensure the file is written on disk. Callers should also sync the directory of
+// the destination file, but may want to do that after linking/copying multiple
+// files. If dst was created and an error occurred, it is removed.
+func LinkOrCopy(log *mlog.Log, dst, src string, srcReaderOpt io.Reader, sync bool) (rerr error) {
+	// Try hardlink first.
+	err := os.Link(src, dst)
+	if err == nil {
+		return nil
+	} else if os.IsNotExist(err) {
+		// No point in trying with regular copy, we would fail again. Either src doesn't
+		// exist or dst directory doesn't exist.
+		return err
+	}
+
+	// File system may not support hardlinks, or link could be crossing file systems.
+	// Do a regular file copy.
+	if srcReaderOpt == nil {
+		sf, err := os.Open(src)
+		if err != nil {
+			return fmt.Errorf("open source file: %w", err)
+		}
+		defer func() {
+			err := sf.Close()
+			log.Check(err, "closing copied source file")
+		}()
+		srcReaderOpt = sf
+	}
+
+	df, err := os.OpenFile(dst, os.O_CREATE|os.O_EXCL|os.O_WRONLY, 0660)
+	if err != nil {
+		return fmt.Errorf("create destination: %w", err)
+	}
+	defer func() {
+		if df != nil {
+			err = os.Remove(dst)
+			log.Check(err, "removing partial destination file")
+			err = df.Close()
+			log.Check(err, "closing partial destination file")
+		}
+	}()
+
+	if _, err := io.Copy(df, srcReaderOpt); err != nil {
+		return fmt.Errorf("copy: %w", err)
+	}
+	if sync {
+		if err := df.Sync(); err != nil {
+			return fmt.Errorf("sync destination: %w", err)
+		}
+	}
+	err = df.Close()
+	df = nil
+	if err != nil {
+		err := os.Remove(dst)
+		log.Check(err, "removing partial destination file")
+		return err
+	}
+	return nil
+}