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_test.go

@@ -0,0 +1,57 @@
+package moxio
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/mjl-/mox/mlog"
+)
+
+func tcheckf(t *testing.T, err error, format string, args ...any) {
+	if err != nil {
+		t.Helper()
+		t.Fatalf("%s: %s", fmt.Sprintf(format, args...), err)
+	}
+}
+
+func TestLinkOrCopy(t *testing.T) {
+	log := mlog.New("linkorcopy")
+
+	// link in same directory. file exists error. link to file in non-existent
+	// directory (exists error). link to file in system temp dir (hopefully other file
+	// system).
+	src := "linkorcopytest-src.txt"
+	f, err := os.Create(src)
+	tcheckf(t, err, "creating test file")
+	defer os.Remove(src)
+	err = LinkOrCopy(log, "linkorcopytest-dst.txt", src, nil, false)
+	tcheckf(t, err, "linking file")
+	err = os.Remove("linkorcopytest-dst.txt")
+	tcheckf(t, err, "remove dst")
+
+	err = LinkOrCopy(log, "bogus/linkorcopytest-dst.txt", src, nil, false)
+	if err == nil || !os.IsNotExist(err) {
+		t.Fatalf("expected is not exist, got %v", err)
+	}
+
+	// Try with copying the file. This can currently only really happen on systems that
+	// don't support hardlinking. Because other code and tests already use os.Rename on
+	// similar files, which will fail for being cross-filesystem (and we do want
+	// users/admins to have the mox temp dir on the same file system as the account
+	// files).
+	dst := filepath.Join(os.TempDir(), "linkorcopytest-dst.txt")
+	err = LinkOrCopy(log, dst, src, nil, true)
+	tcheckf(t, err, "copy file")
+	err = os.Remove(dst)
+	tcheckf(t, err, "removing dst")
+
+	// Copy based on open file.
+	_, err = f.Seek(0, 0)
+	tcheckf(t, err, "seek to start")
+	err = LinkOrCopy(log, dst, src, f, true)
+	tcheckf(t, err, "copy file from reader")
+	err = os.Remove(dst)
+	tcheckf(t, err, "removing dst")
+}