syscallcompat: move quirks logic here & fix darwin

We need to look at f_fstypename acc. to
https://stackoverflow.com/a/52299141/1380267 :

> As filesystem type numbers are now assigned at runtime in
> recent versions of MacOS, you must use f_fstypename to
> determine the type.

https://github.com/rfjakob/gocryptfs/issues/585

1 files changed, 41 insertions, 0 deletions

diff --git a/internal/syscallcompat/quirks_darwin.go b/internal/syscallcompat/quirks_darwin.go
new file mode 100644
index 0000000..f4e7e71
--- /dev/null
+++ b/internal/syscallcompat/quirks_darwin.go
@@ -0,0 +1,41 @@
+package syscallcompat
+
+import (
+	"golang.org/x/sys/unix"
+
+	"github.com/rfjakob/gocryptfs/internal/tlog"
+)
+
+func DetectQuirks(cipherdir string) (q uint64) {
+	const (
+		// From https://github.com/rfjakob/gocryptfs/issues/585#issuecomment-887370065
+		FstypenameExfat = "exfat"
+	)
+
+	var st unix.Statfs_t
+	err := unix.Statfs(cipherdir, &st)
+	if err != nil {
+		tlog.Warn.Printf("DetectQuirks: Statfs on %q failed: %v", cipherdir, err)
+		return 0
+	}
+
+	// Convert null-terminated st.Fstypename int8 array to string
+	var buf []byte
+	for _, v := range st.Fstypename {
+		if v == 0 {
+			break
+		}
+		buf = append(buf, byte(v))
+	}
+	fstypename := string(buf)
+	tlog.Debug.Printf("DetectQuirks: Fstypename=%q\n", fstypename)
+
+	// On MacOS ExFAT, all empty files share inode number 1:
+	// https://github.com/rfjakob/gocryptfs/issues/585
+	if fstypename == FstypenameExfat {
+		logQuirk("ExFAT detected, disabling hard links. See https://github.com/rfjakob/gocryptfs/issues/585 for why.")
+		q |= QuirkDuplicateIno1
+	}
+
+	return q
+}