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

3 files changed, 97 insertions, 0 deletions

diff --git a/internal/syscallcompat/quirks.go b/internal/syscallcompat/quirks.go
new file mode 100644
index 0000000..60d584d
--- /dev/null
+++ b/internal/syscallcompat/quirks.go
@@ -0,0 +1,20 @@
+package syscallcompat
+
+import (
+	"github.com/rfjakob/gocryptfs/internal/tlog"
+)
+
+const (
+	// QuirkBrokenFalloc means the falloc is broken.
+	// Preallocation on Btrfs is broken ( https://github.com/rfjakob/gocryptfs/issues/395 )
+	// and slow ( https://github.com/rfjakob/gocryptfs/issues/63 ).
+	QuirkBrokenFalloc = uint64(1 << iota)
+	// QuirkDuplicateIno1 means that we have duplicate inode numbers.
+	// On MacOS ExFAT, all empty files share inode number 1:
+	// https://github.com/rfjakob/gocryptfs/issues/585
+	QuirkDuplicateIno1
+)
+
+func logQuirk(s string) {
+	tlog.Info.Printf(tlog.ColorYellow + "DetectQuirks: " + s + tlog.ColorReset)
+}
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
+}
diff --git a/internal/syscallcompat/quirks_linux.go b/internal/syscallcompat/quirks_linux.go
new file mode 100644
index 0000000..ffdbfab
--- /dev/null
+++ b/internal/syscallcompat/quirks_linux.go
@@ -0,0 +1,36 @@
+package syscallcompat
+
+import (
+	"golang.org/x/sys/unix"
+
+	"github.com/rfjakob/gocryptfs/internal/tlog"
+)
+
+// DetectQuirks decides if there are known quirks on the backing filesystem
+// that need to be workarounded.
+//
+// Tested by tests/root_test.TestBtrfsQuirks
+func DetectQuirks(cipherdir string) (q uint64) {
+	const (
+		// From Linux' man statfs
+		BTRFS_SUPER_MAGIC = 0x9123683e
+	)
+
+	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
+	}
+
+	// Preallocation on Btrfs is broken ( https://github.com/rfjakob/gocryptfs/issues/395 )
+	// and slow ( https://github.com/rfjakob/gocryptfs/issues/63 ).
+	//
+	// Cast to uint32 avoids compile error on arm: "constant 2435016766 overflows int32"
+	if uint32(st.Type) == BTRFS_SUPER_MAGIC {
+		logQuirk("Btrfs detected, forcing -noprealloc. See https://github.com/rfjakob/gocryptfs/issues/395 for why.")
+		q |= QuirkBrokenFalloc
+	}
+
+	return q
+}