Ensure FAT file timestamps match when files were written

fatfs/go_fatfs_callbacks.go

@@ -83,15 +83,30 @@ func go_fatfs_disk_ioctl(drv unsafe.Pointer, cmd uint8, param unsafe.Pointer) in
 
 //export go_fatfs_get_fattime
 func go_fatfs_get_fattime() (t uint32) {
+	const (
+		epochYear      = 1980 // years are stored as an offset from 1980
+		yearShift      = 25   // year offset, bits 25-31
+		monthShift     = 21   // month, bits 21-24
+		dayShift       = 16   // day of month, bits 16-20
+		hourShift      = 11   // hour, bits 11-15
+		minuteShift    = 5    // minute, bits 5-10
+		monthMask      = 0x0f // month, 4 bits
+		dayMask        = 0x1f // day of month, 5 bits
+		hourMask       = 0x1f // hour, 5 bits
+		minuteMask     = 0x3f // minute, 6 bits
+		secondMask     = 0x1f // two-second units, 5 bits
+		secondsPerUnit = 2    // FAT timestamps have two-second resolution
+	)
+
 	now := time.Now()
 	year, month, day := now.Date()
 	hour, minute, second := now.Hour(), now.Minute(), now.Second()
-	t |= uint32(year-1980) << 24
-	t |= (uint32(month) & 0xF) << 20
-	t |= (uint32(day) & 0x1F) << 15
-	t |= (uint32(hour) & 0x1F) << 10
-	t |= (uint32(minute) & 0x3F) << 4
-	t |= (uint32(second) / 2) & 0xF
+	t |= uint32(year-epochYear) << yearShift
+	t |= (uint32(month) & monthMask) << monthShift
+	t |= (uint32(day) & dayMask) << dayShift
+	t |= (uint32(hour) & hourMask) << hourShift
+	t |= (uint32(minute) & minuteMask) << minuteShift
+	t |= (uint32(second) / secondsPerUnit) & secondMask
 	return t
 }
 

fatfs/go_fatfs_test.go

@@ -3,8 +3,10 @@
 package fatfs
 
 import (
+	"encoding/binary"
 	"os"
 	"testing"
+	"time"
 
 	"tinygo.org/x/tinyfs"
 )
@@ -15,6 +17,41 @@ const (
 	testBlockCount = 4096
 )
 
+func TestStoredModificationTime(t *testing.T) {
+	fs, dev, unmount := createTestFS(t)
+	defer unmount()
+	if typ := Type(fs.fs.fs_type); typ != TypeFAT12 && typ != TypeFAT16 {
+		t.Fatalf("expected FAT12 or FAT16, got %s", typ)
+	}
+	before := time.Now().Truncate(2 * time.Second)
+	f, err := fs.OpenFile("TIME.TXT", os.O_CREATE|os.O_WRONLY|os.O_TRUNC)
+	check(t, err)
+	_, err = f.Write([]byte("timestamp"))
+	check(t, err)
+	check(t, f.Close())
+	after := time.Now()
+
+	root := make([]byte, int(fs.fs.n_rootdir)*32)
+	_, err = dev.ReadAt(root, int64(fs.fs.dirbase)*SectorSize)
+	check(t, err)
+	for offset := 0; offset < len(root); offset += 32 {
+		entry := root[offset : offset+32]
+		if string(entry[:11]) != "TIME    TXT" {
+			continue
+		}
+		// FAT directory entries store modification time at bytes 22-25.
+		clock := binary.LittleEndian.Uint16(entry[22:24])
+		date := binary.LittleEndian.Uint16(entry[24:26])
+		got := time.Date(1980+int(date>>9), time.Month(date>>5&15), int(date&31),
+			int(clock>>11), int(clock>>5&63), int(clock&31)*2, 0, time.Local)
+		if got.Before(before) || got.After(after) {
+			t.Fatalf("stored modification time = %s, want between %s and %s", got, before, after)
+		}
+		return
+	}
+	t.Fatal("TIME.TXT not found in root directory")
+}
+
 func TestType_String(t *testing.T) {
 	expectString(t, "fatfs: (1) A hard error occurred in the low level disk I/O layer", FileResultErr.Error())
 }