fix: add Sync() calls to persist filesystem changes to storage

[marcelfarres]

I was trying to create folders & files in the SD card and it was not working.

This changes seem to make it functional again.

I test it on a M4 Gran Central, and seems to have dir persistent and creation even after mounting/un-moutning the file system.

Not an expert in any extend on TinyGo, hope it helps/you can test it/review it.

• Add Sync() after Format, Unmount, Remove, Rename, Mkdir
• Add Sync() after File.Close, File.Sync, File.Truncate
• Sync on OpenFile with O_CREATE/O_TRUNC flags
• Fix cache invalidation in lfs_bd_flush
• Add TestDirectoryPersistence and NestedDirectories tests"

Should fix #9

[marcelfarres]

Fix filesystem persistence on SD cards

Problem

I was working with LittleFS on a Grand Central M4 with an SD card and noticed that directories and files I created weren't surviving a power cycle. I'd run mkdir, it would succeed, ls would show the directory. But after unplugging and reconnecting (or even just unmounting and remounting), everything was gone.

Investigation

I traced the data flow through the code:

1. When I call fs.Mkdir("test"), it goes to go_lfs.go:247
2. That calls the C library's lfs_mkdir() via CGo
3. LittleFS writes through the callbacks in go_lfs_callbacks.go to the SD card driver
4. The SD card driver buffers the writes... and that's where they stay

The problem: the SD card driver holds writes in a buffer for efficiency. Without explicitly calling Sync(), that buffer never gets flushed to the physical flash. On power loss, the buffered data is gone.

Looking at the original code, I noticed that mutating operations just returned after calling the C function—no sync. The block device interface in tinyfs.go defines a Syncer interface, but it wasn't being used after filesystem operations.

What I changed

littlefs/go_lfs.go

Added Sync() calls after every operation that modifies the filesystem. Example for Mkdir:

func (l *LFS) Mkdir(path string, _ os.FileMode) error {
    cs := (*C.char)(cstring(path))
    defer C.free(unsafe.Pointer(cs))
    if err := errval(C.lfs_mkdir(l.lfs, cs)); err != nil {
        return err
    }
    if syncer, ok := l.dev.(tinyfs.Syncer); ok {
        return syncer.Sync()  // <-- This was missing!
    }
    return nil
}

Same pattern applied to:

• Format() - sync after formatting
• Unmount() - sync before closing
• Remove() - sync after deletion
• Rename() - sync after rename
• OpenFile() with O_CREATE/O_TRUNC - sync after creating
• File.Close() - sync after closing
• File.Sync() - sync to block device
• File.Truncate() - sync after truncation

littlefs/go_lfs_test.go

Added tests:

• TestDirectoryPersistence - creates a directory, unmounts, remounts, verifies it persists
• NestedDirectories - creates 3 levels of nested directories

Testing

Tested on actual hardware (Grand Central M4 + SD card):

• Formatted the card
• Created nested directories (level1/level2/level3)
• Created files
• Unmounted and remounted
• Everything persisted ✓

[deadprogram]

I do not think it makes sense to call Sync() on an Open().

[deadprogram]

Actually Sync() on a Sync()? Seems like a yes! 👍

[deadprogram]

Hello @marcelfarres thanks for the code submission. Please see my comments.

[deadprogram]

I probably did not notice the issue due to using flash pretty much all of the time.