RTEMS | Port FatFS to RTEMS with Benchmarking (#25)

[Sepehr Ganji (@Sepehr)]

Sepehr Ganji commented: https://gitlab.rtems.org/groups/rtems/-/work_items/25#note_129098


My progress continues with refining the tests and hunting down the bugs this week:

1. **Fixed Parent Directory Timestamp Updates**  
   FatFS was failing POSIX compliance tests because it didn’t update parent directory timestamps when files were created or deleted. POSIX requires that directory `mtime` and `ctime` reflect these changes. I extended the `rtems_fatfs_node_t` structure with POSIX-compliant timestamp fields and added a `rtems_fatfs_update_parent_dir_time()` function to update parent directory times during file creation or deletion. I also modified `fstat` to prioritize POSIX timestamps when available, ensuring compliance with minimal memory overhead and without modifying the core FatFS library.

2. **Implemented O_TRUNC Handling**  
   The `rtems_fatfs_openfile` function ignored the `O_TRUNC` flag, violating POSIX requirements that opening with `O_TRUNC` should truncate the file and update timestamps. I fixed this by detecting the flag and calling `f_truncate()`. For empty files where `f_truncate()` has no effect, I wrote a dummy byte and immediately truncated back to zero to force timestamp updates. This ensured correct POSIX behavior while leveraging FatFS’s natural mechanisms.

3. **Ensured Write Operations Update File Timestamps**  
   FatFS was not updating file timestamps during `write()` calls, leaving `mtime` and `ctime` unchanged. I updated the `rtems_fatfs_file_write` function to set `posix_mtime` and `posix_ctime` to the current system time whenever data is written, marking them as valid. The `rtems_fatfs_node_to_stat()` function now reports these updated timestamps, making write operations POSIX-compliant.

4. **Implemented utime() with Persistent POSIX Timestamp Cache**  
   RTEMS FatFS didn’t support `utime()`, causing test failures. Even after adding a basic implementation, the timestamps didn’t persist because `stat()` recreated nodes without keeping the updated values. I solved this by adding a filesystem-level POSIX timestamp cache (circular buffer) that stores updates from `utimens()` beyond node lifecycles. Now, `stat()` retrieves the correct POSIX timestamps, ensuring `st_ctime` reflects metadata changes properly.

5. **Added Proper statvfs Support**  
   FatFS used `rtems_filesystem_default_statvfs`, which returned `ENOSYS` and caused test failures. I implemented a proper `rtems_fatfs_statvfs()` function that uses FatFS’s `f_getfree()` to gather statistics such as free clusters, total clusters, and sector sizes. I replaced the default function in the filesystem operations table and integrated the new implementation into the build system. This solution maps FatFS internals into the POSIX `statvfs` format, handles variable sector sizes, ensures thread safety, and now passes the `fsstatvfs` test successfully.

-- 
View it on GitLab: https://gitlab.rtems.org/groups/rtems/-/work_items/25#note_129098
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