On 31.03.2019 23:08, ttcoder@netcourrier.com wrote:
Correction: "Mail" is probably a quite bad example, as it relies heavily on extended attributes. There are probably many candidates all over the difficulty spectrum (from easy to hard) that would make more sense to port. Unless someone came up with a hack to wrap xattr's into something else, ouch.

Cedric

xattrs? So, Haiku uses Linux terminology? OS/2 calls them "EA's". What are extended attributes needed for in Mail
applications? Indexing? Storing keywords? OS/2 mostly uses EA's to store object properties for desktop objects, and
also to emulate POSIX file attributes. And also icons, keywords for search, tokenized images (i.e., bytecode) for scripts written in REXX language etc. In OS/2, EA's are limited in size to at least, 64 KB. With JFS, they theoretically could be larger, but for compatibility with older HPFS file system, the limit is set to 64 KB. IIRC, in ext2fs, max. extended attribute size is only 4 KB, which could be too little for some purposes.  In Sculpt, ext2 is used as current file system, so, its limits apply. (this is for Linux ext2 driver, not sure about a NetBSD driver in rump kernel). On other file sysytems, EA support may be missing, but it can be emulated. For example, on FAT, OS/2 uses a file called "ea data. sf" in the root directory, which contains EA's of all files on the partition in special format. All file metainfo is accessed from a directory entry (on FAT), or its inode on most other file systems. So, on FAT, a directory entry contains a 16-bit handle to an EA stored in "ea data. sf" file. Though, in FAT32 direntry, the two bytes used to reference a EA, are reused for high word of first cluster number of the file. So, for implementation of EA's in FAT32, two bits of another reserved byte of a direntry is reused. (one bit means "this file has EA's", another means "the EA's are critical", i.e., they are mandatory to be existed when opening the file). So, EA's on FAT32 are laying around near the file itself, in the same directory, in a hidden file. The disadvantage is that EA files are laying around everywhere, like a trash. So, if a file system doesn't support EA's, they can be emulated like this.

If I correctly understood, there is no EA/xattr support in Genode VFS, so far. But for my project (which is "OS/2 personality" on top of Genode), I plan to make a server (like Genode's VFS server) supporting OS/2-like IFS (Installable File Systems), like plugins. They, of course, support EA's. But then only OS/2 applications will be able to take advantage of using EA's. Serving OS/2 file systems to Genode applications is planned too. You can go similar way. 


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