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docs: extend object type documentation
Extend the object type documentation with file endings used for the individual type. Also clarify in which situation content type objects are used and why they do not match the SHA256 hash today. Signed-off-by: Stefan Agner <stefan.agner@toradex.com>
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@ -31,13 +31,16 @@ regenerate it from source code.
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A dirtree contains a sorted array of (filename, checksum)
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pairs for content objects, and a second sorted array of
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(filename, dirtree checksum, dirmeta checksum), which are
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subdirectories.
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subdirectories. These type of objects are stored as files
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ending with `.dirtree` in the objects directory.
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### Dirmeta objects
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In git, tree objects contain the metadata such as permissions
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for their children. But OSTree splits this into a separate
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object to avoid duplicating extended attribute listings.
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These type of objects are stored as files ending with `.dirmeta`
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in the objects directory.
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### Content objects
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@ -45,7 +48,13 @@ Unlike the first three object types which are metadata, designed to be
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`mmap()`ed, the content object has a separate internal header and
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payload sections. The header contains uid, gid, mode, and symbolic
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link target (for symlinks), as well as extended attributes. After the
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header, for regular files, the content follows.
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header, for regular files, the content follows. These parts toghether
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form the SHA256 hash for content objects. The content type objects in
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this format exist only in `archive` OSTree repositories. Today the
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content part is gzip'ed and the objects are stored as files ending
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with `.filez` in the objects directory. Because the SHA256 hash is
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formed over the uncompressed content, these files do not match the
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hash they are named as.
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The OSTree data format intentionally does not contain timestamps. The reasoning
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is that data files may be downloaded at different times, and by different build
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