Documentation
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Overview ¶
Package rootfsblob holds the rootfs blob's self-describing format contract: the byte patterns the HOST writes into the cached blob and the GUEST reads back to decide how to materialize it.
It lives in internal/ for the reason internal/protocol does — both worlds hardcode this independently and a mismatch breaks the pairing — but it is a different contract from the wire protocol, so it is a different package. The blob is not sent over the wire at all: the host attaches it as a read-only virtio-blk device and the guest reads it off the block layer.
Why the format is sniffed rather than announced. The codec is a host-side choice that already rides the content-cache identifier (digest + codec + flatten generation), so a given blob's format is fixed at the moment it is written and can never change under a running guest. Sending it out of band as well would create a second source of truth that could disagree with the bytes — and the one path where that matters, a cache entry produced by a different ossein version, is exactly where the out-of-band channel would be least trustworthy. The bytes are authoritative; this package is the single place both sides agree on how to read them.
Index ¶
Constants ¶
const EROFSSuperOffset = 1024
EROFSSuperOffset is where an EROFS image carries its superblock, and erofsMagic is EROFS_SUPER_MAGIC_V1 (0xE0F5E1E2) as it appears there, little-endian.
const SniffLen = EROFSSuperOffset + 4
SniffLen is how many leading bytes Sniff needs to classify every format. A reader must be able to buffer at least this much before dispatching.
Variables ¶
This section is empty.
Functions ¶
This section is empty.
Types ¶
type Format ¶
type Format int
Format is what a rootfs blob contains.
const ( // FormatTar is a bare uncompressed tar. No codec produces one any more — // it is the fallback for anything unrecognized, which is deliberate: an // unknown blob is offered to the tar reader, whose error names an actual // structural problem, rather than to a speculative decoder whose error // would name the wrong layer. A blob reaching this branch means the cache // served something no current codec writes. FormatTar Format = iota // FormatLZ4 is a tar in an lz4 frame — the guest decodes and extracts. FormatLZ4 // FormatEROFS is a mountable filesystem image — the guest mounts it // read-only under a tmpfs-upper overlay and extracts nothing. FormatEROFS )
func Sniff ¶
Sniff classifies the head of a rootfs blob. head may be shorter than SniffLen — a short device read, or simply a blob smaller than a superblock — in which case the formats whose marker lies beyond it are correctly ruled out rather than guessed at.
EROFS is tested first and at a FIXED offset, so it can never be confused with a compressed stream whose framing lives at byte 0, nor matched by an archive that happens to contain those four bytes anywhere else.
Source Files