[Rpm-maint] [rpm-software-management/rpm] API improvement to accommodate for RPM CoW (PR#1470) (Discussion #2057)

[David Disseldorp]

If what you mean is that the payload shouldn't be used before it's been checked against a digest in the cryptographically verified header, then yes. The question is what quantifies as "used".
The payload data needs to go somewhere while we're calculating the digest, in which case we could "stream" it to Btrfs in it's compressed, yet-to-be-verified form via `BTRFS_IOC_ENCODED_WRITE`. However, doing so would allow any concurrent (pre-verification) reads to effectively fuzz the btrfs/zstd decompression code path.
One protection mechanism might be to write out the payload in its regular (compressed) form before attempting to *switch* those regular extents to compressed extents on btrfs via some new `copy_file_range()` flag. The regular->compressed extent switch would only be attempted following successful payload verification.
I'm very much open to any other suggestions :-)

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