[ma2bd]

Motivation

The web wallet decompresses bytecodes with ruzstd, whose StreamingDecoder decodes a single
zstd frame and then keeps returning Ok(0). Our wasm32 code did not account for that, as
noted in #2710:

• decompress looped while !decoder.get_ref().is_empty(), so a bytecode made of more than one
  frame made it loop forever, freezing the tab, instead of merely decoding the first frame.
• decompressed_size_at_most measured the first frame only, so it underestimated the size of a
  multi-frame bytecode.

Validators use the zstd crate, which accepts several concatenated frames, both for the size
check in linera-execution/src/policy.rs and for decompression in linera-storage/src/lib.rs.
Nothing that we compress produces multiple frames, but nothing rejects them either: a module
published as two frames is valid to a validator and hangs every web client that loads it.

Proposal

Decode frame by frame on wasm32, creating a StreamingDecoder per frame and stepping over
skippable frames using the length in their header, which is what zstd does. The two targets
now accept the same bytecodes, so this is not a change in validation.

The frame loop is shared by decompress and decompressed_size_at_most, and replaces the
TODO(#2710) comments in both.

Also drops the two #[cfg(with_metrics)] blocks from the wasm32 impl: with_metrics is
defined as all(not(target_arch = "wasm32"), feature = "metrics"), so they were dead code, and
the one in decompress referred to an unqualified BYTECODE_DECOMPRESSION_LATENCY that would
not have compiled had it ever been enabled.

Test Plan

Three new unit tests in linera-base cover a bytecode of two frames followed by a skippable
frame, and check the new code against zstd on the same bytes: all frames are decompressed, all
frames count towards the size limit, and trailing garbage is an error rather than a hang. They
run on the usual targets rather than on wasm32, where Bytecode::compress is unavailable, so
CI covers them; ruzstd is a dev-dependency off wasm32 for that purpose.

Checked with cargo test -p linera-base, cargo clippy -p linera-base --all-targets --all-features and cargo clippy -p linera-base --target wasm32-unknown-unknown.

Release Plan

• These changes should be backported to the latest testnet branch, then
  • be released in a new SDK.

Links

• Fixes Bytecodes compressed into multiple frames can't be decompressed in the web wallet #2710
• reviewer checklist