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fix(byte-storage): copy a large payload only after the fallback decode accepts the envelope (LAB-12208) - #154
The fallback envelope decode copied a bin or str payload of LARGE_PAYLOAD (64 KiB) or more out of the input as soon as rmp_serde reached it, before the rest of the envelope had decoded. An envelope that passes the pre-scan and then fails in a later field (an array of 3 or 5 elements, an integer or non-UTF-8 format, a 7-element checksum) had already made a payload-sized allocation when it was rejected. The one-pass decode copies only after it has read all four fields; the fallback now does the same.
This corrects the premise of the reply on #150 that the large-payload reject path held by construction: it held for the one-pass decode, not for the fallback.
Change
The borrowed payload visitor always borrows a bin or str payload.
decode_envelope_full applies the copy threshold (copy_large_payload, which calls the same borrow_below_large the one-pass decode uses) only once rmp_serde::from_slice returns Ok.
Accepted envelopes decode as before: a payload under LARGE_PAYLOAD is borrowed, a longer one is owned, and a legacy int-array payload is owned. The accept/reject set, the decoded fields and every error message are unchanged. No wire-format change.
Test
New rejected_envelope_reads_do_not_copy_a_large_payload in src/read_allocation_probe.rs builds five envelopes inline, each with a bin32 payload of LARGE_PAYLOAD and of 2 * LARGE_PAYLOAD bytes, and a str32 payload of LARGE_PAYLOAD bytes (the visitor's visit_borrowed_str). Through both retrieve and retrieve_data it asserts DeserializationFailed raised by rmp_serde, not by the pre-scan (a pre-scan rejection never reaches the payload, so the count would pass regardless), and fewer than LARGE_PAYLOAD bytes requested from the allocator.
Red at ca626d1 (the test alone, on main at a55af62), green at head. Bytes requested at the 65,536 B payload; retrieve and retrieve_data request the same, and so do bin32 and str32. At 131,072 B the red counts grow by exactly 65,536 B and the green counts do not change.
Envelope
Error
Before
After
fixarray(3), ends before format
invalid length 3, expected struct StorageEnvelope with 4 elements
65,789
253
fixarray(5), trailing nil
array had incorrect length, expected 4
65,679
143
integer format
invalid type: integer `7`, expected a string
65,662
126
non-UTF-8 format
string found to be invalid utf8: …
65,851
315
7-element checksum
invalid length 7, expected an array of length 8
65,667
131
Deleting the copy_large_payload call fails the existing bin_payload_is_borrowed_and_legacy_payload_is_owned, so the copy on an accepted envelope is pinned too.
No protocol vector: LARGE_PAYLOAD is internal to this crate, and the protocol's allocation requirement covers only the size-cap and ratio reject vectors.
Local: cargo fmt --check; clippy with --all-features, --all-features --all-targets and --features encryption --all-targets; cargo test with --all-features, --features ffi and --features encryption. The byte_storage_corrupted_envelope fuzz target builds and smoke-ran for 60 s with no failure.
Docs: SECURITY.md → Envelope decode bounds, the read_allocation_probe.rs module doc and the README's test paragraph now say a large payload is copied only once the envelope decode accepts the envelope. They also say what still allocates: an envelope the decode accepts is copied even when the size, ratio or checksum check after it rejects the envelope (by design, unchanged), and a legacy int-array payload is decoded into an owned buffer as it is read.
A complete envelope with a bin32 payload of LARGE_PAYLOAD or more passes the
pre-scan and reaches rmp_serde, whose payload visitor copies the payload
before a later field rejects the envelope. Fails on main: every case
requests at least the payload's length.
…e accepts the envelope
The fallback decode's payload visitor copied a bin or str payload of
LARGE_PAYLOAD or more as soon as rmp_serde reached it, so an envelope that
then failed in a later field (a short array, a bad format, a 7-byte
checksum) had already copied its payload. The one-pass decode copies only
after it has read all four fields.
The visitor now always borrows, and decode_envelope_full applies the copy
threshold once rmp_serde returns Ok. Accepted envelopes decode as before:
a payload under LARGE_PAYLOAD is borrowed and a longer one is owned.
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README.md
src/read_allocation_probe.rs
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Summary by CodeRabbit
Bug Fixes
Rejected messages with large binary or string payloads are checked without copying those payloads into additional memory, reducing unnecessary memory use.
Large binary and string payloads are copied only after an envelope is accepted. Legacy integer-array payloads continue to be decoded into owned buffers.
Documentation
Clarified memory handling for accepted and rejected messages, including legacy integer-array payloads and when large payloads are copied.
Walkthrough
The full MessagePack decode now borrows byte and string payloads during deserialisation. After successful envelope deserialisation, it copies payloads of at least 64 KiB. Allocation tests check rejected envelopes with large payloads.
Changes
Payload decoding
Layer / File(s)
Summary
Decode and verify deferred copying src/byte_storage.rs, src/read_allocation_probe.rs, README.md, SECURITY.md
The full decode copies large borrowed payloads only after successful deserialisation. Tests check allocation requests for rejected envelopes across retrieve and retrieve_data. The documentation describes this behaviour and notes that legacy integer-array payloads are allocated during decoding.
No actionable merge-blocking issue is established by the supplied evidence. The allocation tests cover rejected byte and string payloads; merge after normal checks pass.
Pre-merge checks: 5
Check name
Status
Explanation
Title check
The title clearly identifies the main change: delaying large-payload copying until fallback envelope decoding accepts the envelope. It is specific and related to the changeset.
Description check
The description directly explains the fallback decoding change, affected payload types, tests, unchanged behaviour, documentation updates, and validation performed.
Docstring Coverage
Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff with cyclomatic complexity of at least 2. Documentat…
Linked Issues check
Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check
Check skipped because no linked issues were found for this pull request.
Full details: Docstring Coverage
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Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff with cyclomatic complexity of at least 2. Documentation-only edits and deletions within surviving functions are included; unchanged diff context and fully deleted functions are excluded. Analyzed 3 functions across 2 files. (1 skipped: 1 unsupported.)
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Add a str32 payload case to the large-payload rejection test.
Every payload in this test uses the bin32 marker (0xc6). The visit_borrowed_str branch in borrow_payload therefore never receives input here. This PR changes that branch, so a regression that copies a str payload there would not fail this test.
A str32 (0xdb) payload at LARGE_PAYLOAD passes the pre-scan. It reaches the payload visitor, and the envelope still fails at a later field. The existing assertions therefore apply to it unchanged.
🧪 Suggested test change
- for payload_len in [LARGE_PAYLOAD, 2 * LARGE_PAYLOAD] {- let mut payload = vec![0xc6];+ for (marker, payload_len) in [+ (0xc6, LARGE_PAYLOAD),+ (0xc6, 2 * LARGE_PAYLOAD),+ (0xdb, LARGE_PAYLOAD),+ ] {+ let mut payload = vec![marker];
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Review comment at @src/read_allocation_probe.rs around lines 168 - 175:
Add a `str32` case to the large-payload rejection test so `borrow_payload`’s
`visit_borrowed_str` branch is exercised. Extend the payload cases with the
`0xdb` marker at `LARGE_PAYLOAD`, while preserving both existing `bin32` cases
and their assertions.
Fix comments: Fix CodeRabbit comments on this PR
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Inline comments:
Review comments at @SECURITY.md:
- Around line 198-199: Update the README allocation claim to limit it to
envelopes rejected by typed decoding, distinguishing those from decoder-accepted
envelopes that may still be copied before `extract_payload` returns
`InputTooLarge`.
---
Nitpick comments:
Review comments at @src/read_allocation_probe.rs:
- Around line 168-175: Add a `str32` case to the large-payload rejection test so
`borrow_payload`’s `visit_borrowed_str` branch is exercised. Extend the payload
cases with the `0xdb` marker at `LARGE_PAYLOAD`, while preserving both existing
`bin32` cases and their assertions.
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Reviewing files that changed from the base of the PR and between a55af62 and aee2146.
Files selected for processing: 4
README.md
SECURITY.md
src/byte_storage.rs
src/read_allocation_probe.rs
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… rejections, add a str32 case
The docs said a rejected envelope copies no large payload. An envelope the
decode accepts is still copied when the size, ratio or checksum check that
follows rejects it, which is by design; the README, SECURITY.md and the
probe module doc now say "rejected by the decode". The probe test also
runs a str32 payload, which reaches the visitor's visit_borrowed_str.
CodeRabbit-Resolved: SECURITY.md:198:Update the README allocati
CodeRabbit-Resolved: read_allocation_probe.rs:168:Add a str32 case to the
04631a1 also adds the nitpick's str32 case to rejected_envelope_reads_do_not_copy_a_large_payload; restoring the old copy in visit_borrowed_str fails it.
Note: CodeRabbit is an incremental review system and does not re-review already reviewed commits. This command is applicable only when automatic reviews are paused.
…copy
A rejected read can still allocate in proportion to the input: rmp_serde's
error message quotes an offending str field. What the decode guarantees is
that it never copies the payload of an envelope it rejects, so SECURITY.md,
the probe module doc and copy_large_payload's doc now say that and no more.
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Resolved src/read_allocation_probe.rs (union: kept both new tests, rejected_envelope_reads_do_not_copy_a_large_payload and main's accepted_envelope_prescan_allocates_nothing); auto-rebased onto main; CI will re-run.
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The fallback envelope decode copied a
binorstrpayload ofLARGE_PAYLOAD(64 KiB) or more out of the input as soon asrmp_serdereached it, before the rest of the envelope had decoded. An envelope that passes the pre-scan and then fails in a later field (an array of 3 or 5 elements, an integer or non-UTF-8format, a 7-element checksum) had already made a payload-sized allocation when it was rejected. The one-pass decode copies only after it has read all four fields; the fallback now does the same.This corrects the premise of the reply on #150 that the large-payload reject path held by construction: it held for the one-pass decode, not for the fallback.
Change
borrowedpayload visitor always borrows abinorstrpayload.decode_envelope_fullapplies the copy threshold (copy_large_payload, which calls the sameborrow_below_largethe one-pass decode uses) only oncermp_serde::from_slicereturnsOk.Accepted envelopes decode as before: a payload under
LARGE_PAYLOADis borrowed, a longer one is owned, and a legacy int-array payload is owned. The accept/reject set, the decoded fields and every error message are unchanged. No wire-format change.Test
New
rejected_envelope_reads_do_not_copy_a_large_payloadinsrc/read_allocation_probe.rsbuilds five envelopes inline, each with abin32payload ofLARGE_PAYLOADand of2 * LARGE_PAYLOADbytes, and astr32payload ofLARGE_PAYLOADbytes (the visitor'svisit_borrowed_str). Through bothretrieveandretrieve_datait assertsDeserializationFailedraised byrmp_serde, not by the pre-scan (a pre-scan rejection never reaches the payload, so the count would pass regardless), and fewer thanLARGE_PAYLOADbytes requested from the allocator.Red at ca626d1 (the test alone, on
mainat a55af62), green at head. Bytes requested at the 65,536 B payload;retrieveandretrieve_datarequest the same, and so dobin32andstr32. At 131,072 B the red counts grow by exactly 65,536 B and the green counts do not change.formatinvalid length 3, expected struct StorageEnvelope with 4 elementsarray had incorrect length, expected 4formatinvalid type: integer `7`, expected a stringformatstring found to be invalid utf8: …invalid length 7, expected an array of length 8Deleting the
copy_large_payloadcall fails the existingbin_payload_is_borrowed_and_legacy_payload_is_owned, so the copy on an accepted envelope is pinned too.No protocol vector:
LARGE_PAYLOADis internal to this crate, and the protocol's allocation requirement covers only the size-cap and ratio reject vectors.Local:
cargo fmt --check; clippy with--all-features,--all-features --all-targetsand--features encryption --all-targets;cargo testwith--all-features,--features ffiand--features encryption. Thebyte_storage_corrupted_envelopefuzz target builds and smoke-ran for 60 s with no failure.Docs: SECURITY.md → Envelope decode bounds, the
read_allocation_probe.rsmodule doc and the README's test paragraph now say a large payload is copied only once the envelope decode accepts the envelope. They also say what still allocates: an envelope the decode accepts is copied even when the size, ratio or checksum check after it rejects the envelope (by design, unchanged), and a legacy int-array payload is decoded into an owned buffer as it is read.Closes LAB-12208