feat(sdk): add a chunked segment writer (experimental) - #3940
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Included review availability: Your plan provides up to 1 included review per hour; 0 remain after this review. 📝 WalkthroughWalkthroughThe SDK adds an experimental chunked writer. It encrypts independently indexed segments, supports out-of-order writes, creates signed manifests, wraps keys for KAS access, and handles retryable and permanently fenced archive failures. ChangesChunked writer SDK
Priority: ⬇️ Low Estimated code review effort: 4 (Complex) | ~60 minutes Change: Feature Sequence Diagram(s)sequenceDiagram
participant Caller
participant ChunkedWriter
participant SegmentCipher
participant ArchiveWriter
participant ManifestBuilder
Caller->>ChunkedWriter: WriteSegment(index, data)
ChunkedWriter->>SegmentCipher: Encrypt segment
ChunkedWriter->>ArchiveWriter: Write encrypted segment
Caller->>ChunkedWriter: Finalize(options)
ChunkedWriter->>ManifestBuilder: Build signed manifest
ChunkedWriter->>ArchiveWriter: Finalize and close archive
ArchiveWriter-->>ChunkedWriter: Return mutation status
ChunkedWriter-->>Caller: Return result or fenced error
Suggested reviewers: Merge Risk: 🔵 Low · up to Concurrent manifest previews can race in stateful custom key splitters. This is bounded to an experimental API but should be documented or serialized. 🚥 Pre-merge checks | ✅ 5✅ Passed checks (5 passed)
✨ Finishing Touches📝 Generate docstrings
🧪 Generate unit tests (beta)
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Inline comments:
In `@sdk/chunked_options.go`:
- Around line 19-24: Clarify the comment above the slice-valued options to state
that cloning is shallow: retained elements such as *policy.Value and
AssertionConfig.SigningKey, including the AssertionKey.Key interface’s
underlying key material, remain caller-owned and must not be mutated after
configuration. Identify that buildChunkedPolicy and signAssertions read these
values during Finalize, so only the slice containers are isolated.
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sdk/chunked_options.gosdk/chunked_test.gosdk/chunked_writer.gosdk/internal/zipstream/segment_writer.gosdk/internal/zipstream/writer.gosdk/key_splitter.gosdk/key_splitter_test.go
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Inline comments:
In `@sdk/chunked_writer.go`:
- Around line 250-254: Update the KeySplitter interface documentation to state
that implementations must be safe for concurrent Split calls, since GetManifest
may invoke the shared splitter concurrently while holding a read lock. Keep the
requirement scoped to concurrency safety for caller-provided splitters accepted
by WithChunkedKeySplitter.
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sdk/chunked_test.gosdk/chunked_writer.gosdk/internal/zipstream/segment_writer_test.go
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Read the whole diff and traced the concurrency, fencing, rollback and manifest paths against main (including #4030's typed integrity helpers and the current zipstream internals). No blocking defects. The hard parts hold up:
- The reservation/commit split is sound, and the ordering constraints are documented at exactly the sites where a "simplification" would break them. I checked the two that would be silent:
CleanupSegmenton an unwritten index is a genuine no-op inSegmentMetadata, so the benign-failure path can't spuriously fence; andwriteLocalFileHeaderwrites only into the caller's buffer, sowriteSegmentErr's "no path out of WriteSegment leaves the writer half-changed" is true rather than aspirational. - The lock-nesting warning in the
WriteSegmentdefer is accurate —Finalizereally does holdw.muacrossarchiveWriter.Finalize, so hoisting the lock aboveCleanupSegmentwould close the cycle. - Segment and root integrity match what the reader recomputes (
tdf.go:1408-1432decodes eachSegment.Hashand concatenates, same asbuildManifest), andDefaultSegmentSize == DefaultEncryptedSegSize - 28satisfies theErrSegSizeMismatchgate for AES-GCM's 12+16 framing. Error.Mutatedis the right shape for the retry-vs-fence decision, andTestSegmentWriterErrorMutatedpins both sides of the boundary for the two sentinels returned from both sides — which is the whole reason the flag can't be inferred.TestSegmentWriterNonMutatingFinalizeIsRetryablethen proves the promise instead of just asserting the flag, which is the version of that test that's actually worth having.Validate+VerifyReconstructionclose a real hole: a bad injected splitter previously surfaced as a root-signature failure at decrypt, reported as tampering, after the plaintext was gone. Constant-time compare and the deliberate refusal to report the mismatch offset are both the right call.- Failure-path coverage is better than the code it covers — cipher panic, cipher failure, archive failure with retry, cleanup failure, close failure, mutating vs non-mutating finalize, in-flight writes, concurrent duplicate indices under
-race. Every sentinel has a test that provokes it.
Eight comments inline. Two are worth settling before #3946 lands CreateTDF on this writer:
- Non-uniform segment sizes (
chunked_writer.go:1014) — nothing requires non-final segments to match segment 0's size, and the manifest advertises segment 0's assegmentSizeDefault. Go's reader maps offsets from cumulative per-segment sizes so it copes; readers that seek byindex * segmentSizeDefaultwon't.TestChunkedRoundTripbakes the non-uniform shape in as the happy path. sealedis a wasted full-segment copy (chunked_writer.go:799) — it rebuilds the exact bufferocryptoalready allocated, to read its last 16 bytes. Now that the algorithm is hardcodedSegmentGMAC,ciphertextgives the identical tag.
The rest are cleanups: a config field no option can set, a policy-JSON divergence from createPolicyObject on the empty-attribute case, a hardcoded literal where the package constant exists, one tautological error message, two unreachable unwrapped error returns, and a needless package-level var.
One more that has no single line to hang off: chunked_test.go uses context.Background() throughout, while segment_writer_test.go in this same PR uses t.Context(). It matters most in TestChunkedConcurrentWrites, TestChunkedGetManifestDuringInFlightWrite and TestChunkedFinalizeRejectsInFlightWrite — those deliberately park a write inside a blocking cipher, so a writer that wedges currently hangs to the package timeout instead of failing where it wedged.
One note on the doc comments, since there are a lot of them and they're load-bearing: I spot-checked the claims that would be expensive to get wrong — the expectedSegments: 1 "harmless because Finalize always derives an order" argument, the EncryptInPlace "nothing is encrypted in place" claim (gcm.Seal(nil, ...), so correct — data really is neither retained nor modified), and the TDFData "second read yields zero bytes and no error" claim. All three check out against the code they describe.
Adds ChunkedWriter, a TDF creation path that accepts segments in any order: callers encrypt and upload each segment independently, then call Finalize to get the ZIP closing bytes. Also adds KeySplitter, the pluggable attribute-to-KAS-split seam Finalize uses. Review fixes folded in on top of the original implementation: - WriteSegment's rollback is now panic-safe (a committed flag plus a single deferred release, instead of an explicit release() call on each error path), so a panic unwinding through an injected cipher or archive-writer seam can no longer leak a segment-index reservation. - release() now also calls archiveWriter.CleanupSegment when the archive itself accepted part of a write before the segment failed, completing the rollback contract zipstream.SegmentWriter documents. - Finalize returns the new sentinel ErrChunkedCloseFailed if archiveWriter.Close fails after archiveWriter.Finalize already succeeded -- the archive is terminally finalized internally at that point regardless, so the writer must refuse further calls rather than look retryable. - WithChunkedIntegrityAlgorithm and WithChunkedSegmentIntegrityAlgorithm now reject any value other than HS256/GMAC instead of letting calculateSignature silently treat an unrecognized value as GMAC. - The default KeySplitter rejects an empty default-KAS URI, which previously propagated silently into KeyAccess.KasURL and produced a TDF with no rewrap endpoint. - Removed the dead maxSegmentIndex field (written, never read). - Documented that WithChunkedSegments trimming does not shrink the archive: CleanupSegment is never called for a dropped index, so the caller must still append its bytes when assembling the file. Second review round -- the data-loss and contradicted-doc findings: - A cancelled context at Finalize no longer bricks the writer. zipstream.Error gains a documented Mutated flag, set per return site in segmentWriter.Finalize against the SetOrder boundary via three constructors (finalizeErr, finalizeErrMutated, writeSegmentErr). chunkedWriter.Finalize pre-checks ctx.Err() before touching the archive and now fences only on a mutation -- or on an error it cannot classify, which fails safe. Context cancellation was the only production-reachable path into ErrChunkedFinalizeFailed, and it is exactly the case where discarding an already-uploaded payload is wrong. - Finalize refuses a racing WriteSegment with the new sentinel ErrChunkedWriteInFlight instead of emitting an archive whose correctness depends on the interleaving. zipstream has already counted the in-flight segment's bytes into the payload entry by the time Finalize can see it, so the trailer's offsets overshoot unless the caller appends bytes the manifest never mentions. The in-flight set is derived from the segment map rather than counted, so it cannot drift. GetManifest deliberately still allows it: a snapshot may be one segment short, an archive may not. - ChunkedFinalizeResult gains ArchiveEncryptedSize and ArchiveTotalSize. EncryptedSize/TotalSize describe the manifest after WithChunkedSegments trimming; the archive totals describe every written segment, which is what the caller must actually append. Sizing an upload from the former silently truncates. - SplitResult.VerifyReconstruction(dek) checks that the shares XOR back to the DEK and that each is the DEK's length -- the half of the splitter contract Validate cannot see. buildChunkedKeyAccessObjects calls it after Validate, before the archive is touched. Without it a splitter returning short or wrong shares produces a TDF that wraps, uploads and rewraps cleanly, then fails the reader's root signature check as if tampered with. - The DEK handed to KeySplitter.Split is now a clone, and the four slice-valued options clone what they are given. A splitter that reuses its argument as scratch space desynchronized the segment signatures from the root signature; a caller reusing an attributes slice could loosen the written policy after the fact. - Removed WithChunkedExcludeVersion and ErrChunkedVersionHexMismatch. The option was dead in both directions: a no-op in legacy mode and an unconditional error outside it, because omitting schemaVersion and hex-encoding signatures can only be chosen together, at construction, which is what WithChunkedTargetMode does. - Doc corrections where the comment contradicted the code: Finalize's duplicated error clause, the "nothing usable was produced" claim on the Close branch (a complete trailer was produced and is being discarded), the unreachable injection seams advertised on NewChunkedWriter, TDFData's single-use reader, GetManifest's overstated concurrency, "must carry a SigningKey", Split.Data and Split.ID, and Experimental banners on unexported types. The lock inversion between WriteSegment's defer and Finalize is now named at both sites. New tests: context cancellation is retryable at both Finalize and WriteSegment, Finalize rejects in-flight writes while GetManifest still succeeds in the same window, a non-mutating trailer failure leaves the writer usable (the existing fencing test is unchanged as the fail-safe guard), archive-vs-manifest totals under trimming, TestSplitResultVerifyReconstruction, DEK and attribute cloning, WithChunkedMimeType, zero-length payload and interior segments, and the manifest deep clone. Signed-off-by: Dave Mihalcik <dmihalcik@virtru.com>
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Proposed Changes
Adds
ChunkedWriter, a TDF creation path that accepts segments in anyorder. Callers encrypt and upload each segment independently -- typically
off-thread or in parallel -- then call
Finalizeto get the ZIP closingbytes. Contrast with
SDK.CreateTDF, which needs the whole plaintext upfront behind an
io.ReadSeeker. This is a rewrite ofhttps://github.com/opentdf/platform/blob/main/sdk/experimental/tdf/writer.go,
absent several features that are added later (including assertions).
This is the first half of DSPX-2604. It adds the implementation only; the
public face of out-of-order writing stays
sdk/experimental/tdf, which isrewired onto this writer in a later PR. Graduating
ChunkedWritertosupported API -- dropping the
Experimental:markers and adding theSDK.NewChunkedWritermethod -- is deliberately deferred to the end of thestack.
What is exported, and why that set
Go forces a compromise here.
sdk/experimental/tdfis a separate package,so it cannot reach unexported symbols in
sdk, and the writer cannot moveunder
sdk/internal/because it needsManifest,KeyAccess,Segment,createKeyAccessandcalculateSignature. So: export the minimum bridgethe adapter needs, mark all of it experimental, unexport everything else.
Exported and
Experimental:-marked:ChunkedWriter,NewChunkedWriter,the two result structs, the two config structs and option types, the
caller-facing
WithChunked*options, the sentinel errors, and theKeySplitter/Split/SplitResult/KASPublicKeygroup that anout-of-package splitter has to implement.
Unexported: the test seams. The clock, the segment cipher and its factory,
the archive writer factory, the entropy source, and the four options that
inject them.
archiveWriterFactoryin particular returns azipstream.SegmentWriterfrominternal/, so no external package couldhave implemented it even when the type was exported -- an exported symbol
no caller can satisfy should not be exported. Those three files (
clock.go,segment.go,archive_writer.go-- 28, 29 and 23 lines) are folded intochunked_writer.gonow that nothing outside the package can see them.Correctness guards included rather than deferred
Three small guards ship with the code they protect, since splitting "add
new code with a known hole" from "fix it" across two PRs of brand-new code
is churn with no review value:
Finalizerejects a write set missing segment 0(
ErrChunkedMissingSegmentZero). Only segment 0 emits the payload's ZIPlocal file header and every recorded offset is measured from it, so a set
without it silently produces a corrupt archive. It cannot be synthesized
at
Finalizetime -- by then the caller has already encrypted and shippedthe bytes.
scheme (
ErrSplitterUnsupportedAlgorithm) instead of letting the emptystring reach
createKeyAccess, where it selects the RSA branch whileocrypto.FromPublicPEMsniffs the PEM and wraps anyway -- producing a KAOthat claims keyType "wrapped" with no ephemeral public key, i.e. a TDF
nothing can decrypt.
is indistinguishable from an unset field, so no default is installed and
the nil surfaces as a panic partway through -- for the key splitter, not
until
Finalize.WriteSegmentalso reserves an index with a negative-size placeholder androlls the reservation back if encryption, signing or the archive write
fails, so a failed attempt cannot leave a placeholder that blocks a retry
or that
Finalizemistakes for a written segment. Release matches onpointer identity and on the placeholder still being unwritten, so it can
never discard a segment another call has since completed.
Deferred to the next PR: the
GetManifest-splits-under-RLock question,which is a behavior change to an exported method rather than a hole in
what lands here.
Integrity algorithms are fixed, not configurable
Rebasing onto #4030 (DSPX-4703) removed
calculateSignatureandintegrityAlgorithmString, which this writer called. Rather than port thecall sites onto the new typed helpers and keep the two options, both
WithChunkedIntegrityAlgorithmandWithChunkedSegmentIntegrityAlgorithmare gone and the algorithms are hardcoded:
RootHS256for the rootsignature,
SegmentGMACfor segment hashes. Neither was a meaningfulchoice.
AEAD never processed, so a "GMAC" root is just a copy of the last segment
hash, forgeable by an attacker with no key.
WithChunkedIntegrityAlgorithm(GMAC)accepted exactly that, producing a file
validateRootSignaturenowrefuses.
RootIntegrityAlghas one value and the option had nothing leftto express.
not AEAD output — a plaintext segment has no tag to read out. There is no
plaintext payload path anywhere in
sdk/: all three writers hardcodeIsEncrypted: true, the cipher is always AES-256-GCM, andenableEncryptionintdf_config.gois a dead field nothing sets orreads. So GMAC always applies, and HS256 would only re-MAC bytes the
cipher already authenticated.
Hardcoding
SegmentGMACalso changes this writer's segment default, whichwas
HS256. GMAC is whatCreateTDFdefaults to (tdf_config.go) and whatall eight golden TDFs in the cross-SDK corpus declare, so this is the
alignment that lets #3946 rewrite
CreateTDFon top of this writer withoutchanging its manifest output.
Side effect worth noting for reviewers of the doc comments:
tdf_config.goand
sdk/experimental/tdf/options.goboth claimSegmentHS256coversplaintext segments "which the experimental writer can produce and the stable
one cannot". That is not true today —
experimental/tdf/writer.goalsohardcodes
IsEncrypted: trueand always encrypts. Left alone here; it is#4030's text, not this PR's.
Tests cover round-trip, out-of-order and sparse writes, segment trimming,
KAO shape for RSA and EC, legacy and current target modes, assertion
signing, deterministic ZIP timestamps, the error contracts above,
archive-failure rollback and retry, the two fixed integrity algorithms
(including that a segment hash really is the AEAD tag and not an HMAC over
the same bytes), and concurrent
WriteSegmentcalls to distinct and toduplicate indices under
-race.Checklist
Testing Instructions
sdk/chunked_test.gois the bulk of the diff. The concurrency cases(
WriteSegmentto distinct and to duplicate indices) are the ones that want-racespecifically.The full DSPX-2604 stack — 20 PRs
mainmainmainmainmainmainmaindspx-2604-base-11= #3932 + #3934 + #3935dspx-2604-base-17= #3944 + #3945dspx-2604-base-19= #3947 + #3939Reviewable in parallel right now, since they sit directly on
mainand depend onnothing else: 01, 02, 04, 05, 06, 07, 08.
Why three PRs have a
dspx-2604-base-*base. A GitHub PR takes one base branch,but 11, 17 and 19 each build on more than one parent. The
base-*branches are emptymerge commits that exist only to join those parents so the PR diff shows exactly its
own change and nothing else. They contain no code, have no PR of their own, and go
away once their parents land — retarget the child onto
mainat that point.Wants a cross-SDK xtest run before merge: 15, 17 (and therefore 20). They touch
the KAS wire format.
Red checks you may see are network flakes, not this stack. Four distinct ones hit
this batch and all clear on re-run:
golangci-lint config verifytiming out onhttps://golangci-lint.run/.../golangci.v2.8.jsonschema.json(fails the wholego (<module>)job and fail-fast cancels its siblings), the bats installer getting a 403,Docker Hub timing out on
keycloak/keycloak:26.4, andbufreporting "the serverhosted at that remote is unavailable" while the Java SDK generates sources. The
govulncheckstep also emits##[error]annotations against the go1.25.11 stdlib, butit is
continue-on-error: trueand never fails a job — 01 bumps the toolchain andclears those annotations.
Summary by CodeRabbit
New Features
Bug Fixes