fix(sdk): read zip entry bytes with io.ReadFull (DSPX-4590)

[dmihalcik-virtru]

Jira: https://virtru.atlassian.net/browse/DSPX-4590

The read-side counterpart to #3936, which fixed the same class of bug on the
write side. Independent of every open stack — no PR in DSPX-2604 (#3941,
#3943-#3949) touches sdk/internal/zipstream, and #4043 leaves readBytes
alone — so this sits directly on main and can land in any order.

The bug

readBytes issued a single Read and, on the nil-error path, returned the
whole make()'d buffer regardless of how many bytes actually arrived:

buf := make([]byte, size)
n, err := readerSeeker.Read(buf)
if errors.Is(err, io.EOF) { return buf[:n], io.EOF }
if err != nil { return buf[:n], ... }
return buf, nil   // full length, zero-padded, n discarded

io.Reader explicitly permits 0 < n < len(p) with a nil error, so the tail
of the buffer comes back as zeros that never existed in the archive —
reported as success.

Demonstrated against a valid fixture with a one-byte-at-a-time ReadSeeker:

want "payload bytes"
got  "p\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00"  err=<nil>  len=13

bytes.Reader never reads short, which is why no existing test caught it.
But NewReader takes a caller-supplied io.ReadSeeker, and the
implementations that do read short in practice — HTTP range requests,
network filesystems — are exactly the ones large archives get served from.

The ErrSegSizeMismatch guard in tdf.go (sdk/tdf.go:1114) cannot catch
this either: make() already gave the buffer the length that check is
looking for, so the check is comparing a value that is correct by
construction. A silently zero-padded segment reaches AES-GCM and fails
there instead, several layers from the cause.

Why this is the same fix as #3936

#3936 ("fill each segment with io.ReadFull and size the buffer to the input")
made this argument for CreateTDFContext's encrypt loop:

io.Reader.Read is permitted to return fewer bytes than the caller asked for
without erroring… A *bytes.Reader or *os.File on a local disk rarely
returns short, which is why this has held up, but any wrapping ReadSeeker —
a decompressor, a network-backed store, an instrumented reader — can
trigger it and there is nothing wrong with the input when it does.

Same defect, opposite consequence. On the write side a short read made the
loop fail loudly on valid input (io.ReadSeeker.Read size mismatch). Here
it silently corrupts: no error, full-length buffer, invented bytes. The
noisy direction got fixed; the quiet one was left, and it is the worse of the
two. This closes it.

The fix

io.ReadFull, which retries the short read and reports
io.ErrUnexpectedEOF when the archive really is truncated. Two related
changes follow from it:

• No data is returned alongside an error. The old code handed back a
  partial buffer with a bare io.EOF — a trap for a caller doing the
  reflexive errors.Is(err, io.EOF) → "normal end of stream", which would
  silently accept a truncated payload. All four production callers
  (sdk.go:489, tdf.go:864, tdf.go:987, tdf.go:1109) already treat the
  error as fatal, so nothing depends on the old shape.
• io.EOF is normalized to io.ErrUnexpectedEOF. io.ReadFull reports
  the bare sentinel when it reads nothing — a read starting exactly at the
  end of a truncated archive — and io.ErrUnexpectedEOF only when it got
  part of the request. Both mean the same thing to a caller that asked for a
  byte count taken from the central directory, and letting the first one out
  would have reopened the trap above. Zero-length reads are unaffected;
  io.ReadFull returns nil for them even at EOF. (Thanks @​opencode for
  catching this.)
• A zero-length read at the end of the archive now succeeds. Any Read
  on an exhausted bytes.Reader reports io.EOF, including a zero-length
  one, so an empty entry stored last used to fail to read back.

Error classification is deliberately left alone — readBytes still does
not wrap errZipFormat. Eight other sites in this file have the same issue
and that belongs in one follow-up, not here.

Reading this next to #3945

#3945 goes the other way on the same sentinel: it makes the encrypt loop
tolerate io.EOF and io.ErrUnexpectedEOF from io.ReadFull, while this
PR makes io.ErrUnexpectedEOF fatal. Both are right, because the contracts
differ. The write side has no declared length — a short final read just means
the caller's stream ended. The read side has a length from the central
directory, so a short read means the archive is truncated.

The zero-length change above is also the mirror of #3945's empty-payload
handling ("An empty payload still gets one empty segment"). zeroLenOKReader
in tdf_segment_defaults_test.go exists because SDK.CreateTDF cannot
currently encrypt a genuinely empty io.Reader; #3945 fixes the write half
and this fixes the read half. Worth rechecking whether that workaround can be
deleted once both have landed — not assuming it, since the comment there
describes more than one quirk.

Tests

chunkedSeeker in reader_test.go serves at most N bytes per Read and
reports no error for the short ones, standing in for the range-backed reader
bytes.Reader cannot model. Five cases: short reads reassembled, a partially
satisfiable read rejected with io.ErrUnexpectedEOF and no partial buffer, a
read starting at EOF rejected with the same sentinel rather than io.EOF, a
zero-length read at EOF succeeding, and the same short read driven end-to-end
through ReadAllFileData.

Each was verified to fail against the implementation it guards — the
first four against the original readBytes, the EOF-normalization case
against this PR with the conversion removed — so they are regression tests
rather than tautologies.

They use testify, unlike the rest of reader_test.go, which is t.Fatalf
throughout — the other three test files in the package use testify, and
require.ErrorIs(err, io.ErrUnexpectedEOF) has no clean t.Fatalf form. Say
the word if you would rather the file stay internally consistent.

• cd sdk && go test ./... -race — green
• golangci-lint run ./internal/zipstream/... — 0 issues
• make fmt — no diff

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Reviewing files that changed from the base of the PR and between c092561 and 8c9b274.

📒 Files selected for processing (2)

• sdk/internal/zipstream/reader.go
• sdk/internal/zipstream/reader_test.go

📝 Walkthrough

Walkthrough

The zipstream reader now reads requested byte counts completely, reports truncated input with io.ErrUnexpectedEOF, and returns no partial buffer on errors. Tests cover short-reading seekers and archive reads.

Changes

Zipstream read integrity

Layer / File(s)	Summary
Exact read behavior and validation sdk/internal/zipstream/reader.go, sdk/internal/zipstream/reader_test.go	readBytes uses io.ReadFull, io.SeekStart, and nil data on errors. Tests cover short-read assembly, truncated input, zero-length reads at EOF, and archive reads through a chunked seeker.

Priority: ⬇️ Low

Estimated code review effort: 3 (Moderate) | ~20 minutes

Change: Bug fix

Suggested reviewers: elizabethhealy

Merge Risk: 🟡 Moderate · up to c0925

Some malformed or truncated archives return a different error depending on whether any entry bytes remain. Normalize immediate EOF to the established truncation error before merging.

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✅ Passed checks (5 passed)

Check name	Status	Explanation
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Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly identifies the SDK zip-entry reading fix and the use of io.ReadFull, which matches the main change.

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_{I’m a rabbit with bytes in a row
Short reads now gather and grow
Truncated paths clearly say “EOF”
No partial carrots are handed off
The zipstream hops cleanly below}

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Actionable comments posted: 1

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• 🪄 Fix CodeRabbit comments on this PR

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Inline comments:
In `@sdk/internal/zipstream/reader.go`:
- Line 388: Update readBytes to normalize io.EOF from io.ReadFull to
io.ErrUnexpectedEOF before wrapping the read error when size is positive,
preserving other errors unchanged. Add a regression test covering
readBytes(bytes.NewReader([]byte("abc")), 3, 1).

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[github-actions]

Benchmark results, click to expand

Benchmark authorization.GetDecisions Results:

Metric	Value
Approved Decision Requests	1000
Denied Decision Requests	0
Total Time	116.809455ms

Benchmark authorization.v2.GetMultiResourceDecision Results:

Metric	Value
Approved Decision Requests	1000
Denied Decision Requests	0
Total Time	54.214386ms

Benchmark Statistics

Name	№ Requests	Avg Duration	Min Duration	Max Duration

Bulk Benchmark Results

Metric	Value
Total Decrypts	100
Successful Decrypts	100
Failed Decrypts	0
Total Time	227.767406ms
Throughput	439.04 requests/second

TDF3 Benchmark Results:

Metric	Value
Total Requests	5000
Successful Requests	5000
Failed Requests	0
Concurrent Requests	50
Total Time	30.163634015s
Average Latency	300.95789ms
Throughput	165.76 requests/second

[github-actions]

X-Test Failure Report

✅ go@main-v0.9.0
cukes-report
✅ java@main-v0.9.0
govulncheck-failure-3
✅ java@main-main
✅ js@main-pull-4087
bats-test-results
✅ java@main-pull-4087
✅ js@main-v0.9.0
✅ js@main-main
govulncheck-failure-1
opentdfplatformQPVE9U.dockerbuild
govulncheck-failure-8
✅ go@main-main

[github-actions]

Benchmark results, click to expand

Benchmark authorization.GetDecisions Results:

Metric	Value
Approved Decision Requests	1000
Denied Decision Requests	0
Total Time	172.536937ms

Benchmark authorization.v2.GetMultiResourceDecision Results:

Metric	Value
Approved Decision Requests	1000
Denied Decision Requests	0
Total Time	97.268176ms

Benchmark Statistics

Name	№ Requests	Avg Duration	Min Duration	Max Duration

Bulk Benchmark Results

Metric	Value
Total Decrypts	100
Successful Decrypts	100
Failed Decrypts	0
Total Time	365.025864ms
Throughput	273.95 requests/second

TDF3 Benchmark Results:

Metric	Value
Total Requests	5000
Successful Requests	5000
Failed Requests	0
Concurrent Requests	50
Total Time	43.513293365s
Average Latency	434.241955ms
Throughput	114.91 requests/second

[github-actions]

X-Test Failure Report

✅ go@pull-4087-v0.9.0
✅ java@main-pull-4087
✅ js@main-main
✅ go@main-v0.9.0
✅ js@main-v0.9.0
govulncheck-failure-1
✅ js@main-pull-4087
✅ java@main-main
cukes-report
opentdfplatformCQ05LH.dockerbuild
✅ java@main-v0.9.0
govulncheck-failure-3
govulncheck-failure-8

[github-actions]

Benchmark results, click to expand

Benchmark authorization.GetDecisions Results:

Metric	Value
Approved Decision Requests	1000
Denied Decision Requests	0
Total Time	224.863798ms

Benchmark authorization.v2.GetMultiResourceDecision Results:

Metric	Value
Approved Decision Requests	1000
Denied Decision Requests	0
Total Time	117.361665ms

Benchmark Statistics

Name	№ Requests	Avg Duration	Min Duration	Max Duration

Bulk Benchmark Results

Metric	Value
Total Decrypts	100
Successful Decrypts	100
Failed Decrypts	0
Total Time	478.269737ms
Throughput	209.09 requests/second

TDF3 Benchmark Results:

Metric	Value
Total Requests	5000
Successful Requests	5000
Failed Requests	0
Concurrent Requests	50
Total Time	57.656434482s
Average Latency	575.3325ms
Throughput	86.72 requests/second

[github-actions]

⚠️ Govulncheck found vulnerabilities ⚠️

The following modules have known vulnerabilities:

• otdfctl
• service
• tests-bdd

See the workflow run for details.

[github-actions]

X-Test Results

bats-test-results
✅ go@main-main
✅ js@main-v0.9.0
govulncheck-failure-3
✅ go@pull-4087-pull-4087
✅ java@main-v0.9.0
govulncheck-failure-8
✅ go@main-pull-4087
✅ go@pull-4087-v0.9.0
✅ go@main-v0.9.0
✅ java@main-main
✅ go@pull-4087-main
✅ js@main-main
opentdfplatformY1L2DV.dockerbuild
✅ java@main-pull-4087
govulncheck-failure-1
✅ js@main-pull-4087
cukes-report