fix: DURATION lost the fractional part of the transaction start time

[fzipi]

Proposed changes

Fixes #3686.

Transaction::m_creationTimeStamp was declared as clock_t but initialised from the double that utils::cpu_seconds() returns, so the fractional seconds of the transaction start were truncated. DURATION is computed as cpu_seconds() - m_creationTimeStamp and was therefore off by up to one second depending on where in the second the transaction was created (for example 0.80 instead of 0.05 for a transaction that started at CPU time 1234.75 and was evaluated 50 ms later).

The field is now a double; Duration::evaluate() already subtracts in double, so nothing else changes. The member keeps its size on LP64 platforms, but it is a public header, so this is an ABI-relevant change like any other edit to Transaction.

Not changed here, as discussed in #2704 and noted in the issue: the clock is still the process CPU time, and cpu_seconds() still returns 0 on macOS.

Tests

make check style run of the full regression suite on this branch: 763 regression tests passed, 20 skipped (macOS, PCRE2 10.49, libxml2 2.15, Lua 5.5, YAJL 2.1). The existing variable-DURATION.json test passes. I did not add a new regression test because the truncation cannot be asserted deterministically through the JSON test harness: with the old code the value is the fractional part of the current CPU time, which falls below 1 most of the time, so a DURATION < 1 check would pass with or without the fix.

🤖 Generated with Claude Code

Summary by CodeRabbit

• Bug Fixes
  • Transaction durations now retain fractional seconds, improving accuracy for transactions measured over short or partial-second intervals. Previously, fractional timing values could be truncated, resulting in an incorrect duration.

[coderabbitai]

📝 Walkthrough

The transaction creation timestamp changes from clock_t to double. This preserves the fractional seconds returned by utils::cpu_seconds() when calculating transaction duration.

Changes: Transaction duration timestamp

Layer / File(s)	Summary
Preserve fractional seconds headers/modsecurity/transaction.h	The timestamp member changes to double. Its documentation notes that integer storage truncated fractional seconds and affected duration.

Priority: ➖ Normal

Estimated code review effort: 2 (Simple) | ~10 minutes

Change: Bug fix · Severity of issue fixed: Medium

Suggested reviewers: airween

Merge Risk: 🟡 Moderate · up to 198e8

Confirm whether this release accepts a binary compatibility break before merging. If it does, document the break and require affected consumers to rebuild.

Pre-merge checks: 5

Passed checks: 5 passed

Check name	Status	Explanation
Description Check		Check skipped - CodeRabbit’s high-level summary is enabled.
Title check		The title clearly and concisely describes the main change: preserving the fractional transaction start time so DURATION is accurate.
Linked Issues check		Issue #3686 requires preservation of fractional transaction start seconds for DURATION. The reviewed change declares Transaction::m_creationTimeStamp as const double. Transaction construction still in…
Out of Scope Changes check		The reported changes are limited to Transaction::m_creationTimeStamp and its documentation. These changes directly implement issue #3686. The PR does not change the clock source, worker CPU-time seman…
Docstring Coverage		No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 1…

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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Actionable comments posted: 1

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• 🪄 Fix CodeRabbit comments on this PR

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Inline comments:
Review comments at @headers/modsecurity/transaction.h:
- Line 440: Preserve the public Transaction binary layout by reverting the
m_creationTimeStamp type change or applying the project’s ABI-preserving
migration path; do not leave prebuilt consumers with incompatible member
offsets.

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📥 Commits

Reviewing files that changed from the base of the PR and between 2d4d903 and 198e891.

📒 Files selected for processing: 1

• headers/modsecurity/transaction.h

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[airween]

Thanks @fzipi, I went through the layout question in detail and I agree with keeping the type change as it is.

On 64bit archs (x86_64/aarch64, macOS) clock_t is long, so swapping it for double leaves the size, the alignment and every member offset of Transaction unchanged. That covers the vast majority of deployments. On i386 Linux the member grows from 4 to 8 bytes (with 4-byte alignment, so no padding). On Windows x64 clock_t is a 4-byte long. On both platforms the later members shift and sizeof(Transaction) grows.

I also considered keeping the old clock_t member and appending a new double at the end of the class. That isn't actually safer: it grows sizeof(Transaction) on every platform, 64bit systems included. A C++ consumer built against the old headers that does new modsecurity::Transaction(...) itself (as our own examples/using_bodies_in_chunks and test/benchmark do) would allocate the old size while the new constructor writes past it. Your version keeps LP64 completely neutral, so it is the better trade-off. There is also precedent: m_isInterrupted (2394fc5, v3.0.15) was inserted mid-class.

I agree that the truncation can't be asserted deterministically through the JSON harness, so no new regression test is needed here.

The broader DURATION semantics (CPU time vs. elapsed time, and the unit) are out of scope for this PR. I'll open a separate issue for them.

Merging this one, thanks again.