#35662 script: prevent partial reinitialization of sighash data

Removes default construction and the public `Init()` method from `PrecomputedTransactionData`, requiring transaction context and optional spent outputs at construction time. Validation and test call sites that defer precomputation now use `std::optional<PrecomputedTransactionData>`, emplacing the object only after a script-cache miss. The PR also skips txdata allocation during block connection when assumevalid bypasses script checks, and eliminates an unintended copy of caller-supplied txdata in the kernel C API.

Problem: Default-constructing `PrecomputedTransactionData` and initializing it later through `Init()` allowed instances to exist uninitialized or be reused across transactions. The old `Init()` safety guard checked only `m_spent_outputs_ready`, which stayed false after forced initialization without spent outputs even though BIP143 sighash hashes were cached. Additionally, the experimental kernel C verification API copied caller txdata on every call due to ternary expression typing, and `ConnectBlock()` allocated unused txdata memory during assumevalid verification.

Evidence of importance:

• PR description: "The old Init() guard only checked m_spent_outputs_ready, which remained false after forced initialization without spent outputs even though force=true cached the BIP143 prevout, sequence, and output hashes for witness-v0 sighash."
• Commit d07c727ff8: "Reproducer: https://godbolt.org/z/PzcKGW9da (GCC 15.2: one copy before, none after)."
• PR description references closed PR #35663 ("validation: avoid preallocating unused txdata during assumevalid"), whose memory optimization is incorporated directly into ConnectBlock().

Scope: Touches 17 files, but most edits are mechanical adaptations across call sites (PSBT, wallet feebumper, fuzz targets, test fixtures) replacing two-step `PrecomputedTransactionData` initialization with direct construction or `std::optional` wrapping.

Risk of a mistake: medium (consensus, validation). A mistake in initializing or binding PrecomputedTransactionData could corrupt sighash computations during script execution, causing invalid transactions to be accepted or valid blocks to be rejected.

Ready · Needs rebase

Conflicts with master; review the logic now, the rebase is mechanical.

Waiting on (from the thread read):

• reviewer: review of the updated stack and kernel API fix from theuni or other reviewers 2026-09-22

Strong

substantive support, no open objection (fametrano)

Thread notes: Added sedited's suggestion to remove Init() and question on staleness, answered by fametrano who also provided a substantive Tested ACK. / Added sedited's suggestion to remove Init entirely and question regarding staleness. Added fametrano's tested ACK and answer to sedited's question.

Objections, suggestions, and questions:

Support:

• sedited (member) (ACK, 2026-09-22): ACK on the changes [not substantive]
• fametrano (contributor) (Tested ACK, 2026-10-25) [both: kernel API, libbitcoinkernel, src/script/interpreter.cpp]: differential sighash testing showed equivalence across 5961 BIP143 and BIP341 key paths, unit and kernel tests passed, and kernel API benchmark showed orders-of-magnitude speedup by eliminating txdata copying

Participants:

• sedited (member, support, 3 statements 2026-07-08 to 2026-09-22): ACKed the changes; earlier objection and suggestion to eliminate Init() were adopted
• fametrano (contributor, support, 1 statements 2026-10-25 to 2026-10-25): Tested ACK with sighash differential testing and kernel API benchmarks

81 lines under test/bench/ci.

• src/test/sighash_tests.cpp +37/-3
• src/validation.cpp +14/-14
• src/script/interpreter.cpp +4/-14
• src/psbt.cpp +5/-9
• src/test/txvalidationcache_tests.cpp +8/-6
• src/kernel/bitcoinkernel.cpp +4/-8
• src/script/interpreter.h +4/-8
• src/script/sign.cpp +2/-5
• src/test/fuzz/script_assets_test_minimizer.cpp +2/-4
• src/test/script_assets_tests.cpp +2/-4
• src/bench/verify_script.cpp +1/-2
• src/signet.cpp +1/-2
• src/test/descriptor_tests.cpp +1/-2
• src/test/fuzz/script_flags.cpp +1/-2
• src/test/fuzz/script_sigcache.cpp +2/-1
• src/test/script_tests.cpp +1/-2
• src/wallet/feebumper.cpp +1/-2

The short text the ranking pass compares across PRs.

Removes default construction and the public Init() method from PrecomputedTransactionData, ensuring instances are immutably bound to a specific transaction and spent outputs upon construction. Validation and tests now hold an std::optional to defer construction until a script-cache miss, while ConnectBlock skips allocating txdata vectors entirely when assumevalid bypasses script verification. It also fixes an unintended copy of caller-supplied txdata on every call in the experimental bitcoinkernel C API, corroborated by a compiler reproducer.

P3 · speedup

Eliminating txdata copying in the experimental kernel C verification API resolves a significant performance issue, yielding orders-of-magnitude speedups in kernel API benchmarks.

Membership: src/kernel/bitcoinkernel.cpp is modified to eliminate an unintended copy of caller-supplied txdata in the kernel C verification API.

Factors: security/stability 1, bug 1, performance 1, user value 0, leverage 0

P3 · cleanup (ranking pass: Moved down: hardening of an error-prone pattern, no active bug, sits with the other defensive validation refactors)

The PR removes an error-prone stateful reinitialization pattern in PrecomputedTransactionData that allowed instances to be partially reinitialized or reused across transactions, and avoids allocating unused txdata during assumevalid block connection.

Membership: Labeled Consensus and modifies src/script/interpreter.{h,cpp} and src/validation.cpp.

Factors: security/stability 1, bug 1, performance 1, user value 0, leverage 0