#35911 Warn on and add missing [[noreturn]]

Enables the -Wmissing-noreturn compiler warning flag in CMake and annotates non-returning functions, methods, and lambdas across the codebase with [[noreturn]]. It also suppresses Clang's -Wc++23-lambda-attributes warning on annotated lambdas and migrates bare C exit and abort calls (abort(), exit(), _Exit()) to their std:: counterparts in <cstdlib>.

Problem: Functions that never return (such as unconditional throwers, exit handlers, and fatal assert stubs) lack [[noreturn]] annotations, which can lead to compiler analysis false positives such as spurious missing-return warnings. Without an enforced compiler warning flag, newly added non-returning functions easily omit the attribute.

Evidence of importance:

• PR description: "Missing [[noreturn]] attributes can cause compiler false positives and other issues; see discussion in https://github.com/bitcoin/bitcoin/pull/35896#discussion_r3726612556."
• Commit 934c041f07 adds -Wmissing-noreturn to warning flags to enforce the attribute systematically.

Scope: Touches bundled third-party or extracted libraries (src/ipc/libmultiprocess commit marked [nomerge], and src/util/subprocess.h), which the author has submitted upstream.

Risk of a mistake: low (build, validation, gui, tests). Mistakes are limited to compile-time attribute and warning flag changes or minor std:: namespace qualification on abort/exit calls.

Changed since the previous assessment: Expanded the standard library refactor commit to replace bare abort() and _Exit() calls across validation, sync, and txdb with std::abort() and std::_Exit(), alongside the previous std::exit() replacements.

Ready · CI failing

CI is red; check whether the failure is the PR's before a deep review.

Mild

objection that needs an answer has none yet (maflcko)

Thread notes: Initial record for draft PR #35911. Concept ACKs from hebasto and stickies-v, plus approval from maflcko. Open compilation/warning issues remain on fuzz tests and Windows (MSVC/clang-cl), and the PR is drafted awaiting upstream subtree updates.

Objections, suggestions, and questions:

Support:

• maflcko (member) (ACK, 2026-08-06): Nice. lgtm. Left some style nits, but feel free to ignore. [not substantive]
• hebasto (member) (Concept ACK, 2026-08-06): Concept ACK. [not substantive]
• stickies-v (member) (Concept ACK, 2026-08-11): Concept ACK [not substantive]

Participants:

• Sjors (member, question, 1 statements 2026-08-06 to 2026-08-06): asked about potential false positives from the warning flag, cited outdated docs
• hebasto (member, support, 2 statements 2026-08-06 to 2026-08-06): Concept ACK; asked to split a commit into a separate PR
• maflcko (member, objection, 7 statements 2026-08-06 to 2026-08-31): supported overall ('Nice. lgtm'), but reported compiler errors in fuzz.cpp and Windows build failures
• stickies-v (member, support, 1 statements 2026-08-11 to 2026-08-11): Concept ACK

118 lines under test/bench/ci.

• src/test/fuzz/fuzz.cpp +50/-41
• src/sync.cpp +8/-7
• src/bitcoind.cpp +7/-6
• src/test/util/net.h +3/-3
• CMakeLists.txt +5/-0
• src/qt/bitcoin.cpp +3/-2
• src/validation.h +3/-2
• src/protocol.cpp +3/-1
• src/test/threadpool_tests.cpp +3/-1
• src/test/util/setup_common.cpp +2/-2
• src/test/util/net.cpp +0/-3
• src/test/util_tests.cpp +2/-1
• src/interfaces/init.h +1/-1
• src/interfaces/mining.h +1/-1
• src/ipc/libmultiprocess/include/mp/proxy-types.h +1/-1
• src/ipc/libmultiprocess/test/mp/test/foo.h +1/-1
• src/qt/bitcoin.h +1/-1
• src/rpc/mining.cpp +1/-1
• src/test/fuzz/dbwrapper.cpp +1/-1
• src/test/fuzz/threadpool.cpp +1/-1
• src/test/fuzz/util/net.h +1/-1
• src/txdb.cpp +1/-1
• src/util/subprocess.h +1/-1
• src/test/fuzz/util/net.cpp +0/-1

The short text the ranking pass compares across PRs.

Enables the -Wmissing-noreturn compiler warning flag in CMake and annotates non-returning functions, methods, and lambdas across the codebase with [[noreturn]]. It also migrates bare C runtime abort and exit calls to std::abort, std::exit, and std::_Exit, while suppressing Clang -Wc++23-lambda-attributes warnings on annotated lambdas. This prevents compiler analysis false positives, such as spurious missing-return warnings, where the compiler cannot tell that a branch unconditionally terminates.

P3 · cleanup (ranking pass: Moved down from 32: prevents spurious diagnostics only; weaker payoff than -Wunused's cited bug class)

Enables the -Wmissing-noreturn compiler warning flag in CMake and annotates non-returning functions to prevent compiler analysis false positives. While systematic warning enforcement improves code hygiene and avoids spurious missing-return diagnostics, it is developer-focused and reasonably deferrable.

Membership: Modifies CMakeLists.txt to enable the -Wmissing-noreturn compiler warning flag and suppress Clang warnings on annotated lambdas.

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