Add complete musl source file coverage and move native tool builds
to build-time custom commands so protoc/grpc_cpp_plugin are built
without musl's -nostartfiles flag.

Assisted-by: Claude Sonnet 4.5 via GitHub Copilot

Add USE_MUSL=1 to AMD_RELEASE, ARM_RELEASE, and AMD_MUSL CI build
configurations using the standard toolchain files instead of the
standalone musl toolchain. Add USE_MUSL support to the aarch64
toolchain, mirroring the x86_64 implementation.

Assisted-by: Claude Opus 4.6 via GitHub Copilot

Add toolchain file and -DUSE_MUSL=1 to FastTest cmake configuration,
and add contrib/musl to the submodule list. This replaces the old
commented-out reference to toolchain-x86_64-musl.cmake.

Assisted-by: Claude Opus 4.6 via GitHub Copilot

…mand

Revert the native tool build from build-time (add_custom_command) back to
configure-time (execute_process) to match the upstream approach. The
add_custom_command approach only declared a stamp file as OUTPUT but not the
actual native binary paths, causing ninja to fail with 'missing and no known
rule to make it' for any cross-compilation build (not just musl).

The condition is extended to also trigger native builds when USE_MUSL is set,
since musl's -nostartfiles breaks standalone executables like protoc.

Remove the now-unnecessary NATIVE_TOOLS_TARGET target-level dependencies from
all proto generator files, since with execute_process the native binaries
already exist on disk before ninja starts.

Assisted-by: Claude Opus 4.6 via GitHub Copilot

Add 14 math functions (log, log2, exp, exp2, expm1, pow, floor, fabs,
scalbn, logf, fabsl, scalbnl, copysignl, fmodl) that were previously
provided by glibc but are needed when building with musl. Also add
explogxf.cpp as a shared dependency for exp/exp2/expm1.

Includes both the source files in the SRCS list and corresponding
x86_64 runtime dispatch entries for CPU-optimized variant selection.

Assisted-by: Claude Opus 4.6 via GitHub Copilot

lexer_test uses plain add_executable instead of clickhouse_add_executable,
so it was missing musl CRT objects (crt1, crti, crtn) while the global
-nostartfiles flag prevented linking glibc's CRT. This caused the binary
to have no _start entry point, resulting in a segfault.

Assisted-by: Claude Opus 4.6 via GitHub Copilot

Include all 232 musl math source files instead of a partial subset.
Previously 129 math files were excluded, assuming LLVM-libc would
provide them, but LLVM-libc only covers ~45 functions. This left 84
math functions (mostly float-precision variants like sinf, cosf, expf)
with no implementation, causing 'undefined symbol: nan' linker errors.

LLVM-libc's optimized versions still take precedence via link ordering
(linked before musl in global-libs).

Also fix ksigaction.h include-override for aarch64: only x86_64 has an
arch-specific ksigaction.h in musl; all other architectures should use
the generic version from src/internal/ksigaction.h.

Assisted-by: Claude Opus 4.6 via GitHub Copilot

With USE_MUSL, -nostartfiles is set globally but only clickhouse_add_executable
adds musl CRT objects (crt1, crti, crtn). Fix decompressor, parquet tools, and
perf_checksum which use plain add_executable and would fail to link with
'cannot find entry symbol _start'.

Assisted-by: Claude Opus 4.6 via GitHub Copilot

With USE_MUSL, the target-built compressor binary cannot run on the host
because it's statically linked against musl. Extend the cross-compilation
check to also trigger when USE_MUSL is set, matching the native build
condition in the root CMakeLists.txt. This ensures the native-built
pre_compressor (without musl) is used on the host, while the target-built
decompressor (with musl) is passed separately for embedding.

Assisted-by: Claude Opus 4.6 via GitHub Copilot

Assisted-by: Claude Opus 4.6 via GitHub Copilot

Assisted-by: Claude Opus 4.6 via GitHub Copilot

# Conflicts:
#	ci/jobs/build_clickhouse.py

Assisted-by: Claude Opus 4.6 via GitHub Copilot

Assisted-by: Claude Opus 4.6 via GitHub Copilot

# Conflicts:
#	CMakeLists.txt

Disabling the trap unconditionally weakened the harmful-function guard
on glibc builds, where mbsrtowcs is not needed by libc internals. Only
musl uses it internally (in getaddrinfo), so limit the exemption to
USE_MUSL as suggested by the AI reviewer.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

The glibc userspace headers from the sysroot were reachable in every
compilation through clang's builtin sysroot search path, so any header
musl does not provide (or does not win in the include order) silently
fell back to glibc declarations without a link-time signal; the
execinfo.h stub was one manually-shadowed example. The AI reviewer
suggested narrowing the fallback to the kernel headers only.

Compile everything with -nostdlibinc and pass the musl include
directories explicitly in the toolchain flags. The kernel (uapi)
headers, the only part of the sysroot that must stay reachable, are
whitelisted through a directory of symlinks created at configure time
in cmake/musl.cmake. A header musl does not provide is now a compile
error: for example, #include <gnu/libc-version.h> fails with "file not
found", while execinfo.h resolves to the stub and linux/asm headers to
the whitelist.

The two target_include_directories(global-libs ...) blocks removed here
were inert: only INTERFACE_LINK_LIBRARIES of global-libs is forwarded
to global-group, so these directories never appeared on any compile
line; the fallback came from the builtin sysroot path, which
-nostdlibinc disables.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>