build: make pkg-config optional for the system libunwind/breakpad options

[mertefesensoy]

Problem

SENTRY_LIBUNWIND_SYSTEM and SENTRY_BREAKPAD_SYSTEM locate their dependencies with find_package(PkgConfig REQUIRED) + pkg_check_modules(... REQUIRED IMPORTED_TARGET ...), and sentry-config.cmake.in repeats that for consumers of a static build:

• CMakeLists.txt:708 — SENTRY_WITH_LIBUNWIND (Linux only) + SENTRY_LIBUNWIND_SYSTEM
• CMakeLists.txt:806 — SENTRY_BACKEND_BREAKPAD + SENTRY_BREAKPAD_SYSTEM
• CMakeLists.txt:937 — the crash daemon's libunwind-ptrace lookup
• sentry-config.cmake.in:19,24 — the same two lookups, at consumer time

Those are the only places in this project that touch pkg-config, they are all Linux-only, and both options default to OFF (the only other user is vendored crashpad, on Linux with CRASHPAD_ENABLE_STACKTRACE=ON). But because the requirement is unconditional and hard, packagers end up declaring the tool for every platform they build sentry-native for. vcpkg's port is the concrete example — it lists pkgconf as a host dependency of the default backend feature for !android & !ios, so vcpkg install sentry-native:x64-windows resolves pkgconf@3.0.3 into the install plan even though nothing in the sentry-native build will ever run it.

That is not free. Since pkgconf 2.9.90 the release tarball contains tests/lib1/të😋st/lib/pkgconfig/utf8.pc (added in pkgconf commit 943a4497a95939c8b9c7aea442b417a193c51220, still present in 3.0.5; vcpkg is on 3.0.3). GitHub's git archive stores that name as raw UTF-8 in the 100-byte ustar name field — the full path is 64 bytes, so no pax path= record is emitted, and git archive emits no hdrcharset either. Windows' tar.exe (libarchive) therefore falls back to archive_string_default_conversion_for_read() and decodes header names with the machine's legacy code page. Where that code page rejects the bytes, archive_mstring_copy_mbs_len_l() clears every string form of the entry name, archive_entry_pathname_w() returns NULL, and extraction aborts with Invalid empty pathname; tar.exe exits non-zero and vcpkg fails the port. The same failure mode has been reported for other ports whose sources contain non-ASCII paths, e.g. microsoft/vcpkg#43984

None of that is sentry-native's bug — but sentry-native is what pulled the tool onto platforms that never needed it. Making the dependency honest is the part that belongs in this repo.

Change

Add cmake/sentry-find-system-library.cmake with a sentry_find_system_library() helper that

• prefers the pkg-config metadata when both the tool and the requested .pc module are present (unchanged behaviour, including transitive Requires: / Libs.private:), and
• otherwise resolves the library and its headers with find_library() / find_path().

Both branches define the same imported target — sentry::libunwind, sentry::libunwind-ptrace, sentry::breakpad-client — so the call sites and the installed config no longer have to know which lookup succeeded. The module is installed next to sentry-config.cmake and reused by it, so consumers of a static build recreate the targets the same way and no longer need pkg-config themselves.

Two incidental improvements fall out of this:

• A missing dependency now reports what was not found and how to fix it, instead of failing inside FindPkgConfig with Could NOT find PkgConfig (missing: PKG_CONFIG_EXECUTABLE).
• The crash daemon's libunwind-ptrace lookup no longer depends on an earlier find_package(PkgConfig) call having run ~230 lines above it in the same file.

The vendored-libunwind default path, non-Linux builds, and builds that do have pkg-config are untouched.

Follow-up (not in this PR)

With this merged, ports/sentry-native/vcpkg.json can drop its pkgconf host dependency outright. I verified with the real resolver that doing so removes pkgconf (and its own vcpkg-tool-meson host dependency) from the install plan on both x64-windows and x64-linux, and that the Linux build still configures, builds, installs and links downstream with no pkg-config on the machine at all. Happy to open that upstream. Worth noting the port's $comment on that dependency is inaccurate today: it attributes pkgconf to the breakpad backend, but the port never enables SENTRY_BREAKPAD_SYSTEM; the real user is the system-libunwind lookup on Linux.

Verification

On Linux, for each of three states — pkg-config available / pkg-config physically removed from the machine / pkg-config present but the .pc module missing:

• SENTRY_BACKEND=native -DSENTRY_LIBUNWIND_SYSTEM=ON, static and shared: configure, build, cmake --install, then find_package(sentry) from a downstream project, build and run it. master fails to configure in the "removed" state with Could NOT find PkgConfig; this branch succeeds. Confirmed sentry-crash links libunwind, libunwind-ptrace and libunwind-generic in every case, and that sentry-targets.cmake carries sentry::libunwind in the static link interface.
• SENTRY_BACKEND=breakpad -DSENTRY_BREAKPAD_SYSTEM=ON against a synthetic breakpad-client package: both branches resolve to the same include root (<prefix>/include/breakpad), matching the client/<os>/handler/exception_handler.h includes in src/backends/sentry_backend_breakpad.cpp.
• The not-found path produces the intended diagnostic.
• Default (vendored libunwind) build and make test-unit still pass.

Supersedes #2006, which closed automatically when its head branch was renamed.

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.43%. Comparing base (69639a0) to head (2a6bac0).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2007      +/-   ##
==========================================
- Coverage   74.69%   74.43%   -0.27%     
==========================================
  Files         104      104              
  Lines       26182    26182              
  Branches     4740     4740              
==========================================
- Hits        19557    19488      -69     
- Misses       5295     5370      +75     
+ Partials     1330     1324       -6     

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[mertefesensoy]

@jpnurmi @JoshuaMoelans
Could one of you take a look, or point me at the right reviewer? Build-system only (CMake + the exported config), CI green on all platforms, branch up to date with master.

[jpnurmi]

The changes look promising, though I've only briefly glanced through them.

It makes me wonder whether this is really best solved independently in each individual project, though. sentry-native is certainly not alone here, and the necessary discovery logic gets pretty noisy and error-prone.

Just an idea: could this pattern be proposed upstream to CMake as a reusable abstraction?