ci: patch obs-studio's Windows CMake to define OBS::w32-pthreads

libobs/cmake/os-windows.cmake unconditionally links OBS::w32-pthreads, but
in the OBS_CMAKE_VERSION>=3.0.0 ("modern") top-level CMakeLists.txt branch
this bootstrap selects, nothing ever adds deps/w32-pthreads (confirmed at
every checked tag, 30.0.2 through 31.1.1) -- only the legacy branch's
add_subdirectory(deps) does, and libobs/CMakeLists.txt itself only adds
deps/libcaption and deps/uthash.

Upstream's own CI never hits this because it leaves ENABLE_UI on, and
UI/cmake/os-windows.cmake happens to add deps/w32-pthreads as a side effect
of building the Qt UI. Building obs-frontend-api instead of libobs (as
upstream's CI does) does not help: UI/obs-frontend-api/CMakeLists.txt only
links OBS::libobs, and UI/CMakeLists.txt returns before reaching the file
that adds w32-pthreads whenever ENABLE_UI is off -- which this bootstrap
deliberately keeps off to avoid a ~100 MB Qt6 download this plugin's plain
obs_properties_* UI does not need.

Add _patch_obs_studio_w32_pthreads() to buildspec_common.cmake: after the
obs-studio archive is extracted and before the OBS sub-configure runs, patch
its libobs/CMakeLists.txt to add deps/w32-pthreads itself, guarded by the
same if(NOT TARGET OBS::w32-pthreads) check UI/cmake/os-windows.cmake already
uses upstream. Idempotent (skips if already patched) and fails loudly if the
expected anchor text is missing, rather than silently no-opping against a
future OBS version with a different libobs/CMakeLists.txt shape.

Verified locally: the CMake string(FIND)/string(REPLACE) patch logic was run
against the real libobs/CMakeLists.txt fetched from the obs-studio 30.0.2
tag, confirmed to produce the intended block, and confirmed idempotent on a
second run. The actual Windows CMake configure/build this unblocks cannot be
verified from this Linux sandbox -- that's what the next CI run is for.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RL8abRmgFXkVASHkkqiJbE

cmake/common/buildspec_common.cmake

@@ -24,6 +24,7 @@
 #      build tree, so it does not trip over the install rules of targets that
 #      were deliberately never built.
 #   7. _resolve_versioned_macos_sdk exists at all -- see its own comment.
+#   8. _patch_obs_studio_w32_pthreads exists at all -- see its own comment.
 #
 
 include_guard(GLOBAL)
@@ -147,6 +148,70 @@ function(_resolve_versioned_macos_sdk out_path)
   endif()
 endfunction()
 
+# _patch_obs_studio_w32_pthreads: Windows-only. Confirmed at every obs-studio
+# tag from 30.0.2 through 31.1.1 (checked directly against
+# libobs/cmake/os-windows.cmake and the top-level CMakeLists.txt at each tag):
+# libobs/cmake/os-windows.cmake unconditionally links `OBS::w32-pthreads`, but
+# that target is defined only by deps/w32-pthreads/CMakeLists.txt, and nothing
+# in the OBS_CMAKE_VERSION>=3.0.0 ("modern") top-level CMakeLists.txt branch --
+# the one -DOBS_CMAKE_VERSION=3.0.0 selects -- ever adds deps/w32-pthreads.
+# libobs/CMakeLists.txt itself only adds deps/libcaption and deps/uthash.
+#
+# Upstream obs-studio's own CI never hits this because it builds with
+# ENABLE_UI left ON: UI/cmake/os-windows.cmake happens to add
+# deps/w32-pthreads too (guarded by `if(NOT TARGET OBS::w32-pthreads)`), which
+# satisfies libobs's link by the time CMake generates -- purely as a side
+# effect of Qt still being in the build, not because anything wires
+# w32-pthreads to libobs on purpose. This bootstrap deliberately builds with
+# ENABLE_UI:BOOL=OFF (see the file header) specifically to avoid pulling in
+# Qt6, so that side effect never happens here, and the gap is exposed.
+#
+# Rather than re-enable ENABLE_UI (and pay for a ~100 MB Qt6 download this
+# plugin's plain obs_properties_* UI does not need) or vendor a full copy of
+# deps/CMakeLists.txt, patch libobs/CMakeLists.txt to add the one missing
+# subdirectory itself, using the exact same existence guard
+# UI/cmake/os-windows.cmake already relies on. Idempotent: running this again
+# against an already-patched tree is a no-op (the search text no longer
+# matches), so it is safe to call on every configure regardless of whether
+# .deps/ was freshly extracted or reused from a previous run.
+function(_patch_obs_studio_w32_pthreads)
+  set(_cmakelists "${dependencies_dir}/${_obs_destination}/libobs/CMakeLists.txt")
+  if(NOT EXISTS "${_cmakelists}")
+    message(FATAL_ERROR "Cannot apply the w32-pthreads workaround: ${_cmakelists} does not exist.")
+  endif()
+
+  file(READ "${_cmakelists}" _contents)
+
+  string(FIND "${_contents}" "deps/w32-pthreads" _already_patched)
+  if(NOT _already_patched EQUAL -1)
+    message(STATUS "libobs/CMakeLists.txt already carries the w32-pthreads workaround - skipping")
+    return()
+  endif()
+
+  set(_needle "if(OS_WINDOWS)\n  include(cmake/os-windows.cmake)")
+  set(_replacement
+      "if(OS_WINDOWS)\n  if(NOT TARGET OBS::w32-pthreads)\n    add_subdirectory(\"\${CMAKE_SOURCE_DIR}/deps/w32-pthreads\" \"\${CMAKE_BINARY_DIR}/deps/w32-pthreads\")\n  endif()\n  include(cmake/os-windows.cmake)"
+  )
+
+  string(FIND "${_contents}" "${_needle}" _pos)
+  if(_pos EQUAL -1)
+    message(
+      FATAL_ERROR
+      "Could not find the expected 'if(OS_WINDOWS) / include(cmake/os-windows.cmake)' block in "
+      "${_cmakelists} to apply the w32-pthreads workaround. obs-studio's libobs/CMakeLists.txt "
+      "layout may have changed since this was written against 30.0.2."
+    )
+  endif()
+
+  string(REPLACE "${_needle}" "${_replacement}" _patched "${_contents}")
+  file(WRITE "${_cmakelists}" "${_patched}")
+  message(
+    STATUS
+    "Patched ${_cmakelists}: added deps/w32-pthreads so OBS::w32-pthreads exists "
+    "(obs-studio's modern Windows CMake path never defines it with ENABLE_UI=OFF)."
+  )
+endfunction()
+
 # _setup_obs_studio: Create obs-studio build project, then build libobs and obs-frontend-api
 function(_setup_obs_studio)
   if(NOT libobs_DIR)
@@ -366,5 +431,9 @@ function(_check_dependencies)
 
   set(CMAKE_PREFIX_PATH ${CMAKE_PREFIX_PATH} CACHE PATH "CMake prefix search path" FORCE)
 
+  if(OS_WINDOWS)
+    _patch_obs_studio_w32_pthreads()
+  endif()
+
   _setup_obs_studio()
 endfunction()