fix: save and restore BUILD_SHARED_LIBS around AWS SDK FetchContent

Author: beekld

cmake/aws-sdk-cpp.cmake

@@ -29,16 +29,36 @@ FetchContent_Declare(aws-sdk-cpp
 # fails to find AWS Core symbols at link time (the AWS SDK's visibility
 # configuration doesn't export them consistently across its FetchContent
 # build). Linking aws-sdk-cpp statically into our shared wrapper sidesteps
-# the issue and matches the redis pattern of pinning dep build shape via
-# a scoped variable rather than overwriting the global BUILD_SHARED_LIBS.
+# the issue.
 #
-# Using a function for scope: `set(BUILD_SHARED_LIBS OFF)` inside the
-# function shadows any cached/parent value during FetchContent_MakeAvailable
-# without persisting after the call, so user `-D LD_BUILD_SHARED_LIBS=ON`
-# for the rest of the project remains untouched.
-function(_ld_fetch_aws_sdk_cpp_static)
-    set(BUILD_SHARED_LIBS OFF)
-    FetchContent_MakeAvailable(aws-sdk-cpp)
-endfunction()
-
-_ld_fetch_aws_sdk_cpp_static()
+# This needs cache manipulation rather than a function-scoped `set()`:
+# aws-sdk-cpp's top-level CMakeLists pins `cmake_policy(SET CMP0077 OLD)`,
+# which makes its `option(BUILD_SHARED_LIBS ... ON)` ignore non-cache
+# variables and unconditionally write the cache. The only way to stop
+# `option()` from setting the cache to ON is to pre-populate the cache
+# with OFF before FetchContent_MakeAvailable runs.
+#
+# The prior cache state is saved before the FORCE-write and restored
+# immediately after FetchContent finishes, so other subprojects (e.g.
+# libs/server-sdk-otel) see whatever BUILD_SHARED_LIBS value was in the
+# cache when this file was first included. An earlier attempt that
+# FORCE-wrote OFF without restoring leaked into those subprojects; the
+# save/restore here is the fix for that.
+if (DEFINED CACHE{BUILD_SHARED_LIBS})
+    set(_LD_AWS_BSL_PREV "${BUILD_SHARED_LIBS}")
+    set(_LD_AWS_BSL_WAS_CACHED TRUE)
+else ()
+    set(_LD_AWS_BSL_WAS_CACHED FALSE)
+endif ()
+
+set(BUILD_SHARED_LIBS OFF CACHE BOOL "" FORCE)
+
+FetchContent_MakeAvailable(aws-sdk-cpp)
+
+if (_LD_AWS_BSL_WAS_CACHED)
+    set(BUILD_SHARED_LIBS "${_LD_AWS_BSL_PREV}" CACHE BOOL "" FORCE)
+else ()
+    unset(BUILD_SHARED_LIBS CACHE)
+endif ()
+unset(_LD_AWS_BSL_PREV)
+unset(_LD_AWS_BSL_WAS_CACHED)