Build static extension_module lib for iOS demo apps (pytorch#2038)

Summary:
Building the demo apps on iOS is broken after pytorch#2022 because it switches the `extension_module` library from a static to a shared one.  AFAIK, the latter requires code signing and it's failing on CI with the following error `Signing for "extension_module" requires a development team`

I'm not quite sure what would be the best way to address this, but keeping the library as a static one for iOS build seems like an easy workaround.

Pull Request resolved: pytorch#2038

Reviewed By: larryliu0820

Differential Revision: D54047521

Pulled By: huydhn

fbshipit-source-id: e752f29bb1fefd355525a40d586f89659119a625

.github/workflows/app-build.yml

@@ -14,6 +14,7 @@ on:
       - build/build_apple_frameworks.sh
       - build/test_ios_ci.sh
       - examples/demo-apps/**
+      - extension/module/**
   workflow_dispatch:
 
 concurrency:

build/executorch-config.cmake

@@ -53,9 +53,12 @@ foreach(lib ${lib_list})
         message("${lib} library is not found.
             If needed rebuild with the proper options in CMakeLists.txt")
     else()
-        if("${lib}" STREQUAL "extension_module")
+        if("${lib}" STREQUAL "extension_module" AND (NOT CMAKE_TOOLCHAIN_IOS))
             add_library(${lib} SHARED IMPORTED)
         else()
+            # Building a share library on iOS requires code signing, so it's
+            # easier to keep all libs as static when CMAKE_TOOLCHAIN_IOS is
+            # used
             add_library(${lib} STATIC IMPORTED)
         endif()
         set_target_properties(

extension/module/CMakeLists.txt

@@ -17,7 +17,12 @@ if(NOT EXECUTORCH_ROOT)
 endif()
 
 list(TRANSFORM _extension_module__srcs PREPEND "${EXECUTORCH_ROOT}/")
-add_library(extension_module SHARED ${_extension_module__srcs})
+if(CMAKE_TOOLCHAIN_IOS)
+  # Building a share library on iOS requires code signing
+  add_library(extension_module STATIC ${_extension_module__srcs})
+else()
+  add_library(extension_module SHARED ${_extension_module__srcs})
+endif()
 target_link_libraries(extension_module PRIVATE executorch)
 target_include_directories(extension_module PUBLIC ${EXECUTORCH_ROOT}/..)
 target_compile_options(extension_module PUBLIC -Wno-deprecated-declarations