backends: Automatically include backend.cmake files for Zephyr

Append to the pw_set_zephyr_backend_ifdef function such that the
function can include the backend declaration prior to setting the
backend.

Change-Id: I1bdb703d710e96b432a690cd14f87dc79c594e35
Signed-off-by: Yuval Peress <peress@google.com>
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/123390
Reviewed-by: Rob Barnes <robbarnes@google.com>
Reviewed-by: Wyatt Hepler <hepler@google.com>

CMakeLists.txt

@@ -21,14 +21,22 @@ cmake_minimum_required(VERSION 3.16)
 include(pw_build/pigweed.cmake)
 
 if(CONFIG_ZEPHYR_PIGWEED_MODULE)
-  pw_set_zephyr_backend_ifdef(CONFIG_PIGWEED_ASSERT pw_assert.assert pw_assert_zephyr.assert)
-  pw_set_zephyr_backend_ifdef(CONFIG_PIGWEED_ASSERT pw_assert.check pw_assert_zephyr.check)
-  pw_set_zephyr_backend_ifdef(CONFIG_PIGWEED_CHRONO_SYSTEM_CLOCK pw_chrono.system_clock pw_chrono_zephyr.system_clock)
-  pw_set_zephyr_backend_ifdef(CONFIG_PIGWEED_INTERRUPT_CONTEXT pw_interrupt.context pw_interrupt_zephyr.context)
-  pw_set_zephyr_backend_ifdef(CONFIG_PIGWEED_LOG pw_log pw_log_zephyr)
-  pw_set_zephyr_backend_ifdef(CONFIG_PIGWEED_SYNC_MUTEX pw_sync.mutex pw_sync_zephyr.mutex_backend)
-  pw_set_zephyr_backend_ifdef(CONFIG_PIGWEED_SYNC_BINARY_SEMAPHORE pw_sync.mutex pw_sync_zephyr.binary_semaphore_backend)
-  pw_set_zephyr_backend_ifdef(CONFIG_PIGWEED_SYS_IO pw_sys_io pw_sys_io_zephyr)
+  pw_set_zephyr_backend_ifdef(CONFIG_PIGWEED_ASSERT
+      pw_assert.assert pw_assert_zephyr.assert pw_assert/backend.cmake)
+  pw_set_zephyr_backend_ifdef(CONFIG_PIGWEED_ASSERT
+      pw_assert.check pw_assert_zephyr.check pw_assert/backend.cmake)
+  pw_set_zephyr_backend_ifdef(CONFIG_PIGWEED_CHRONO_SYSTEM_CLOCK
+      pw_chrono.system_clock pw_chrono_zephyr.system_clock pw_chrono/backend.cmake)
+  pw_set_zephyr_backend_ifdef(CONFIG_PIGWEED_INTERRUPT_CONTEXT
+      pw_interrupt.context pw_interrupt_zephyr.context pw_interrupt/backend.cmake)
+  pw_set_zephyr_backend_ifdef(CONFIG_PIGWEED_LOG
+      pw_log pw_log_zephyr pw_log/backend.cmake)
+  pw_set_zephyr_backend_ifdef(CONFIG_PIGWEED_SYNC_MUTEX
+      pw_sync.mutex pw_sync_zephyr.mutex_backend pw_sync/backend.cmake)
+  pw_set_zephyr_backend_ifdef(CONFIG_PIGWEED_SYNC_BINARY_SEMAPHORE
+      pw_sync.mutex pw_sync_zephyr.binary_semaphore_backend pw_sync/backend.cmake)
+  pw_set_zephyr_backend_ifdef(CONFIG_PIGWEED_SYS_IO
+      pw_sys_io pw_sys_io_zephyr pw_sys_io/backend.cmake)
 
   if(CONFIG_NANOPB AND "${dir_pw_third_party_nanopb}" STREQUAL "")
     set(dir_pw_third_party_nanopb "${ZEPHYR_NANOPB_MODULE_DIR}" CACHE PATH "" FORCE)

pw_build/docs.rst

@@ -866,6 +866,14 @@ work with, since their values only change when they're assigned, but then
 persist accross CMake invocations. These variables should be set in one of the
 following ways:
 
+* Prior to setting a backend, your application should include
+  ``$ENV{PW_ROOT}/backends.cmake``. This file will setup all the backend targets
+  such that any misspelling of a facade or backend will yield a warning.
+
+  .. note::
+    Zephyr developers do not need to do this, backends can be set automatically
+    by enabling the appropriate Kconfig options.
+
 * Call ``pw_set_backend`` to set backends appropriate for the target in the
   target's toolchain file. The toolchain file is provided to ``cmake`` with
   ``-DCMAKE_TOOLCHAIN_FILE=<toolchain file>``.

pw_build/pigweed.cmake

@@ -725,14 +725,15 @@ function(pw_set_backend NAME BACKEND)
   set("${NAME}" "${BACKEND}" CACHE STRING "backend variable for a facade" FORCE)
 endfunction(pw_set_backend)
 
-# Zephyr specific wrapper for pw_set_backend, selects the default zephyr backend based on a Kconfig while
-# still allowing the application to set the backend if they choose to
-function(pw_set_zephyr_backend_ifdef COND FACADE BACKEND)
-  get_property(result CACHE "${FACADE}_BACKEND" PROPERTY TYPE)
+# Zephyr specific wrapper for pw_set_backend.
+function(pw_set_zephyr_backend_ifdef COND FACADE BACKEND BACKEND_DECL)
   if(${${COND}})
-    if("${result}" STREQUAL "")
-      pw_set_backend("${FACADE}" "${BACKEND}")
+    if(NOT EXISTS "${BACKEND_DECL}")
+      message(FATAL_ERROR
+          "Can't find backend declaration file '${BACKEND_DECL}'")
     endif()
+    include("${BACKEND_DECL}")
+    pw_set_backend("${FACADE}" "${BACKEND}")
   endif()
 endfunction()