build: raise error on missing or multiple main function in a scratch target

CHANGES.md

@@ -44,6 +44,7 @@ Changes from ns-3.40 to ns-3-dev
   * The `restrict` warning has been disabled in GCC versions 12.1-12.3.1.
 * Raised minimum CMake version to 3.13.
 * Raised minimum C++ version to C++20.
+* Added guard rails for scratch targets missing or containing more than one `main` function.
 
 ### Changed behavior
 

scratch/CMakeLists.txt

@@ -14,14 +14,24 @@ function(create_scratch source_files)
     file(READ ${source_file} source_file_contents)
     string(REGEX MATCHALL "main[(| (]" main_position "${source_file_contents}")
     if(CMAKE_MATCH_0)
-      set(scratch_src ${source_file})
+      list(APPEND scratch_src ${source_file})
     endif()
   endforeach()
 
-  if(NOT scratch_src)
-    return()
+  list(LENGTH scratch_src scratch_src_len)
+
+  # If there is no main function, raise an error
+  if(${scratch_src_len} EQUAL 0)
+    message(FATAL_ERROR "The following scratch source files do not contain a main function: ${source_files}")
   endif()
 
+  # If there are multiple main functions, raise an error
+  if(${scratch_src_len} GREATER 1)
+    message(FATAL_ERROR "The following scratch source files contain ${scratch_src_len} files with a main function: ${scratch_src}")
+  endif()
+
+  # If there is a single main function, continue normally
+
   # Get parent directory name
   get_filename_component(scratch_dirname ${scratch_src} DIRECTORY)
   string(REPLACE "${CMAKE_CURRENT_SOURCE_DIR}" "" scratch_dirname
@@ -69,10 +79,9 @@ foreach(scratch_src ${single_source_file_scratches})
   create_scratch(${scratch_src})
 endforeach()
 
-# Scan *.cc files in ns-3-dev/scratch subdirectories and build a target for each
-# subdirectory
+# Scan ns-3-dev/scratch subdirectories
 file(
-  GLOB_RECURSE scratch_subdirectories
+  GLOB scratch_subdirectories
   CONFIGURE_DEPENDS
   LIST_DIRECTORIES true
   ${CMAKE_CURRENT_SOURCE_DIR}/**
@@ -84,6 +93,7 @@ foreach(entry ${scratch_subdirectories})
   endif()
 endforeach()
 
+# Build scratches per directory or following CMakeLists.txt instructions
 foreach(subdir ${scratch_subdirectories})
   if(EXISTS ${subdir}/CMakeLists.txt)
     # If the subdirectory contains a CMakeLists.txt file