Support nested build directories in sibling search

cmake/Modules/OpmSiblingSearch.cmake

@@ -1,26 +1,67 @@
 option (SIBLING_SEARCH "Search sibling directories before system paths" ON)
 mark_as_advanced (SIBLING_SEARCH)
 
+# NOTE: opm-grid and opm-simulators duplicate this logic inline in their
+#    CMakeLists.txt because they need to locate opm-common *before*
+#    find_package(opm-common), so this module is not yet available.
+#    If you change the search logic here, update the inline copies too.
+# DESCRIPTION:
+# Try to infer <module>_DIR (i.e., the build directory of a sibling module)
+# by inspecting the current project's source/build layout. This is useful
+# when working with multiple sibling repositories (e.g., opm-grid,
+# opm-simulators) without explicitly setting *_DIR variables.
+#
+# The macro attempts three strategies, in order:
+#
+# 1) Matching relative build paths inside sibling repositories
+#    Assumes each module is built in the same relative location within its
+#    own source tree.
+#    Example:
+#      /work/opm-grid/builds/debug
+#      /work/opm-simulators/builds/debug
+#    → reuse the relative path ("builds/debug") for the sibling repo.
+#
+# 2) Sibling build directories with similar naming patterns
+#    Assumes build directories are siblings in the same parent directory,
+#    and their names differ only by module name.
+#    Example:
+#      /work/builds/build-opm-grid-debug
+#      /work/builds/build-opm-simulators-debug
+#    → replace PROJECT_NAME with <module> in the leaf directory name.
+#
+# 3) Common build root containing per-module subdirectories
+#    Assumes all modules are built under a shared directory, with each
+#    module having its own subdirectory named after the module.
+#    Example:
+#      /work/build/opm-grid/
+#      /work/build/opm-simulators/
+#    → use <parent>/<module> if it looks like a CMake build dir
+#      (contains CMakeCache.txt).
+#
+# The first matching strategy that finds an existing directory is used.
+# If none match, <module>_DIR is left unset.
 macro(create_module_dir_var module)
   if(SIBLING_SEARCH AND NOT ${module}_DIR)
-    # guess the sibling dir
-    get_filename_component(_leaf_dir_name ${PROJECT_BINARY_DIR} NAME)
-    get_filename_component(_parent_full_dir ${PROJECT_BINARY_DIR} DIRECTORY)
-    get_filename_component(_parent_dir_name ${_parent_full_dir} NAME)
-    #Try if <module-name>/<build-dir> is used
-    get_filename_component(_modules_dir ${_parent_full_dir} DIRECTORY)
     set(_clone_dir "${module}")
-    if(IS_DIRECTORY ${_modules_dir}/${_clone_dir}/${_leaf_dir_name})
-      set(${module}_DIR ${_modules_dir}/${_clone_dir}/${_leaf_dir_name})
+    # Strategy 1: Compute relative path from source to build dir, apply to sibling.
+    # Handles both flat (repo/build) and nested (repo/builds/debug) layouts.
+    get_filename_component(_source_parent "${PROJECT_SOURCE_DIR}" DIRECTORY)
+    file(RELATIVE_PATH _build_rel "${PROJECT_SOURCE_DIR}" "${PROJECT_BINARY_DIR}")
+    set(_sibling_build "${_source_parent}/${_clone_dir}/${_build_rel}")
+    if(NOT _build_rel MATCHES "^\\.\\." AND IS_DIRECTORY "${_sibling_build}")
+      set(${module}_DIR "${_sibling_build}")
     else()
-      string(REPLACE ${PROJECT_NAME} ${_clone_dir} _module_leaf ${_leaf_dir_name})
+      # Fallback strategies for out-of-source or non-standard layouts
+      get_filename_component(_leaf_dir_name "${PROJECT_BINARY_DIR}" NAME)
+      get_filename_component(_parent_full_dir "${PROJECT_BINARY_DIR}" DIRECTORY)
+      # Strategy 2: Build dirs named <prefix><module><postfix>
+      string(REPLACE "${PROJECT_NAME}" "${_clone_dir}" _module_leaf "${_leaf_dir_name}")
       if(NOT _leaf_dir_name STREQUAL _module_leaf
-          AND IS_DIRECTORY ${_parent_full_dir}/${_module_leaf})
-        # We are using build directories named <prefix><module-name><postfix>
-        set(${module}_DIR ${_parent_full_dir}/${_module_leaf})
-      elseif(IS_DIRECTORY ${_parent_full_dir}/${_clone_dir} AND
-          EXISTS ${_parent_full_dir}/${_clone_dir}/CMakeCache.txt)
-        # All modules are in a common build dir
+          AND IS_DIRECTORY "${_parent_full_dir}/${_module_leaf}")
+        set(${module}_DIR "${_parent_full_dir}/${_module_leaf}")
+      # Strategy 3: All modules in a common build dir
+      elseif(IS_DIRECTORY "${_parent_full_dir}/${_clone_dir}" AND
+          EXISTS "${_parent_full_dir}/${_clone_dir}/CMakeCache.txt")
         set(${module}_DIR "${_parent_full_dir}/${_clone_dir}")
       endif()
     endif()