CMake: add a way of defining an external defered driver by setting on…

…e or several ADD_EXTERNAL_DEFERRED_PLUGIN_XXX CMake variables that will cause the pointed file to be built in libgdal, and a DeclareDeferredXXX() C function in it to be called by GDALAllRegister()

.github/workflows/alpine/build.sh

@@ -2,11 +2,32 @@
 
 set -e
 
+cat << EOF > /tmp/foo.cpp
+#include <cstdio>
+extern "C" void DeclareDeferredFOO(void);
+void DeclareDeferredFOO()
+{
+    FILE* f = fopen("/tmp/DeclareDeferredFOO_has_been_run.bin", "wb");
+    if (f)
+      fclose(f);
+}
+EOF
+
 cmake ${GDAL_SOURCE_DIR:=..} \
   -DCMAKE_BUILD_TYPE=Release \
   -DUSE_CCACHE=ON \
   -DCMAKE_INSTALL_PREFIX=/usr \
   -DIconv_INCLUDE_DIR=/usr/include/gnu-libiconv \
   -DIconv_LIBRARY=/usr/lib/libiconv.so \
+  -DADD_EXTERNAL_DEFERRED_PLUGIN_FOO=/tmp/foo.cpp \
   -DCMAKE_C_FLAGS=-Werror -DCMAKE_CXX_FLAGS=-Werror -DWERROR_DEV_FLAG="-Werror=dev"
 make -j$(nproc)
+
+apps/gdalinfo
+
+if test -f /tmp/DeclareDeferredFOO_has_been_run.bin; then
+  echo "DeclareDeferredFOO() has been run"
+else
+  echo "DeclareDeferredFOO() has NOT been run"
+  exit 1
+fi

doc/source/development/building_from_source.rst

@@ -2226,6 +2226,21 @@ OGR_REGISTER_DRIVER_<driver_name>_FOR_LATER_PLUGIN can be declared at
 libgdal build time without requiring the dependent libraries needed to build
 the pluging later to be available.
 
+Out-of-tree deferred loaded plugins
++++++++++++++++++++++++++++++++++++
+
+Out-of-tree drivers can also benefit from the deferred loading capability, provided
+libgdal is built with CMake variable(s) pointing to external code containing the
+code for registering a proxy driver.
+
+This can be done with the following option:
+
+.. option:: ADD_EXTERNAL_DEFERRED_PLUGIN_<driver_name>:FILEPATH=/path/to/some/file.cpp
+
+The pointed file must declare a ``void DeclareDeferred<driver_name>(void)``
+method with C linkage that takes care of creating a GDALPluginDriverProxy
+instance and calling :cpp:func:`GDALDriverManager::DeclareDeferredPluginDriver` on it.
+
 .. _building-python-bindings:
 
 Python bindings options

frmts/CMakeLists.txt

@@ -1,6 +1,32 @@
 # CMake4GDAL project is distributed under MIT license. See accompanying file LICENSE.txt.
 
-add_library(gdal_frmts OBJECT gdalallregister.cpp)
+set(FRMT_SOURCES gdalallregister.cpp)
+
+get_cmake_property(_variableNames VARIABLES)
+set(EXTERNAL_DEFERRED_PLUGINS_SOURCE_CODE "")
+foreach (_variableName ${_variableNames})
+    unset(MATCHED)
+    string(REGEX MATCH "ADD_EXTERNAL_DEFERRED_PLUGIN_(.*)" MATCHED ${_variableName})
+    if (NOT MATCHED)
+        continue()
+    endif()
+    if (EXTERNAL_DEFERRED_PLUGINS_SOURCE_CODE STREQUAL "")
+        set(EXTERNAL_DEFERRED_PLUGINS_SOURCE_CODE "// File generated by frmts/CMakeLists.txt. DO NOT EDIT !\nextern void DeclareDeferredPlugins();\nvoid DeclareDeferredPlugins()\n{\n")
+    endif()
+    string(APPEND EXTERNAL_DEFERRED_PLUGINS_SOURCE_CODE "    // Plugin ${CMAKE_MATCH_1}\n    extern void DeclareDeferred${CMAKE_MATCH_1}();\n    DeclareDeferred${CMAKE_MATCH_1}();\n")
+    list(APPEND FRMT_SOURCES "${${_variableName}}")
+endforeach()
+if (NOT EXTERNAL_DEFERRED_PLUGINS_SOURCE_CODE STREQUAL "")
+    string(APPEND EXTERNAL_DEFERRED_PLUGINS_SOURCE_CODE "}\n")
+    file(WRITE "${CMAKE_CURRENT_BINARY_DIR}/external_deferred_plugins.c" "${EXTERNAL_DEFERRED_PLUGINS_SOURCE_CODE}")
+    list(APPEND FRMT_SOURCES "${CMAKE_CURRENT_BINARY_DIR}/external_deferred_plugins.c")
+endif()
+
+add_library(gdal_frmts OBJECT ${FRMT_SOURCES})
+if (NOT EXTERNAL_DEFERRED_PLUGINS_SOURCE_CODE STREQUAL "")
+    target_compile_definitions(gdal_frmts PRIVATE -DHAVE_EXTERNAL_DEFERRED_PLUGINS)
+endif()
+
 include(GdalDriverHelper)
 include(CMakeDependentOption)
 

frmts/gdalallregister.cpp

@@ -40,6 +40,10 @@
 static char *szConfiguredFormats = "GDAL_FORMATS";
 #endif
 
+#if defined(HAVE_EXTERNAL_DEFERRED_PLUGINS)
+extern "C" void DeclareDeferredPlugins(void);
+#endif
+
 /************************************************************************/
 /*                          GDALRegisterPlugin()                        */
 /*                                                                      */
@@ -124,6 +128,10 @@ void CPL_STDCALL GDALAllRegister()
 {
     auto poDriverManager = GetGDALDriverManager();
 
+#if defined(HAVE_EXTERNAL_DEFERRED_PLUGINS)
+    DeclareDeferredPlugins();
+#endif
+
 #if defined(DEFERRED_ARROW_DRIVER)
     DeclareDeferredOGRArrowPlugin();
 #endif