[potentially breaking] update to use pybind11 submodule

.gitmodules

@@ -13,3 +13,6 @@
 [submodule "utils/kokkos-kernels"]
 	path = utils/kokkos-kernels
 	url = https://github.com/kokkos/kokkos-kernels.git
+[submodule "utils/pybind11"]
+	path = utils/pybind11
+	url = https://github.com/pybind/pybind11.git

CHANGELOG.md

@@ -9,6 +9,7 @@
 ### Changed (changing behavior/API/variables/...)
 
 ### Infrastructure (changes irrelevant to downstream codes)
+- [[PR653]](https://github.com/lanl/singularity-eos/pull/653) Move pybind11 to a submodule rather than fetching it via cmake fetchcontent
 
 ### Deprecated (soon to be removed behavior/API/variables/...)
 

CMakeLists.txt

@@ -183,6 +183,7 @@ include(singularity-eos/hdf5)
 include(singularity-eos/kokkos)
 include(singularity-eos/spiner)
 include(singularity-eos/ports-of-call)
+include(singularity-eos/pybind11)
 
 add_library(singularity-eos INTERFACE)
 add_library(singularity-eos::singularity-eos ALIAS singularity-eos)
@@ -294,6 +295,10 @@ endif()
 # Must happen before building singularity-utils (which needs ports-of-call and spiner)
 # Dependency order: Kokkos -> ports-of-call -> spiner -> singularity-utils
 
+if (SINGULARITY_BUILD_PYTHON)
+  find_package(Python COMPONENTS Interpreter Development REQUIRED)
+endif()
+
 if(SINGULARITY_SUBMODULE_MODE)
   # add all submodules
   message(STATUS "singularity-eos configuring in submodule mode.")
@@ -318,6 +323,11 @@ if(SINGULARITY_SUBMODULE_MODE)
   if(SINGULARITY_USE_EIGEN)
     singularity_import_eigen()
   endif()
+
+  # And pybind11 (if needed)
+  if (SINGULARITY_BUILD_PYTHON)
+    singularity_import_pybind11()
+  endif()
 else()
   # use system packages
 
@@ -341,6 +351,11 @@ else()
   if(SINGULARITY_USE_EIGEN)
     singularity_find_eigen()
   endif()
+
+  # And pybind11 (if needed)
+  if (SINGULARITY_BUILD_PYTHON)
+    singularity_find_pybind11()
+  endif()
 endif()
 
 # Build singularity-utils first - it's a dependency for both

cmake/singularity-eos/pybind11.cmake

@@ -0,0 +1,24 @@
+# ------------------------------------------------------------------------------#
+# © 2026. Triad National Security, LLC. All rights reserved.  This program
+# was produced under U.S. Government contract 89233218CNA000001 for Los Alamos
+# National Laboratory (LANL), which is operated by Triad National Security, LLC
+# for the U.S.  Department of Energy/National Nuclear Security Administration.
+# All rights in the program are reserved by Triad National Security, LLC, and
+# the U.S. Department of Energy/National Nuclear Security Administration. The
+# Government is granted for itself and others acting on its behalf a
+# nonexclusive, paid-up, irrevocable worldwide license in this material to
+# reproduce, prepare derivative works, distribute copies to the public, perform
+# publicly and display publicly, and to permit others to do so.
+# ------------------------------------------------------------------------------#
+
+macro(singularity_import_pybind11)
+  if(NOT TARGET pybind11::headers)
+    message(STATUS "Using pybind11 submodule")
+    add_subdirectory(utils/pybind11)
+  endif()
+endmacro()
+
+macro(singularity_find_pybind11)
+  message(STATUS "Searching for system pybind11")
+  find_package(pybind11 REQUIRED)
+endmacro()

python/CMakeLists.txt

@@ -1,19 +1,15 @@
-find_package(Python COMPONENTS Interpreter Development REQUIRED)
-find_package(pybind11 CONFIG QUIET)
-
-if(NOT pybind11_FOUND)
-  message(STATUS "No pybind11 installation found, fetching from GitHub.")
-  include(FetchContent)
-  FetchContent_Declare(pybind11
-    GIT_REPOSITORY https://github.com/pybind/pybind11
-    GIT_TAG        v2.9.2
-  )
-  FetchContent_GetProperties(pybind11)
-  if(NOT pybind11_POPULATED)
-    FetchContent_Populate(pybind11)
-    add_subdirectory(${pybind11_SOURCE_DIR} ${pybind11_BINARY_DIR})
-  endif()
-endif()
+# ------------------------------------------------------------------------------#
+# © 2022-2026. Triad National Security, LLC. All rights reserved.  This program
+# was produced under U.S. Government contract 89233218CNA000001 for Los Alamos
+# National Laboratory (LANL), which is operated by Triad National Security, LLC
+# for the U.S.  Department of Energy/National Nuclear Security Administration.
+# All rights in the program are reserved by Triad National Security, LLC, and
+# the U.S. Department of Energy/National Nuclear Security Administration. The
+# Government is granted for itself and others acting on its behalf a
+# nonexclusive, paid-up, irrevocable worldwide license in this material to
+# reproduce, prepare derivative works, distribute copies to the public, perform
+# publicly and display publicly, and to permit others to do so.
+# ------------------------------------------------------------------------------#
 
 pybind11_add_module(singularity_eos
   module.cpp