build(before-code-freeze): Export the stub's dynamic symbols when libpython is static (#451)

* build: Export the stub's dynamic symbols when libpython is static

The manylinux base container builds CPython with --disable-shared, so on
that platform triton_python_backend_stub links libpython3.x.a and the
Py_* symbols live in the executable rather than a shared library. Python
C extension modules are dlopen'ed and deliberately leave Py_* undefined
-- every one of the 77 lib-dynload modules in the image does, math.so
alone with 63 undefined symbols and no libpython in DT_NEEDED -- so they
can only bind against the stub's dynamic symbol table. Without the
export the stub links and starts, then fails on the first `import math`.

Set ENABLE_EXPORTS, which makes CMake add the platform's flag
(-Wl,--export-dynamic on Linux, per Platform/Linux.cmake), matching how
CPython links its own interpreter.

Gate it on the resolved library being an archive rather than on the
platform. A shared libpython needs no export, so the Debian build is
unaffected, and the condition stays correct if a target moves between
the two linkage modes.

The condition reads PYTHON_LIBRARY, not PYTHON_LIBRARIES: pybind11
arrives through FetchContent_MakeAvailable, which uses add_subdirectory,
so only the cache entry find_library creates crosses back into this
scope. PYTHON_LIBRARIES is a plain variable in pybind11's directory and
is empty here, which would have made this a silent no-op.

* docs: Correct the stale RHEL_BUILD rationale

The comment said RHEL_BUILD existed because pybind would otherwise pick
up Python 3.6 in the RHEL base container, so PYBIND11_PYTHON_VERSION was
set to force 3.12. That pin has been dropped from build.py -- pybind11
prefers the newest entry of its own Python_ADDITIONAL_VERSIONS list, and
the manylinux base container puts a single interpreter first on PATH.

RHEL_BUILD itself stays: it still selects the stub's link settings
further down. Only the justification was wrong.

(cherry picked from commit 96d6021)

Author: mc-nv

CMakeLists.txt

@@ -1,4 +1,4 @@
-# Copyright 2020-2025, NVIDIA CORPORATION & AFFILIATES. All rights reserved.
+# Copyright 2020-2026, NVIDIA CORPORATION & AFFILIATES. All rights reserved.
 #
 # Redistribution and use in source and binary forms, with or without
 # modification, are permitted provided that the following conditions
@@ -93,9 +93,15 @@ FetchContent_Declare(
   GIT_SHALLOW ON
 )
 
-# RHEL base container has multiple version of Python installed. By default
-# it seems like pybind will pickup v3.6, so we specifically assign it to
-# search for 3.12 here.
+# Detect a RHEL-family build. Used further down to adjust how the stub
+# executable is linked.
+#
+# This used to also justify passing PYBIND11_PYTHON_VERSION, because pybind
+# would otherwise pick up an older interpreter present in the RHEL base
+# container. That is no longer needed: pybind11 prefers the newest entry of
+# its own Python_ADDITIONAL_VERSIONS list, and the manylinux base container
+# puts a single interpreter first on PATH. Set PYBIND11_PYTHON_VERSION
+# explicitly if a build ever has to target a specific version again.
 set(RHEL_BUILD OFF)
 if(LINUX)
   file(STRINGS "/etc/os-release" DISTRO_ID_LIKE REGEX "ID_LIKE")
@@ -285,6 +291,30 @@ target_compile_options(
 )
 target_compile_definitions(triton-python-backend-stub PRIVATE TRITON_PB_STUB)
 
+# When libpython is a static archive -- the manylinux base container builds
+# CPython with --disable-shared -- the Py_* symbols land inside the stub
+# executable instead of a shared library. Python C extension modules are
+# dlopen'ed and deliberately leave Py_* undefined, so they can only resolve
+# against the stub's dynamic symbol table. ENABLE_EXPORTS makes CMake add the
+# platform's flag for that (-Wl,--export-dynamic on Linux), the same way
+# CPython links its own interpreter binary.
+#
+# A shared libpython needs none of this, since modules bind to it directly.
+# Key off the linkage rather than the platform so this stays correct if a
+# target ever moves between the two.
+#
+# PYTHON_LIBRARY rather than PYTHON_LIBRARIES: pybind11 is pulled in with
+# FetchContent_MakeAvailable, which uses add_subdirectory, so only the cache
+# entry that find_library creates survives into this scope. PYTHON_LIBRARIES
+# is a plain variable set inside pybind11's directory and reads empty here.
+if(PYTHON_LIBRARY MATCHES "\\.a$")
+  set_target_properties(
+    triton-python-backend-stub
+    PROPERTIES
+      ENABLE_EXPORTS TRUE
+  )
+endif()
+
 # RHEL assets are not released in a container environment nor do the current
 # Python lib versions in the manylinux base container match those currently
 # available for RHEL8 package managers. Therefore, we package the correct