add libstdcxx version-bounds option

Author: samtkaplan

docs/src/pythoncall.md

@@ -285,6 +285,24 @@ into it. If you want to use a pre-existing Conda environment, see the previous s
 If `conda`, `mamba` or `micromamba` is not in your `PATH` you will also need to set
 `JULIA_CONDAPKG_EXE` to its path.
 
+#### If you installed a newer version of libstdc++
+On Linux, Julia comes bundled with its own copy of *libstdc++*.  Therefore, when interacting with
+CondaPkg, PythonCall injects a dependency to bound the allowed versions of the `libstdcxx-ng`
+Conda package. To override this value (e.g. if the user replaced the bundled libstdc++ with something
+newer), use:
+
+```julia
+[PythonCall]
+ENV["JULIA_CONDAPKG_LIBSTDCXX_VERSION_BOUND"] = ">=3.4,<=12"
+```
+
+To figure out installed version, run
+```bash
+strings /path/to/julia/lib/julia/libstdc++.so.6 | grep GLIBCXX
+```
+Then look at <https://gcc.gnu.org/onlinedocs/gcc-12.1.0/libstdc++/manual/manual/abi.html>
+for the GCC version compatible with the GLIBCXX version.
+
 ## [Installing Python packages](@id python-deps)
 
 Assuming you haven't [opted out](@ref pythoncall-config), PythonCall uses

src/cpython/context.jl

@@ -30,6 +30,31 @@ function _atpyexit()
     return
 end
 
+# By default, ensure libstdc++ in the Conda environment is compatible with
+# the one linked in Julia. This is platform/version dependent, so needs to
+# occur at runtime.
+#
+# Allow the user to override the default.  This is useful when the version
+# of libstdcxx linked in Julia is customized in the local installation of
+# Julia.
+#
+# To figure out cxx_version for a given Julia version, run
+#   strings /path/to/julia/lib/julia/libstdc++.so.6 | grep GLIBCXX
+# then look at
+#   https://gcc.gnu.org/onlinedocs/gcc-12.1.0/libstdc++/manual/manual/abi.html
+# for the highest GCC version compatible with the highest GLIBCXX version.
+function get_libstdcxx_version_bound()
+    if Base.VERSION <= v"1.6.2"
+        # GLIBCXX_3.4.26
+        cxx_version = ">=3.4,<9.2"
+    else
+        # GLIBCXX_3.4.29
+        # checked up to v1.8.0
+        cxx_version = ">=3.4,<11.4"
+    end
+    get(ENV, "JULIA_CONDAPKG_LIBSTDCXX_VERSION_BOUND", cxx_version)
+end
+
 function init_context()
 
     CTX.is_embedded = haskey(ENV, "JULIA_PYTHONCALL_LIBPTR")
@@ -60,23 +85,7 @@ function init_context()
                 exe_path::String
             else
                 if Sys.islinux()
-                    # Ensure libstdc++ in the Conda environment is compatible with the one
-                    # linked in Julia. This is platform/version dependent, so needs to occur at
-                    # runtime.
-                    #
-                    # To figure out cxx_version for a given Julia version, run
-                    #   strings /path/to/julia/lib/julia/libstdc++.so.6 | grep GLIBCXX
-                    # then look at
-                    #   https://gcc.gnu.org/onlinedocs/gcc-12.1.0/libstdc++/manual/manual/abi.html
-                    # for the highest GCC version compatible with the highest GLIBCXX version.
-                    if Base.VERSION <= v"1.6.2"
-                        # GLIBCXX_3.4.26
-                        cxx_version = ">=3.4,<9.2"
-                    else
-                        # GLIBCXX_3.4.29
-                        # checked up to v1.8.0
-                        cxx_version = ">=3.4,<11.4"
-                    end
+                    cxx_version = get_libstdcxx_version_bound()
                     CondaPkg.add("libstdcxx-ng", version=cxx_version, channel="conda-forge", temp=true, file=joinpath(@__DIR__, "..", "..", "CondaPkg.toml"), resolve=false)
                 end
                 # By default, we use Python installed by CondaPkg.

test/context.jl

@@ -0,0 +1,14 @@
+@testitem "libstdc++ version" begin
+    cxxversion = PythonCall.get_libstdcxx_version_bound()
+
+    if VERSION <= v"1.6.2"
+        @test cxx_version == ">=3.4,<9.2"
+    else
+        @test cxx_version == ">=3.4,<11.4"
+    end
+
+    ENV["JULIA_CONDAPKG_LIBSTDCXX_VERSION_BOUND"] = ">=3.4,<=12"
+
+    cxxversion = PythonCall.get_libstdcxx_version_bound()
+    @test cxx_version == ">3.4,<=12"
+end