libomnibus changing weak symbol to undefined?

[teh]

I'm building a cxx_python_extension, but when I'm trying to import it I'm getting an undefined symbol:

ImportError: ...buck-out/v2/gen/root/291b6c3a26d6a3e9/__hello__/hello#link-tree/libomnibus.so: undefined symbol: _ITM_registerTMCloneTable

AFAICT this is happens because libomnibus changes w to U. I think the nm command should probably exclude weak symbols (with --no-weak) but I don't understand whether this is by design or a bug?

example output from nm:

ls buck-out/v2/gen/root/291b6c3a26d6a3e9/__hello__/hello#link-tree/*.so | xargs -tn1 nm | rg _ITM_registerTMCloneTable
nm buck-out/v2/gen/root/291b6c3a26d6a3e9/__hello__/hello#link-tree/libc10.so
                 w _ITM_registerTMCloneTable
nm buck-out/v2/gen/root/291b6c3a26d6a3e9/__hello__/hello#link-tree/libomnibus.so
                 U _ITM_registerTMCloneTable
nm buck-out/v2/gen/root/291b6c3a26d6a3e9/__hello__/hello#link-tree/libtorch_cpu.so
                 w _ITM_registerTMCloneTable
nm buck-out/v2/gen/root/291b6c3a26d6a3e9/__hello__/hello#link-tree/libtorch_python.so
                 w _ITM_registerTMCloneTable

[ndmitchell]

I thought the open source code defaulted to turning off Omnibus? We generally don't recommend using omnibus mode. Did you do something to explicitly enable it?

[teh]

I didn't turn it out explicitly but I can reproduce how it got turned on: by depending on an extension that in turn depends on a prebuilt_cxx_library:

mkdir hello
touch hello/main.py
touch hello/main.cc

python_binary(
    name="hello",
    main_module="hello.main",
    deps=[":hello_lib"],
)

python_library(
    name="hello_lib",
    srcs=["hello/main.py"],
    visibility=["PUBLIC"],
    deps=[":hello_ext"],
)

cxx_python_extension(
    name="hello_ext",
    srcs=["hello/main.cc"],
    visibility=["PUBLIC"],
    deps=[":pybind11"],
)

prebuilt_cxx_library(
    name="pybind11",
    header_dirs=["deps/include/"],
    exported_headers=glob(["deps/include/pybind11/**/*.h"]),
    header_only=True,
    visibility=["PUBLIC"],
)

[ndmitchell]

Omnibus is fairly experimental - it has advantages if you have absolutely huge C++ codebases, but normally can be safely turned off. Unfortunately our open source toolchain was defaulting to it being turned on, which was a mistake. I'm just putting up a diff which reads:

--- a/buck2/prelude/toolchains/python.bzl
+++ b/buck2/prelude/toolchains/python.bzl
@@ -62,7 +62,7 @@
             make_pex_inplace = ctx.attrs.make_pex_inplace[RunInfo],
             compile = RunInfo(args = ["echo", "COMPILEINFO"]),
             package_style = "inplace",
-            native_link_strategy = "merged",
+            native_link_strategy = "separate",
         ),
         PythonPlatformInfo(name = "x86_64"),
     ]

Hopefully that resolves the problem by not using Omnibus.

[ndmitchell]

abd3850 applies that change - can you give it another go?

[teh]

I can confirm that stops building libomnibus, thank you! I think the original bug (don't rewrite weak symbols to undefined symbols) is still valid so will leave closing of issue to you.