Cannot link with libomp5-10 with clang-10 on ubuntu-20.04

[mahrud]

Description
Starting exactly 10 days ago our nightly builds using Clang10 on Ubuntu-20.04 began failing to link with OpenMP despite no other changes being made to our source code. The only difference I found seems to be that starting 20210315.1, libomp5-11 is installed by default rather than libomp5-10, so it is removed when the older version is being installed. However, I couldn't reproduce this issue locally (e.g. by installing clang-11 and libomp5-11, then installing clang-10 and libomp5-10), so I don't think this is an Ubuntu bug.

Area for Triage:
C/C++

Question, Bug, or Feature?:
Bug

Virtual environments affected

• Ubuntu 20.04

Image version
Version: 20210315.1

Expected behavior
The build succeeds on version 20210309.1 and we see:

2021-03-18T06:24:41.8561445Z libomp5-10 is already the newest version (1:10.0.0-4ubuntu1).
2021-03-18T06:24:41.8562371Z libomp5-10 set to manually installed.
...
2021-03-18T06:26:28.8525076Z -- Found OpenMP_C: -fopenmp=libomp (found version "4.5") 

Actual behavior
The build fails starting the day after on version 20210315.1:

2021-03-19T06:24:37.6647483Z The following packages will be REMOVED:
2021-03-19T06:24:37.6658177Z   libomp-11-dev libomp5-11
...
2021-03-19T06:24:52.3045871Z Removing libomp-11-dev (1:11.0.0-2~ubuntu20.04.1) ...
2021-03-19T06:24:52.3752235Z Removing libomp5-11:amd64 (1:11.0.0-2~ubuntu20.04.1) ...
...
2021-03-19T06:25:08.0828994Z Selecting previously unselected package libomp5-10:amd64.
2021-03-19T06:25:08.1115424Z Preparing to unpack .../109-libomp5-10_1%3a10.0.0-4ubuntu1_amd64.deb ...
2021-03-19T06:25:08.1128257Z Unpacking libomp5-10:amd64 (1:10.0.0-4ubuntu1) ...
...
2021-03-19T06:25:16.4675353Z Setting up libomp5-10:amd64 (1:10.0.0-4ubuntu1) ...

Note that the right version of clang is detected:

2021-03-19T06:26:08.0149860Z -- The C compiler identification is Clang 10.0.0
2021-03-19T06:26:08.6670147Z -- Check for working C compiler: /usr/lib/ccache/clang-10 - skipped

The resulting CMake error is:

  Could NOT find OpenMP_C (missing: OpenMP_C_FLAGS OpenMP_C_LIB_NAMES)

And digging in CMakeError.txt, this is where things start to go wrong:

[2/2] Linking C executable cmTC_d8702
FAILED: cmTC_d8702 
: && /usr/lib/ccache/clang-10 -fopenmp=libomp -v  CMakeFiles/cmTC_d8702.dir/OpenMPTryFlag.c.o -o cmTC_d8702  -v && :
clang version 10.0.0-4ubuntu1 
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin
Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/10
Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/7
Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/7.5.0
Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/8
Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/9
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/10
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/7
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/7.5.0
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/8
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/9
Selected GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/10
Candidate multilib: .;@m64
Selected multilib: .;@m64
 "/usr/bin/ld" -z relro --hash-style=gnu --build-id --eh-frame-hdr -m elf_x86_64 -dynamic-linker /lib64/ld-linux-x86-64.so.2 -o cmTC_d8702 /usr/bin/../lib/gcc/x86_64-linux-gnu/10/../../../x86_64-linux-gnu/crt1.o /usr/bin/../lib/gcc/x86_64-linux-gnu/10/../../../x86_64-linux-gnu/crti.o /usr/bin/../lib/gcc/x86_64-linux-gnu/10/crtbegin.o -L/usr/bin/../lib/gcc/x86_64-linux-gnu/10 -L/usr/bin/../lib/gcc/x86_64-linux-gnu/10/../../../x86_64-linux-gnu -L/usr/bin/../lib/gcc/x86_64-linux-gnu/10/../../../../lib64 -L/lib/x86_64-linux-gnu -L/lib/../lib64 -L/usr/lib/x86_64-linux-gnu -L/usr/lib/../lib64 -L/usr/lib/x86_64-linux-gnu/../../lib64 -L/usr/bin/../lib/gcc/x86_64-linux-gnu/10/../../.. -L/usr/lib/llvm-10/bin/../lib -L/lib -L/usr/lib -L/usr/lib/llvm-10/lib CMakeFiles/cmTC_d8702.dir/OpenMPTryFlag.c.o -lomp -lgcc --as-needed -lgcc_s --no-as-needed -lpthread -lc -lgcc --as-needed -lgcc_s --no-as-needed /usr/bin/../lib/gcc/x86_64-linux-gnu/10/crtend.o /usr/bin/../lib/gcc/x86_64-linux-gnu/10/../../../x86_64-linux-gnu/crtn.o
/usr/bin/ld: cannot find -lomp
clang: error: linker command failed with exit code 1 (use -v to see invocation)
ninja: build stopped: subcommand failed.

Repro steps

1. install clang-10 and libomp5-10 on ubuntu-20.04 environment
2. build a cmake project containing find_package(OpenMP REQUIRED)

Public repo: https://github.com/Macaulay2/M2/actions
Succeeding build: https://github.com/Macaulay2/M2/runs/2137307839?check_suite_focus=true
Failing build: https://github.com/Macaulay2/M2/runs/2146146178?check_suite_focus=true

[mahrud]

Ah, actually I was able to reproduce this locally in a docker container with the following steps:

RUN apt-get install -y clang-10 llvm-10 && apt-get clean
RUN apt-get install -y clang-11 llvm-11 && apt-get clean
RUN apt-get install -y libomp5-10 && apt-get clean

This probably indicates that the issue is indeed with Ubuntu, and commenting the last line (to use the newer libomp with clang-10) seems to prevent the error, so I'll close this.

[mahrud]

This is apparently related to this Debian issue: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=811283