[libc] [llvm] [libc][NFC] clean internal fd handling (PR #143991)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `libc-x86_64-debian` running on `libc-x86_64-debian` while building `libc,utils` at step 4 "annotate".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/43/builds/23372

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 4 (annotate) failure: 'python ../llvm-zorg/zorg/buildbot/builders/annotated/libc-linux.py ...' (failure)
...
[       OK ] LlvmLibcSysIoctlTest.InvalidCommandAndFIONREAD (39 us)
Ran 1 tests.  PASS: 1  FAIL: 0
[2141/2141] Running unit test libc.test.utils.FPUtil.x86_long_double_test
[==========] Running 1 test from 1 test suite.
[ RUN      ] LlvmLibcX86LongDoubleTest.is_nan
[       OK ] LlvmLibcX86LongDoubleTest.is_nan (8 ms)
Ran 1 tests.  PASS: 1  FAIL: 0
@@@BUILD_STEP libc-fuzzer@@@
Running: ninja libc-fuzzer
[1/4] Linking CXX executable libc/fuzzing/stdio/libc.fuzzing.stdio.printf_parser_fuzz
FAILED: libc/fuzzing/stdio/libc.fuzzing.stdio.printf_parser_fuzz 
: && /usr/bin/clang++ -fvisibility-inlines-hidden -Werror=date-time -Werror=unguarded-availability-new -Wall -Wextra -Wno-unused-parameter -Wwrite-strings -Wcast-qual -Wmissing-field-initializers -Wimplicit-fallthrough -Wcovered-switch-default -Wno-noexcept-type -Wnon-virtual-dtor -Wdelete-non-virtual-dtor -Wsuggest-override -Wno-comment -Wstring-conversion -Wmisleading-indentation -Wctad-maybe-unsupported -fdiagnostics-color -ffunction-sections -fdata-sections -fsanitize=fuzzer -O3 -DNDEBUG  libc/fuzzing/stdio/CMakeFiles/libc.fuzzing.stdio.printf_parser_fuzz.dir/printf_parser_fuzz.cpp.o -o libc/fuzzing/stdio/libc.fuzzing.stdio.printf_parser_fuzz  libc/src/__support/OSUtil/linux/CMakeFiles/libc.src.__support.OSUtil.linux.linux_util.dir/./exit.cpp.o  libc/src/__support/OSUtil/linux/CMakeFiles/libc.src.__support.OSUtil.linux.linux_util.dir/./fcntl.cpp.o && :
/usr/bin/ld: libc/fuzzing/stdio/CMakeFiles/libc.fuzzing.stdio.printf_parser_fuzz.dir/printf_parser_fuzz.cpp.o: in function `__llvm_libc_20_0_0_git::printf_core::Parser<__llvm_libc_20_0_0_git::internal::MockArgList>::get_next_section()':
printf_parser_fuzz.cpp:(.text._ZN22__llvm_libc_20_0_0_git11printf_core6ParserINS_8internal11MockArgListEE16get_next_sectionEv[_ZN22__llvm_libc_20_0_0_git11printf_core6ParserINS_8internal11MockArgListEE16get_next_sectionEv]+0x10b8): undefined reference to `__llvm_libc_20_0_0_git::libc_errno'
/usr/bin/ld: printf_parser_fuzz.cpp:(.text._ZN22__llvm_libc_20_0_0_git11printf_core6ParserINS_8internal11MockArgListEE16get_next_sectionEv[_ZN22__llvm_libc_20_0_0_git11printf_core6ParserINS_8internal11MockArgListEE16get_next_sectionEv]+0x10bd): undefined reference to `__llvm_libc_20_0_0_git::Errno::operator int()'
/usr/bin/ld: libc/fuzzing/stdio/libc.fuzzing.stdio.printf_parser_fuzz: hidden symbol `_ZN22__llvm_libc_20_0_0_git10libc_errnoE' isn't defined
/usr/bin/ld: final link failed: bad value
clang: error: linker command failed with exit code 1 (use -v to see invocation)
[2/4] Linking CXX executable libc/fuzzing/stdlib/libc.fuzzing.stdlib.atof_differential_fuzz
[3/4] Linking CXX executable libc/fuzzing/math/libc.fuzzing.math.math_differential_fuzz
[4/4] Linking CXX executable libc/fuzzing/math/libc.fuzzing.math.nextafter_differential_fuzz
ninja: build stopped: subcommand failed.
['ninja', 'libc-fuzzer'] exited with return code 1.
The build step threw an exception...
Traceback (most recent call last):
  File "/home/llvm-libc-buildbot/home/sivachandra/libc-x86_64-debian/libc-x86_64-debian/build/../llvm-zorg/zorg/buildbot/builders/annotated/libc-linux.py", line 176, in step
    yield
  File "/home/llvm-libc-buildbot/home/sivachandra/libc-x86_64-debian/libc-x86_64-debian/build/../llvm-zorg/zorg/buildbot/builders/annotated/libc-linux.py", line 167, in main
    run_command(['ninja', 'libc-fuzzer'])
  File "/home/llvm-libc-buildbot/home/sivachandra/libc-x86_64-debian/libc-x86_64-debian/build/../llvm-zorg/zorg/buildbot/builders/annotated/libc-linux.py", line 191, in run_command
    util.report_run_cmd(cmd, cwd=directory)
  File "/home/llvm-libc-buildbot/home/sivachandra/libc-x86_64-debian/libc-x86_64-debian/llvm-zorg/zorg/buildbot/builders/annotated/util.py", line 49, in report_run_cmd
    subprocess.check_call(cmd, shell=shell, *args, **kwargs)
  File "/usr/lib/python3.11/subprocess.py", line 413, in check_call
    raise CalledProcessError(retcode, cmd)
subprocess.CalledProcessError: Command '['ninja', 'libc-fuzzer']' returned non-zero exit status 1.
@@@STEP_FAILURE@@@

```

</details>

https://github.com/llvm/llvm-project/pull/143991