Open Radar

 
Summary: 
If you call open(2) and read(2) on a file that is concurrently being unlinked with unlink(2), sometimes you see the file in a strange state.  That doesn't seem to conform to POSIX semantics, and doesn't happen on other operating systems.

Steps to Reproduce:
Please see test program at https://github.com/macdice/unlinktest

Expected Results:
The test program should produce no output, because the file should either be seen as not existing (ENOENT) or existing and containing 11 bytes.

Actual Results:
On macOS it sometimes reports that it succeeded in opening the file but found it to be empty.

Version/Build:
macOS 10.14.1 build 18B75 (also tested on macOS 10.14 build 18A391).  Compiler is "Apple LLVM version 10.0.0 (clang-1000.11.45.5)", target is "x86_64-apple-darwin18.2.0", uname -a is "Darwin XXXXXX 18.2.0 Darwin Kernel Version 18.2.0: Fri Oct  5 19:41:49 PDT 2018; root:xnu-4903.221.2~2/RELEASE_X86_64 x86_64".

Configuration:
Seen on a mid 2014 MBP and also (in the more complex example that triggered our investigation) a build farm server in the PostgreSQL build/test farm (not sure which model/configuration, sorry).  Using apfs, not tested on any other filesystem.