IntrinsicError: File exists (os error 17) when attempting to overwrite a symlink in dist/

[huonw]

Describe the bug

If pants export-codegen ...'s is run on a target where the output includes a symlink, the second attempt to write that to dist/codegen fails with IntrinsicError: File exists (os error 17), potentially being unable to overwrite the symlink.

I'm guessing this applies to more than just export-codegen and shell_command, but that and shell_command are the easiest way I know to repeatedly attempt to write a symlink to a single path.

Reproducer:

cd $(mktemp -d)

cat > pants.toml <<EOF
[GLOBAL]
pants_version = "2.16.0rc1"
backend_packages = ["pants.backend.shell"]

[anonymous-telemetry]
enabled = false
EOF

cat > BUILD <<EOF
shell_command(
    name="symlink",
    command="ln -s whatever.txt link.txt",
    tools=["ln"],
    output_files=["link.txt"],
)

shell_command(
    name="normal",
    command="touch file.txt",
    tools=["touch"],
    output_files=["file.txt"],
)
EOF

pants export-codegen :symlink
# OK: ... Writing generated files to dist/codegen

pants export-codegen :symlink
# BUG: ... IntrinsicError: File exists (os error 17)

# delete the existing symlink, and try again:
rm dist/codegen/link.txt
pants export-codegen :symlink
# OK: ... Writing generated files to dist/codegen

# non-link works fine, for comparison:
pants export-codegen :normal
# OK: ... Writing generated files to dist/codegen

pants export-codegen :normal
# OK: ... Writing generated files to dist/codegen

All of the export-codegens work, except for the BUG one:

16:42:31.28 [INFO] Writing generated files to dist/codegen
16:42:31.28 [ERROR] 1 Exception encountered:

Engine traceback:
  in `export-codegen` goal

IntrinsicError: File exists (os error 17)

Pants version
2.16.0rc1

OS
macOS

Additional info
#17758 is mildly related, since both that and this are pants failing to correctly update the contents of dist/.