fish shell uses posix_spawn sometimes because of its performance benefits. We can't use it in the following cases:

1. No analog to tcsetpgrp, so it's no good if job control is enabled

2. No analog to fchdir, meaning you have to synchronize with fchdir elsewhere in the progarm

3. Error codes do not convey enough information for good error messages (e.g. if a file doesn't exist, posix_spawn doesn't tell you which file)

4. Inconsistent behavior around dup2 fd redirections and CLO_EXEC.

5. Inconsistent behavior for shebangless scripts

These are basically deal-breakers so fish also supports a fork/exec path. However the performance benefits of posix_spawn are too real to ignore so fish uses posix_spawn when it can, and fork/exec when it must.