Functions returning structs are not thread-safe

[Elias Martenson]

When DEFCFUN is used to declare a function that returns a struct, the resulting function calls CFFI:FOREIGN-FUNCALL which expands into a call to CFFI:PREPARE-FUNCTION. This function is not thread-safe since it can alter the content of *CIF-TABLE*.

Note that this code-path is only followed for functions that are called through libffi (i.e. functions that returns structs). Plain functions are not affected by this.

PREPARE-FUNCTION should use a mutex or other mechanism to prevent parallel access to *CIF-TABLE*.

Another potential solution would be to run DEFCFUN during compile-time, although I don't know the implications of this.

[Luís Oliveira (luismbo)]

Liam's fix at https://github.com/cffi/cffi/commit/bb5577baffae0137ae4087ee95cbd2df31b7ca87 (and discussion on IRC) suggests this is only an issue during compilation, but this bug report suggests otherwise. Which is which?