Thread-safety: initialization and tear-down should be automatic and thread-safe

[nicowilliams]

For initialization (onig_init()) use pthread_once() (POSIX) or InitOnceExecuteOnce() (WIN32).

For finalization (onig_end()) either do nothing (it's OK to leak some global state like Unicode tables) or use .fini section (ELF) or C++ destructors, or DllMain() (WIN32). This includes lazy initialization of the cclass table (which could have its own once-initializer), and the EUC/JIS Hiragana and Katakana property lists initialization (which could have its own once-initializer).

You can then leave onig_init() and onig_end() as empty stubs. For static-link archives it would help to have a --disable-thread option to not require -lpthread and to instead use a dumb once-initializer.

For USE_PARSE_TREE_NODE_RECYCLE use pthread_key_create() and pthread_setspecific() (POSIX) or thread-local storage and DllMain() (WIN32). Or just delete the USE_PARSE_TREE_NODE_RECYCLE code?

I think you can then drop all the THREAD_ATOMIC_START and THREAD_ATOMIC_END business and just declare that the remainder of thread safety is the application's job.

(I don't understand ONIG_STATE_DEC() though. Can you explain why it exists? reg->state doesn't look like a reference count...)

[nicowilliams]

I should explain why this is important.

Libraries aren't used just by programs. Sometimes (often!) libraries are used by other libraries. Ensuring that there is only one call (or non-racing calls) to a library's initializer or finalizer is then impossible. The situation for finalizers is worse because a library might call another's finalizer in its own, but this might happen as a result of a last dlcose(), and if the lower-level library was still in use by other callers in the same process... boom.

For example, jq uses Oniguruma in its libjq.

[kkos]

For ONIG_STATE_DEC() question.
This is because it avoids that regexp recompile and regexp search rise at the same time.
Regexp recompile API existed in old GNU regex. I supported it for compatibility.
However, I do not support it now. (USE_RECOMPILE_API is not defined.)
So, ONIG_STATE_INC() etc... is empty declarations.

[kkos]

I have removed all THREAD_ macro in develop branch.