Thread safety Part 1

[DennisHeimbigner]

Since changes to the netcdf-c library appear to be slowing down for the moment,
I am going to create a PR that is an initial step to thread safety support.
Specifically, I am going to start collecting all global variables and computed
constants into a single file.
There are some complexities.

1. Some global variables are used by all (or almost all) dispatchers.
   These will need to be protected by a global mutex.
2. Some global variables are used by only a single dispatcher.
   Thread safe access to them will need to be done by that dispatcher's code.
3. Computed constants will be initialized by a single initialization routine
   that uses a separate mutex. Once these constants are initialized, they will
   be accessible without any synchronization.

[WardF]

That sounds good; I'm going to push through the outstanding PR's as they stand at the moment, but once that's done we can wait for this :)

[DennisHeimbigner]

No need to hold up most merges. It is only really big ones that may cause trouble.

[edhartnett]

Dennis, I look forward to seeing this work develop.

Also, how do you plan to test?

[wkliao]

This will be a great feature in NetCDF. I am also looking forward to see it.

FYI. There is a test program in PnetCDF for testing one-file-per-thread safety.
https://github.com/Parallel-NetCDF/PnetCDF/blob/master/test/testcases/tst_pthread.c
Feel free to make use of it.

[DennisHeimbigner]

Thanks. What is the general state of thread support in pnetcdf?

[wkliao]

Starting from 1.11.0, PnetCDF supports one-file-per-thread safety.
It does not support multi-threading for accessing the same file.