facebook / zstd

Describe the bug

ZDICT_trainFromBuffer_cover is not thread safe due to the use of global state:

Lines 235 to 236 in 78955f5

    
           /* We need a global context for qsort... */ 
        
           static COVER_ctx_t *g_coverCtx = NULL;

Calling this function from multiple threads may result in segfault/access violation, use-after-free and other badness.

To Reproduce
Call ZDICT_trainFromBuffer_cover from multiple threads

Desktop (please complete the following information):

	/* We need a global context for qsort... */
	static COVER_ctx_t *g_coverCtx = NULL;