[Huang-404-Q]

Problem

POOL_create_advanced assigns ctx->customMem only after the queue has been allocated and its mutex/cond initialized, but the init failure path runs earlier:

ctx = ZSTD_customCalloc(sizeof(POOL_ctx), customMem);   /* zero-initialized */
...
if (error) { POOL_free(ctx); return NULL; }             /* customMem is still empty here */
...
ctx->customMem = customMem;                             /* too late */

ZSTD_customCalloc memsets the context, so POOL_free(ctx) sees an empty allocator and calls the default free() on memory that came from the caller's customAlloc. With a custom allocator that is not malloc-compatible this is undefined behavior; with a counting allocator the allocation is never returned through customFree.

Fix

Assign customMem immediately after the allocation that produces the context, before anything can fail.

Verification

Built lib/common/pool.c on Linux with a wrapper that makes the second pthread_cond_init return ENOMEM, and counted allocations through a ZSTD_customMem:

# before
allocs=2 custom_frees=0
MISMATCH: 2 allocations, 0 custom frees -> default free() used

# after
allocs=2 custom_frees=2
OK

Reproducer: gcc -O0 -DZSTD_MULTITHREAD -I lib -I lib/common -Wl,--wrap=pthread_cond_init repro.c lib/common/threading.c -lpthread

This only reproduces on POSIX: the Windows build maps these init calls to InitializeCriticalSection/InitializeConditionVariable and always returns 0.

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