Per Copilot autofix:

In general, when using snprintf in a loop to build up a string, you must check each call’s return value before using it: if the return value is negative or greater than or equal to the remaining buffer size, you should stop writing and handle the error, rather than unconditionally adding the return value to your running length. This prevents len from being corrupted by a “would-have-written” size that is larger than the remaining space.

For this specific function, the safest fix with minimal functional change is:

    Track the remaining space explicitly (remaining = max_ranks_len;) and always pass remaining as the size argument.
    After each snprintf, if n < 0 or n >= remaining, treat it as failure: free the buffer, set *ranks = NULL, optionally log the error, and return early.
    Only if n is valid, subtract it from remaining and increment len by n.
    Because we are now checking after each call, the final if (len >= max_ranks_len - 1) becomes unnecessary, but we can keep a light sanity check if desired. To minimize changes, we can retain len and the final check, but make sure we never let len exceed max_ranks_len during the loop.

Concretely, in fill_seq_ranks_array in test/test_server.c, we will:

    Introduce an int n; and a int remaining = max_ranks_len;.
    Replace the two snprintf calls to:
        Call snprintf(*ranks + len, remaining, ...).
        Check if (n < 0 || n >= remaining) { free(*ranks); *ranks = NULL; TEST_ERROR(...); return; }.
        Update len += n; remaining -= n;.
    Keep allocations and the if (0 >= nprocs) early-return logic unchanged.

No new methods or external libraries are required; we only reuse malloc, free, snprintf, and the existing TEST_ERROR macro.

Signed-off-by: Ralph Castain <rhc@pmix.org>