Further investigation in the internals of select() showed me I initially misunderstood how FD_SET worked.
This restriction was to ensure the number of the FDs themselves do not go over 1023, because if an accept()ed connection receives such a number, it will be impossible to insert it in a FD_SET and to manage it via select().
The only proper way to avoid this bug would be to prevent accepting of connections after 900 or 1000.
Alternatively, rejecting connections outside of select() when the FD number is above FD_SETSIZE.
Neither are desirable solutions, but the alternative is to stop using select(), which means a significant rewrite of Sendmail.
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ラピー ステファン Lapie Stephane
株式会社朝日ネット システム部