dbm_nextkey: Always return an error if we've reached the end of the database

[bnovkov]

Summary

POSIX.1 states that dbm_nextkey must return an invalid key
(i.e., key.dptr == NULL) after the end of the database was reached.
The current implementation of hash_seq will incorrectly restart
the key sequence after the end of the database is reached.

Fix this by checking the "current bucket" index when R_NEXT is passed.

Sponsored by: Klara, Inc.

[bnovkov]

Remove stray newline.

[markj]

lib/libc/db/hash/hash.c

718

Is it legal to call this routine with flag == 0? The check above suggests so, but within libc there are no such calls. Are consumers allowed to call db->seq() directly?

[bnovkov]

Address @markj 's comments.

lib/libc/db/hash/hash.c

718

Good catch, thanks.
Consumers can call db->seq() directly, I've placed an additional check for flag == 0.

[markj]

lib/libc/db/hash/hash.c

707

The !flag test is redundant.