zizhong opened a new pull request #7117:
URL: https://github.com/apache/trafficserver/pull/7117
GDB showed the dead loop is because the next field of an element pointing to
itself.
```
#1 HostDBCache::is_pending_dns_for_hash (this=this@entry=0xbd3e20 <hostDB>,
hash=...) at HostDB.cc:247
247 HostDB.cc: No such file or directory.
(gdb) info locals
c = 0x2b63897745e0
q = @0x2b638704bdc0: {<DLL<HostDBContinuation, Continuation::Link_link>> =
{head = 0x2b63897745e0}, tail = 0x2b63897745e0}
(gdb) p q->head->link->next
$7 = (Continuation *) 0x2b63897745e0
(gdb) p q->head->link->next->link->next
$8 = (Continuation *) 0x2b63897745e0
```
Tried a test build and found the incorrect next field was caused by
enqueuing the same element.
There is the logic that if HostDB Continuation timeout is applied, we retry
the same continuation which causes the enqueuing the same element.
`pending_dns` is basically an entry of hash map implemented with a linked
list. Not checking the same element is the wrong implementation of a hashmap.
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