Weird failures on lorikeet

Hi Andrew,

I noticed your animal lorikeet failed in the last two runs:
https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=lorikeet&dt=2018-02-21%2009%3A47%3A17
TRAP: FailedAssertion("!(((PageHeader) (page))->pd_special >= (__builtin_offsetof (PageHeaderData, pd_linp)))", File:
"/home/andrew/bf64/root/HEAD/pgsql/src/include/storage/bufpage.h",Line: 313)
 

https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=lorikeet&dt=2018-02-20%2012%3A46%3A17
TRAP: FailedAssertion("!(PMSignalState->PMChildFlags[slot] == 1)", File:
"/home/andrew/bf64/root/HEAD/pgsql.build/../pgsql/src/backend/storage/ipc/pmsignal.c",Line: 229)
 
2018-02-20 08:07:14.054 EST [5a8c1c3b.21d0:3] LOG:  select() failed in postmaster: Bad address
2018-02-20 08:07:14.073 EST [5a8c1c3b.21d0:4] LOG:  database system is shut down

The difference between the last successfull and the last failing build
is a single comment typo commit.

It kinda looks like there might be some underlying issue on the machine
with shared memory going away or such?

Greetings,

Andres Freund