[PATCH] Fix memory leak of primary_sysid in walreceiver

Hi, Hackers,

In WalReceiverMain(), the outer streaming loop calls
walrcv_identify_system() once per iteration to verify the primary's
system identifier:

primary_sysid = walrcv_identify_system(wrconn, &primaryTLI);
...
if (strcmp(primary_sysid, standby_sysid) != 0)
ereport(ERROR, ...);

walrcv_identify_system() (libpqrcv_identify_system() in
libpqwalreceiver.c) returns a pstrdup()'d string, but the caller
never frees it. Each streaming restart therefore leaks the string.
The error path is unaffected because the surrounding memory context
is reset on ERROR.

The attached patch adds a pfree(primary_sysid) right after the
comparison.

This dates back to commit 78c8c814390 ("Refactor libpqwalreceiver",
2016), so it should be a back-patch candidate as well.

No new tests are added; the fix only releases resources and does not
change observable behavior. `make check` and the streaming replication
TAP test (src/test/recovery/t/001_stream_rep.pl) pass with the patch
applied.

Patch attached.

Regards,
DaeMyung Kang