Re: Synchronizing slots from primary to standby

Here are some review comments for v750001.

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Commit message

1.
This patch provides support for non-replication connection
in libpqrcv_connect().

~

1a.
/connection/connections/

~

1b.
Maybe there needs to be a few more sentences just to describe what you
mean by "non-replication connection".

~

1c.
IIUC although the 'replication' parameter is added, in this patch
AFAICT every call to the connect function is still passing that
argument as true. If that's correct, probably this patch comment
should emphasise that this patch doesn't change any functionality at
all but is just preparation for later patches which *will* pass false
for the replication arg.

~~~

2.
This patch also implements a new API libpqrcv_get_dbname_from_conninfo()
to extract database name from the given connection-info

~

/extract database name/the extract database name/

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.../libpqwalreceiver/libpqwalreceiver.c

3.
+ * Apart from walreceiver, the libpq-specific routines here are now being used
+ * by logical replication worker as well.

/worker/workers/

~~~

4. libpqrcv_connect

 /*
- * Establish the connection to the primary server for XLOG streaming
+ * Establish the connection to the primary server.
+ *
+ * The connection established could be either a replication one or
+ * a non-replication one based on input argument 'replication'. And further
+ * if it is a replication connection, it could be either logical or physical
+ * based on input argument 'logical'.

That first comment ("could be either a replication one or...") seemed
a bit meaningless (e.g. it like saying "this boolean argument can be
true or false") because it doesn't describe what is the meaning of a
"replication connection" versus what is a "non-replication
connection".

~~~

5.
/* We can not have logical without replication */
Assert(replication || !logical);

if (replication)
{
keys[++i] = "replication";
vals[i] = logical ? "database" : "true";

if (!logical)
{
/*
* The database name is ignored by the server in replication mode,
* but specify "replication" for .pgpass lookup.
*/
keys[++i] = "dbname";
vals[i] = "replication";
}
}

keys[++i] = "fallback_application_name";
vals[i] = appname;
if (logical)
{
...
}

~

The Assert already says we cannot be 'logical' if not 'replication',
therefore IMO it seemed strange that the code was not refactored to
bring that 2nd "if (logical)" code to within the scope of the "if
(replication)".

e.g. Can't you do something like this:

Assert(replication || !logical);

if (replication)
{
  ...
  if (logical)
  {
    ...
  }
  else
  {
    ...
  }
}
keys[++i] = "fallback_application_name";
vals[i] = appname;

~~~

6. libpqrcv_get_dbname_from_conninfo

+ for (PQconninfoOption *opt = opts; opt->keyword != NULL; ++opt)
+ {
+ /*
+ * If multiple dbnames are specified, then the last one will be
+ * returned
+ */
+ if (strcmp(opt->keyword, "dbname") == 0 && opt->val &&
+ opt->val[0] != '\0')
+ dbname = pstrdup(opt->val);
+ }

Should you also pfree the old dbname instead of gathering a bunch of
strdups if there happened to be multiple dbnames specified ?

SUGGESTION
if (strcmp(opt->keyword, "dbname") == 0 && opt->val && *opt->val)
{
   if (dbname)
    pfree(dbname);
   dbname = pstrdup(opt->val);
}

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src/include/replication/walreceiver.h

7.
/*
 * walrcv_connect_fn
 *
 * Establish connection to a cluster.  'logical' is true if the
 * connection is logical, and false if the connection is physical.
 * 'appname' is a name associated to the connection, to use for example
 * with fallback_application_name or application_name.  Returns the
 * details about the connection established, as defined by
 * WalReceiverConn for each WAL receiver module.  On error, NULL is
 * returned with 'err' including the error generated.
 */
typedef WalReceiverConn *(*walrcv_connect_fn) (const char *conninfo,
   bool replication,
   bool logical,
   bool must_use_password,
   const char *appname,
   char **err);

~

The comment is missing any description of the new parameter 'replication'.

~~~

8.
+/*
+ * walrcv_get_dbname_from_conninfo_fn
+ *
+ * Returns the dbid from the primary_conninfo
+ */
+typedef char *(*walrcv_get_dbname_from_conninfo_fn) (const char *conninfo);
+

/dbid/database name/

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Kind Regards,
Peter Smith.
Fujitsu Australia