Improve PQhost() to return useful data for default Unix-socket connections.

Previously, if no host information had been specified at connection time,
PQhost() would return NULL (unless you are on Windows, in which case you
got "localhost").  This is an unhelpful definition for a couple of reasons:
it can cause corner-case crashes in applications (cf commit c5ef8ce53d),
and there's no well-defined way for applications to find out the socket
directory path that's actually in use.  As an example of the latter
problem, psql substituted DEFAULT_PGSOCKET_DIR for NULL in a couple of
places, but this is subtly wrong because it's conceivable that psql is
using a libpq shared library that was built with a different setting.

Hence, change PQhost() to return DEFAULT_PGSOCKET_DIR when appropriate,
and strip out the now-dead substitutions in psql.  (There is still one
remaining reference to DEFAULT_PGSOCKET_DIR in psql, in prompt.c, which
I don't see a nice way to get rid of.  But it only controls a prompt
abbreviation decision, so it seems noncritical.)

Also update the docs for PQhost, which had never previously mentioned
the possibility of a socket directory path being returned.  In passing
fix the outright-incorrect code comment about PGconn.pgunixsocket.

doc/src/sgml/libpq.sgml

@@ -1463,6 +1463,10 @@ char *PQpass(const PGconn *conn);
      <listitem>
       <para>
        Returns the server host name of the connection.
+       This can be a host name, an IP address, or a directory path if the
+       connection is via Unix socket.  (The path case can be distinguished
+       because it will always be an absolute path, beginning
+       with <literal>/</>.)
 <synopsis>
 char *PQhost(const PGconn *conn);
 </synopsis>

src/bin/psql/command.c

@@ -322,8 +322,6 @@ exec_command(const char *cmd,
 			PQconninfoOption *option;
 
 			host = PQhost(pset.db);
-			if (host == NULL)
-				host = DEFAULT_PGSOCKET_DIR;
 			/* A usable "hostaddr" overrides the basic sense of host. */
 			connOptions = PQconninfo(pset.db);
 			if (connOptions == NULL)
@@ -1750,16 +1748,14 @@ do_connect(char *dbname, char *user, char *host, char *port)
 	/*
 	 * Any change in the parameters read above makes us discard the password.
 	 * We also discard it if we're to use a conninfo rather than the
-	 * positional syntax.  Note that currently, PQhost() can return NULL for a
-	 * default Unix-socket connection, so we have to allow NULL for host.
+	 * positional syntax.
 	 */
 	if (has_connection_string)
 		keep_password = false;
 	else
 		keep_password =
 			(user && PQuser(o_conn) && strcmp(user, PQuser(o_conn)) == 0) &&
-			((host && PQhost(o_conn) && strcmp(host, PQhost(o_conn)) == 0) ||
-			 (host == NULL && PQhost(o_conn) == NULL)) &&
+			(host && PQhost(o_conn) && strcmp(host, PQhost(o_conn)) == 0) &&
 			(port && PQport(o_conn) && strcmp(port, PQport(o_conn)) == 0);
 
 	/*
@@ -1890,8 +1886,6 @@ do_connect(char *dbname, char *user, char *host, char *port)
 		{
 			char	   *host = PQhost(pset.db);
 
-			if (host == NULL)
-				host = DEFAULT_PGSOCKET_DIR;
 			/* If the host is an absolute path, the connection is via socket */
 			if (is_absolute_path(host))
 				printf(_("You are now connected to database \"%s\" as user \"%s\" via socket in \"%s\" at port \"%s\".\n"),

src/interfaces/libpq/fe-connect.c

@@ -5353,7 +5353,10 @@ PQhost(const PGconn *conn)
 	else
 	{
 #ifdef HAVE_UNIX_SOCKETS
-		return conn->pgunixsocket;
+		if (conn->pgunixsocket != NULL && conn->pgunixsocket[0] != '\0')
+			return conn->pgunixsocket;
+		else
+			return DEFAULT_PGSOCKET_DIR;
 #else
 		return DefaultHost;
 #endif

src/interfaces/libpq/libpq-int.h

@@ -302,10 +302,9 @@ struct pg_conn
 	char	   *pghostaddr;		/* the numeric IP address of the machine on
 								 * which the server is running.  Takes
 								 * precedence over above. */
-	char	   *pgport;			/* the server's communication port */
-	char	   *pgunixsocket;	/* the Unix-domain socket that the server is
-								 * listening on; if NULL, uses a default
-								 * constructed from pgport */
+	char	   *pgport;			/* the server's communication port number */
+	char	   *pgunixsocket;	/* the directory of the server's Unix-domain
+								 * socket; if NULL, use DEFAULT_PGSOCKET_DIR */
 	char	   *pgtty;			/* tty on which the backend messages is
 								 * displayed (OBSOLETE, NOT USED) */
 	char	   *connect_timeout;	/* connection timeout (numeric string) */