Allow building without default socket directory

We have code paths for Unix socket support and no Unix socket support.
Now add a third variant: Unix socket support but do not use a Unix
socket by default in the client or the server, only if you explicitly
specify one.  This will be useful when we enable Unix socket support
on Windows.

To implement this, tweak things so that setting DEFAULT_PGSOCKET_DIR
to "" has the desired effect.  This mostly already worked like that;
only a few places needed to be adjusted.  Notably, the reference to
DEFAULT_PGSOCKET_DIR in UNIXSOCK_PATH() could be removed because all
callers already resolve an empty socket directory setting with a
default if appropriate.
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://www.postgresql.org/message-id/75f72249-8ae6-322a-63df-4fe03eeccb9f@2ndquadrant.com

src/include/libpq/pqcomm.h

@@ -68,10 +68,10 @@ typedef struct
 /* Configure the UNIX socket location for the well known port. */
 
 #define UNIXSOCK_PATH(path, port, sockdir) \
+	   (AssertMacro(sockdir), \
+		AssertMacro(*(sockdir) != '\0'), \
 		snprintf(path, sizeof(path), "%s/.s.PGSQL.%d", \
-				((sockdir) && *(sockdir) != '\0') ? (sockdir) : \
-				DEFAULT_PGSOCKET_DIR, \
-				(port))
+				 (sockdir), (port)))
 
 /*
  * The maximum workable length of a socket path is what will fit into

src/include/pg_config_manual.h

@@ -191,6 +191,11 @@
  * directory.  But if you just hate the idea of sockets in /tmp,
  * here's where to twiddle it.  You can also override this at runtime
  * with the postmaster's -k switch.
+ *
+ * If set to an empty string, then AF_UNIX sockets are not used by default: A
+ * server will not create an AF_UNIX socket unless the run-time configuration
+ * is changed, a client will connect via TCP/IP by default and will only use
+ * an AF_UNIX socket if one is explicitly specified.
  */
 #define DEFAULT_PGSOCKET_DIR  "/tmp"
 

src/interfaces/libpq/fe-connect.c

@@ -1095,12 +1095,17 @@ connectOptions2(PGconn *conn)
 			if (ch->host)
 				free(ch->host);
 #ifdef HAVE_UNIX_SOCKETS
-			ch->host = strdup(DEFAULT_PGSOCKET_DIR);
-			ch->type = CHT_UNIX_SOCKET;
-#else
-			ch->host = strdup(DefaultHost);
-			ch->type = CHT_HOST_NAME;
+			if (DEFAULT_PGSOCKET_DIR[0])
+			{
+				ch->host = strdup(DEFAULT_PGSOCKET_DIR);
+				ch->type = CHT_UNIX_SOCKET;
+			}
+			else
 #endif
+			{
+				ch->host = strdup(DefaultHost);
+				ch->type = CHT_HOST_NAME;
+			}
 			if (ch->host == NULL)
 				goto oom_error;
 		}