Restructure libpq code to remove some duplicity

There was some duplicate code to run SHOW transaction_read_only to
determine whether the server is read-write or read-only.  Reduce it by
adding another state to the state machine.

Author: Hari Babu Kommi
Reviewed-by: Takayuki Tsunakawa, Álvaro Herrera
Discussion: https://postgr.es/m/CAJrrPGe_qgdbbN+yBgEVpd+YLHXXjTruzk6RmTMhqrFig+32ag@mail.gmail.com

src/interfaces/libpq/fe-connect.c

@@ -3434,6 +3434,13 @@ keep_going:						/* We will come back to here until there is
 					return PGRES_POLLING_WRITING;
 				}
 
+				/* Almost there now ... */
+				conn->status = CONNECTION_CHECK_TARGET;
+				goto keep_going;
+			}
+
+		case CONNECTION_CHECK_TARGET:
+			{
 				/*
 				 * If a read-write connection is required, see if we have one.
 				 *
@@ -3476,7 +3483,7 @@ keep_going:						/* We will come back to here until there is
 			}
 
 		case CONNECTION_SETENV:
-
+			{
 				/*
 				 * Do post-connection housekeeping (only needed in protocol 2.0).
 				 *
@@ -3502,41 +3509,11 @@ keep_going:						/* We will come back to here until there is
 						goto error_return;
 				}
 
-			/*
-			 * If a read-write connection is required, see if we have one.
-			 * (This should match the stanza in the CONNECTION_AUTH_OK case
-			 * above.)
-			 *
-			 * Servers before 7.4 lack the transaction_read_only GUC, but by
-			 * the same token they don't have any read-only mode, so we may
-			 * just skip the test in that case.
-			 */
-			if (conn->sversion >= 70400 &&
-				conn->target_session_attrs != NULL &&
-				strcmp(conn->target_session_attrs, "read-write") == 0)
-			{
-				if (!saveErrorMessage(conn, &savedMessage))
-					goto error_return;
-
-				conn->status = CONNECTION_OK;
-				if (!PQsendQuery(conn,
-								 "SHOW transaction_read_only"))
-				{
-					restoreErrorMessage(conn, &savedMessage);
-					goto error_return;
-				}
-				conn->status = CONNECTION_CHECK_WRITABLE;
-				restoreErrorMessage(conn, &savedMessage);
-				return PGRES_POLLING_READING;
+				/* Almost there now ... */
+				conn->status = CONNECTION_CHECK_TARGET;
+				goto keep_going;
 			}
 
-			/* We can release the address list now. */
-			release_conn_addrinfo(conn);
-
-			/* We are open for business! */
-			conn->status = CONNECTION_OK;
-			return PGRES_POLLING_OK;
-
 		case CONNECTION_CONSUME:
 			{
 				conn->status = CONNECTION_OK;

src/interfaces/libpq/libpq-fe.h

@@ -67,7 +67,8 @@ typedef enum
 								 * connection. */
 	CONNECTION_CONSUME,			/* Wait for any pending message and consume
 								 * them. */
-	CONNECTION_GSS_STARTUP		/* Negotiating GSSAPI. */
+	CONNECTION_GSS_STARTUP,		/* Negotiating GSSAPI. */
+	CONNECTION_CHECK_TARGET		/* Check if we have a proper target connection */
 } ConnStatusType;
 
 typedef enum