Add PQping and PQpingParams to libpq to allow detection of the server's

status, including a status where the server is running but refuses a
postgres connection.

Have pg_ctl use this new function.  This fixes the case where pg_ctl
reports that the server is not running (cannot connect) but in fact it
is running.

src/interfaces/libpq/exports.txt

@@ -157,3 +157,5 @@ PQescapeLiteral           154
 PQescapeIdentifier        155
 PQconnectdbParams         156
 PQconnectStartParams      157
+PQping                    158
+PQpingParams              159

src/interfaces/libpq/fe-connect.c

@@ -285,6 +285,7 @@ static bool connectOptions1(PGconn *conn, const char *conninfo);
 static bool connectOptions2(PGconn *conn);
 static int	connectDBStart(PGconn *conn);
 static int	connectDBComplete(PGconn *conn);
+static PGPing internal_ping(PGconn *conn);
 static PGconn *makeEmptyPGconn(void);
 static void fillPGconn(PGconn *conn, PQconninfoOption *connOptions);
 static void freePGconn(PGconn *conn);
@@ -375,6 +376,20 @@ PQconnectdbParams(const char **keywords,
 
 }
 
+PGPing
+PQpingParams(const char **keywords,
+				  const char **values,
+				  int expand_dbname)
+{
+	PGconn	   *conn = PQconnectStartParams(keywords, values, expand_dbname);
+	PGPing		ret;
+
+	ret = internal_ping(conn);
+	PQfinish(conn);
+
+	return ret;
+}
+
 /*
  *		PQconnectdb
  *
@@ -408,6 +423,18 @@ PQconnectdb(const char *conninfo)
 	return conn;
 }
 
+PGPing
+PQping(const char *conninfo)
+{
+	PGconn	   *conn = PQconnectStart(conninfo);
+	PGPing		ret;
+
+	ret = internal_ping(conn);
+	PQfinish(conn);
+
+	return ret;
+}
+
 /*
  *		PQconnectStartParams
  *
@@ -2513,6 +2540,32 @@ error_return:
 }
 
 
+/*
+ * internal_ping
+ *	Determine if a server is running and if we can connect to it.
+ */
+PGPing
+internal_ping(PGconn *conn)
+{
+	if (conn && conn->status != CONNECTION_BAD)
+	{
+		(void) connectDBComplete(conn);
+
+		/*
+		 *	If the connection needs a password, we can consider the
+		 *	server as accepting connections.
+		 */
+	    if (conn && (conn->status != CONNECTION_BAD ||
+		    PQconnectionNeedsPassword(conn)))
+			return PQACCESS;
+		else
+			return PQREJECT;
+	}
+	else
+		return PQNORESPONSE;
+}
+
+
 /*
  * makeEmptyPGconn
  *	 - create a PGconn data structure with (as yet) no interesting data

src/interfaces/libpq/libpq-fe.h

@@ -107,6 +107,13 @@ typedef enum
 	PQERRORS_VERBOSE			/* all the facts, ma'am */
 } PGVerbosity;
 
+typedef enum
+{
+	PQACCESS,					/* connected to server */
+	PQREJECT,					/* server rejected access */
+	PQNORESPONSE				/* server did not respond */
+} PGPing;
+
 /* PGconn encapsulates a connection to the backend.
  * The contents of this struct are not supposed to be known to applications.
  */
@@ -403,6 +410,9 @@ extern int	PQendcopy(PGconn *conn);
 extern int	PQsetnonblocking(PGconn *conn, int arg);
 extern int	PQisnonblocking(const PGconn *conn);
 extern int	PQisthreadsafe(void);
+extern PGPing PQping(const char *conninfo);
+extern PGPing PQpingParams(const char **keywords,
+				  const char **values, int expand_dbname);
 
 /* Force the write buffer to be written (or at least try) */
 extern int	PQflush(PGconn *conn);