Check data directory not used by an active instance before cloning

This involves adding some infrastructure to parse pg_control in a
reasonably version-independent fashion. This will probably be useful
in other contexts where we need to verify the status of a
data directory where the instance isn't running.

controldata.c

@@ -0,0 +1,110 @@
+/*
+ * controldata.c
+ * Copyright (c) 2ndQuadrant, 2010-2017
+ *
+ * Portions Copyright (c) 1996-2016, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ */
+
+#include <unistd.h>
+#include <sys/stat.h>
+#include <fcntl.h>
+
+#include "postgres_fe.h"
+#include "port/pg_crc32c.h"
+
+#include "repmgr.h"
+#include "controldata.h"
+
+static ControlFileInfo *get_controlfile(char *DataDir);
+
+DBState
+get_db_state(char *data_directory)
+{
+	ControlFileInfo *control_file_info;
+	DBState state;
+
+	control_file_info = get_controlfile(data_directory);
+
+	if (control_file_info->control_file_processed == true)
+		state = control_file_info->control_file->state;
+	else
+		/* if we were unable to parse the control file, assume DB is shut down */
+		state = DB_SHUTDOWNED;
+
+	pfree(control_file_info->control_file);
+	pfree(control_file_info);
+	return state;
+}
+
+const char *
+describe_db_state(DBState state)
+{
+	switch (state)
+	{
+		case DB_STARTUP:
+			return _("starting up");
+		case DB_SHUTDOWNED:
+			return _("shut down");
+		case DB_SHUTDOWNED_IN_RECOVERY:
+			return _("shut down in recovery");
+		case DB_SHUTDOWNING:
+			return _("shutting down");
+		case DB_IN_CRASH_RECOVERY:
+			return _("in crash recovery");
+		case DB_IN_ARCHIVE_RECOVERY:
+			return _("in archive recovery");
+		case DB_IN_PRODUCTION:
+			return _("in production");
+	}
+	return _("unrecognized status code");
+}
+
+
+/*
+ * we maintain our own version of get_controlfile() as we don't care if
+ * the file isn't readable
+ */
+static ControlFileInfo *
+get_controlfile(char *DataDir)
+{
+	ControlFileInfo *control_file_info;
+	int			fd;
+	char		ControlFilePath[MAXPGPATH];
+	pg_crc32c	crc;
+
+	control_file_info = palloc0(sizeof(ControlFileInfo));
+	control_file_info->control_file_processed = false;
+	control_file_info->control_file = palloc0(sizeof(ControlFileData));
+
+	snprintf(ControlFilePath, MAXPGPATH, "%s/global/pg_control", DataDir);
+
+	if ((fd = open(ControlFilePath, O_RDONLY | PG_BINARY, 0)) == -1)
+	{
+		log_debug("could not open file \"%s\" for reading: %s",
+				  ControlFilePath, strerror(errno));
+		return control_file_info;
+	}
+
+	if (read(fd, control_file_info->control_file, sizeof(ControlFileData)) != sizeof(ControlFileData))
+	{
+		log_debug("could not read file \"%s\": %s",
+				  ControlFilePath, strerror(errno));
+		return control_file_info;
+	}
+
+	close(fd);
+
+	control_file_info->control_file_processed = true;
+
+	/*
+	 * We don't check the CRC here as we're potentially checking a pg_control
+	 * file from a different PostgreSQL version to the one repmgr was compiled
+	 * against. However we're only interested in the first few fields, which
+	 * should be constant across supported versions
+	 *
+	 * XXX double-check this
+	 */
+
+	return control_file_info;
+}