Expose new function get_controlfile_by_exact_path().

This works just like get_controlfile(), but expects the path to the
control file rather than the path to the data directory that contains
the control file. This makes more sense in cases where the caller
has already constructed the path to the control file itself.

Amul Sul and Robert Haas, reviewed by Michael Paquier

src/bin/pg_combinebackup/pg_combinebackup.c

@@ -534,7 +534,7 @@ check_control_files(int n_backups, char **backup_dirs)
 
 		controlpath = psprintf("%s/%s", backup_dirs[i], "global/pg_control");
 		pg_log_debug("reading \"%s\"", controlpath);
-		control_file = get_controlfile(backup_dirs[i], &crc_ok);
+		control_file = get_controlfile_by_exact_path(controlpath, &crc_ok);
 
 		/* Control file contents not meaningful if CRC is bad. */
 		if (!crc_ok)

src/common/controldata_utils.c

@@ -50,10 +50,25 @@
  */
 ControlFileData *
 get_controlfile(const char *DataDir, bool *crc_ok_p)
+{
+	char		ControlFilePath[MAXPGPATH];
+
+	snprintf(ControlFilePath, MAXPGPATH, "%s/global/pg_control", DataDir);
+
+	return get_controlfile_by_exact_path(ControlFilePath, crc_ok_p);
+}
+
+/*
+ * get_controlfile_by_exact_path()
+ *
+ * As above, but the caller specifies the path to the control file itself,
+ * rather than the path to the data directory.
+ */
+ControlFileData *
+get_controlfile_by_exact_path(const char *ControlFilePath, bool *crc_ok_p)
 {
 	ControlFileData *ControlFile;
 	int			fd;
-	char		ControlFilePath[MAXPGPATH];
 	pg_crc32c	crc;
 	int			r;
 #ifdef FRONTEND
@@ -64,7 +79,6 @@ get_controlfile(const char *DataDir, bool *crc_ok_p)
 	Assert(crc_ok_p);
 
 	ControlFile = palloc_object(ControlFileData);
-	snprintf(ControlFilePath, MAXPGPATH, "%s/global/pg_control", DataDir);
 
 #ifdef FRONTEND
 	INIT_CRC32C(last_crc);

src/include/common/controldata_utils.h

@@ -13,6 +13,8 @@
 #include "catalog/pg_control.h"
 
 extern ControlFileData *get_controlfile(const char *DataDir, bool *crc_ok_p);
+extern ControlFileData *get_controlfile_by_exact_path(const char *ControlFilePath,
+													  bool *crc_ok_p);
 extern void update_controlfile(const char *DataDir,
 							   ControlFileData *ControlFile, bool do_sync);