pg_combinebackup: Use new routine to retrieve data of PG_VERSION

pg_combinebackup's custom logic to retrieve the version number of a data
folder's PG_VERSION can be replaced by the facility introduced in
cd0be131ba.  This removes some code.

One thing specific to this tool is that backend versions older than v10
are not supported.  The new code does the same checks as the previous
code.

Author: Michael Paquier <michael@paquier.xyz>
Reviewed-by: Masahiko Sawada <sawada.mshk@gmail.com>
Discussion: https://postgr.es/m/aOiirvWJzwdVCXph@paquier.xyz

src/bin/pg_combinebackup/pg_combinebackup.c

@@ -34,6 +34,7 @@
 #include "common/relpath.h"
 #include "copy_file.h"
 #include "fe_utils/option_utils.h"
+#include "fe_utils/version.h"
 #include "getopt_long.h"
 #include "lib/stringinfo.h"
 #include "load_manifest.h"
@@ -117,7 +118,6 @@ static void process_directory_recursively(Oid tsoid,
 										  manifest_data **manifests,
 										  manifest_writer *mwriter,
 										  cb_options *opt);
-static int	read_pg_version_file(char *directory);
 static void remember_to_cleanup_directory(char *target_path, bool rmtopdir);
 static void reset_directory_cleanup_list(void);
 static cb_tablespace *scan_for_existing_tablespaces(char *pathname,
@@ -153,7 +153,7 @@ main(int argc, char *argv[])
 	int			c;
 	int			n_backups;
 	int			n_prior_backups;
-	int			version;
+	uint32		version;
 	uint64		system_identifier;
 	char	  **prior_backup_dirs;
 	cb_options	opt;
@@ -162,6 +162,7 @@ main(int argc, char *argv[])
 	StringInfo	last_backup_label;
 	manifest_data **manifests;
 	manifest_writer *mwriter;
+	char	   *pgdata;
 
 	pg_logging_init(argv[0]);
 	progname = get_progname(argv[0]);
@@ -271,7 +272,12 @@ main(int argc, char *argv[])
 	}
 
 	/* Read the server version from the final backup. */
-	version = read_pg_version_file(argv[argc - 1]);
+	pgdata = argv[argc - 1];
+	version = get_pg_version(pgdata, NULL);
+	if (GET_PG_MAJORVERSION_NUM(version) < 10)
+		pg_fatal("server version too old");
+	pg_log_debug("read server version %u from file \"%s/%s\"",
+				 GET_PG_MAJORVERSION_NUM(version), pgdata, "PG_VERSION");
 
 	/* Sanity-check control files. */
 	n_backups = argc - optind;
@@ -1155,59 +1161,6 @@ process_directory_recursively(Oid tsoid,
 	closedir(dir);
 }
 
-/*
- * Read the version number from PG_VERSION and convert it to the usual server
- * version number format. (e.g. If PG_VERSION contains "14\n" this function
- * will return 140000)
- */
-static int
-read_pg_version_file(char *directory)
-{
-	char		filename[MAXPGPATH];
-	StringInfoData buf;
-	int			fd;
-	int			version;
-	char	   *ep;
-
-	/* Construct pathname. */
-	snprintf(filename, MAXPGPATH, "%s/PG_VERSION", directory);
-
-	/* Open file. */
-	if ((fd = open(filename, O_RDONLY, 0)) < 0)
-		pg_fatal("could not open file \"%s\": %m", filename);
-
-	/* Read into memory. Length limit of 128 should be more than generous. */
-	initStringInfo(&buf);
-	slurp_file(fd, filename, &buf, 128);
-
-	/* Close the file. */
-	if (close(fd) != 0)
-		pg_fatal("could not close file \"%s\": %m", filename);
-
-	/* Convert to integer. */
-	errno = 0;
-	version = strtoul(buf.data, &ep, 10);
-	if (errno != 0 || *ep != '\n')
-	{
-		/*
-		 * Incremental backup is not relevant to very old server versions that
-		 * used multi-part version number (e.g. 9.6, or 8.4). So if we see
-		 * what looks like the beginning of such a version number, just bail
-		 * out.
-		 */
-		if (version < 10 && *ep == '.')
-			pg_fatal("%s: server version too old", filename);
-		pg_fatal("%s: could not parse version number", filename);
-	}
-
-	/* Debugging output. */
-	pg_log_debug("read server version %d from file \"%s\"", version, filename);
-
-	/* Release memory and return result. */
-	pfree(buf.data);
-	return version * 10000;
-}
-
 /*
  * Add a directory to the list of output directories to clean up.
  */