Disable silently generation of manifests with servers <= 12 in pg_basebackup

Since 0d8c9c12, pg_basebackup would generate an error if connected to a
backend version older than 12 where backup manifests are not supported.
Avoiding this error is possible by using the --no-manifest option.

This error handling could be confusing for some users, where patching a
backup script that interacts with multiple backend versions would cause
the addition of --no-manifest to potentially not generate a backup
manifest even for Postgres 13 and newer versions.  As we want to
encourage the use of backup manifests as much as possible, this commit
silently disables manifests where not supported, instead of generating
an error.

While on it, rework a bit the code to make it more consistent with the
surroundings when generating the BASE_BACKUP command.

Per discussion with Andres Freund, Stephen Frost, Robert Haas, Álvaro
Herrera, Kyotaro Horiguchi, Tom Lane, David Steele, and me.

Author: Michael Paquier
Discussion: https://postgr.es/m/20200410080910.GZ1606@paquier.xyz

src/bin/pg_basebackup/pg_basebackup.c

@@ -108,6 +108,11 @@ typedef void (*WriteDataCallback) (size_t nbytes, char *buf,
  */
 #define MINIMUM_VERSION_FOR_TEMP_SLOTS 100000
 
+/*
+ * Backup manifests are supported from version 13.
+ */
+#define MINIMUM_VERSION_FOR_MANIFESTS	130000
+
 /*
  * Different ways to include WAL
  */
@@ -1770,7 +1775,7 @@ BaseBackup(void)
 	char	   *basebkp;
 	char		escaped_label[MAXPGPATH];
 	char	   *maxrate_clause = NULL;
-	char	   *manifest_clause;
+	char	   *manifest_clause = NULL;
 	char	   *manifest_checksums_clause = "";
 	int			i;
 	char		xlogstart[64];
@@ -1836,15 +1841,6 @@ BaseBackup(void)
 
 	if (manifest)
 	{
-		if (serverMajor < 1300)
-		{
-			const char *serverver = PQparameterStatus(conn, "server_version");
-
-			pg_log_error("backup manifests are not supported by server version %s",
-						 serverver ? serverver : "'unknown'");
-			exit(1);
-		}
-
 		if (manifest_force_encode)
 			manifest_clause = "MANIFEST 'force-encode'";
 		else
@@ -1853,13 +1849,6 @@ BaseBackup(void)
 			manifest_checksums_clause = psprintf("MANIFEST_CHECKSUMS '%s'",
 												 manifest_checksums);
 	}
-	else
-	{
-		if (serverMajor < 1300)
-			manifest_clause = "";
-		else
-			manifest_clause = "MANIFEST 'no'";
-	}
 
 	if (verbose)
 		pg_log_info("initiating base backup, waiting for checkpoint to complete");
@@ -1883,7 +1872,7 @@ BaseBackup(void)
 				 maxrate_clause ? maxrate_clause : "",
 				 format == 't' ? "TABLESPACE_MAP" : "",
 				 verify_checksums ? "" : "NOVERIFY_CHECKSUMS",
-				 manifest_clause,
+				 manifest_clause ? manifest_clause : "",
 				 manifest_checksums_clause);
 
 	if (PQsendQuery(conn, basebkp) == 0)
@@ -2589,6 +2578,10 @@ main(int argc, char **argv)
 	 */
 	umask(pg_mode_mask);
 
+	/* Backup manifests are supported in 13 and newer versions */
+	if (PQserverVersion(conn) < MINIMUM_VERSION_FOR_MANIFESTS)
+		manifest = false;
+
 	/*
 	 * Verify that the target directory exists, or create it. For plaintext
 	 * backups, always require the directory. For tar backups, require it