MDEV-26794 MariaBackup does not recognize added providers upon prepare of incremental backup

prefer backup-my.cnf from the incremental-dir over the one in target-dir

extra/mariabackup/encryption_plugin.cc

@@ -55,12 +55,13 @@ static char XTRABACKUP_EXE[] = "xtrabackup";
   Read "plugin-load" value from backup-my.cnf during prepare phase.
   The value is stored during backup phase.
 */
-static std::string get_encryption_plugin_from_cnf()
+static std::string get_encryption_plugin_from_cnf(const char *dir)
 {
-  FILE *f = fopen("backup-my.cnf", "r");
+  std::string path = dir + std::string("/backup-my.cnf");
+  FILE *f = fopen(path.c_str(), "r");
   if (!f)
   {
-    die("Can't open backup-my.cnf for reading");
+    die("Can't open %s for reading", path.c_str());
   }
   char line[512];
   std::string plugin_load;
@@ -185,9 +186,9 @@ const char *encryption_plugin_get_config()
 extern int finalize_encryption_plugin(st_plugin_int *plugin);
 
 
-void encryption_plugin_prepare_init(int argc, char **argv)
+void encryption_plugin_prepare_init(int argc, char **argv, const char *dir)
 {
-  std::string plugin_load= get_encryption_plugin_from_cnf();
+  std::string plugin_load= get_encryption_plugin_from_cnf(dir ? dir : ".");
   if (plugin_load.size())
   {
     msg("Loading plugins from %s", plugin_load.c_str());

extra/mariabackup/encryption_plugin.h

@@ -2,6 +2,6 @@
 #include <string>
 extern void encryption_plugin_backup_init(MYSQL *mysql);
 extern const char* encryption_plugin_get_config();
-extern void encryption_plugin_prepare_init(int argc, char **argv);
+extern void encryption_plugin_prepare_init(int argc, char **argv, const char *dir);
 
 //extern  void encryption_plugin_init(int argc, char **argv);

extra/mariabackup/xtrabackup.cc

@@ -5774,7 +5774,7 @@ static bool xtrabackup_prepare_func(char** argv)
 	}
 
 	int argc; for (argc = 0; argv[argc]; argc++) {}
-	encryption_plugin_prepare_init(argc, argv);
+	encryption_plugin_prepare_init(argc, argv, xtrabackup_incremental_dir);
 
 	xtrabackup_target_dir= mysql_data_home_buff;
 	xtrabackup_target_dir[0]=FN_CURLIB;		// all paths are relative from here
@@ -6332,12 +6332,17 @@ void handle_options(int argc, char **argv, char ***argv_server,
           {
             prepare= true;
           }
-          else if (!strncmp(argv[i], "--target-dir", optend - argv[i]) &&
+          else if (!strncmp(argv[i], "--incremental-dir", optend - argv[i]) &&
                    *optend)
           {
             target_dir= optend + 1;
           }
-          else if (!*optend && argv[i][0] != '-')
+          else if (!strncmp(argv[i], "--target-dir", optend - argv[i]) &&
+                   *optend && !target_dir)
+          {
+            target_dir= optend + 1;
+          }
+          else if (!*optend && argv[i][0] != '-' && !target_dir)
           {
             target_dir= argv[i];
           }

mysql-test/suite/mariabackup/incremental_compressed.result

+#
+# MDEV-26794 MariaBackup does not recognize added providers upon prepare of incremental backup
+#
+CREATE TABLE t (a INT) ENGINE=InnoDB;
+INSERT INTO t VALUES (1),(2);
+INSTALL SONAME 'provider_snappy';
+SET GLOBAL innodb_compression_algorithm= snappy;
+CREATE TABLE t_snappy (a INT) ENGINE=InnoDB page_compressed=1;
+INSERT INTO t_snappy VALUES (3),(4);
+# restart: --innodb_buffer_pool_load_at_startup=0
+# Prepare initial backup
+# Prepare incremental backup
+DROP TABLE t;

mysql-test/suite/mariabackup/incremental_compressed.test

+--echo #
+--echo # MDEV-26794 MariaBackup does not recognize added providers upon prepare of incremental backup
+--echo #
+
+if (!$PROVIDER_SNAPPY_SO) {
+  skip "Needs provider_snappy plugin";
+}
+
+--source include/have_innodb.inc
+
+--let $targetdir=$MYSQLTEST_VARDIR/tmp/backup
+--let $incdir=$MYSQLTEST_VARDIR/tmp/inc
+
+CREATE TABLE t (a INT) ENGINE=InnoDB;
+INSERT INTO t VALUES (1),(2);
+
+--exec $XTRABACKUP --defaults-file=$MYSQLTEST_VARDIR/my.cnf --backup --target-dir=$targetdir
+
+INSTALL SONAME 'provider_snappy';
+
+SET GLOBAL innodb_compression_algorithm= snappy;
+
+CREATE TABLE t_snappy (a INT) ENGINE=InnoDB page_compressed=1;
+INSERT INTO t_snappy VALUES (3),(4);
+
+# disable buffer pool load to avoid MDEV-26794 warnings
+--let $restart_parameters= --innodb_buffer_pool_load_at_startup=0
+--source include/restart_mysqld.inc
+
+--exec $XTRABACKUP --defaults-file=$MYSQLTEST_VARDIR/my.cnf --backup --incremental-basedir=$targetdir --target-dir=$incdir
+
+--echo # Prepare initial backup
+--exec $XTRABACKUP --prepare --target-dir=$targetdir
+
+--echo # Prepare incremental backup
+--exec $XTRABACKUP --prepare --target-dir=$targetdir --incremental-dir=$incdir > $MYSQLTEST_VARDIR/prepare.log 2>&1
+
+DROP TABLE t;