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MDEV-13318 introduced a condition to Mariabackup that can cause it to hang if the server goes idle after a particular kind of redo log write where the last written redo log block has no payload after the 12-byte header. In log0recv.cc we would treat blocks with less than 12 bytes of length as corrupted, but a block with exactly 12 bytes is fine. However, Mariabackup would disregard blocks that have exactly 12 bytes of length.
The problem was found in the MDEV-23855 branch via a test failure like this:
mariabackup.incremental_encrypted '4k,innodb' w2 [ fail ] timeout after 900 seconds
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Test ended at 2020-10-19 08:16:36
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Test case timeout after 900 seconds
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== /dev/shm/var/2/log/incremental_encrypted.log ==
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call mtr.add_suppression("InnoDB: New log files created");
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CREATE TABLE t(i INT) ENGINE INNODB ENCRYPTED=YES;
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INSERT INTO t VALUES(1);
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# Create full backup , modify table, then create incremental/differential backup
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SET GLOBAL innodb_flush_log_at_trx_commit = 1;
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INSERT INTO t VALUES(2);
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SELECT * FROM t;
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i
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1
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2
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== /dev/shm/var/2/tmp/analyze-timeout-mysqld.1.err ==
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mysqltest: Could not open connection 'default' after 500 attempts: 2002 Can't connect to local MySQL server through socket '/dev/shm/var/tmp/2/mysqld.1.sock' (111)
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The MariaDB server process is idle, and the reason for the hang was that mariabackup --backup in xtrabackup_copy_logfile() would never reach the end of the log, and instead kept complaining that the last block is corrupted, which is it not:
[00] 2020-10-19 12:56:20 >> log scanned up to (59392)
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[00] 2020-10-19 12:56:21 garbage block: 59392,12
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[00] 2020-10-19 12:56:21 >> log scanned up to (59392)
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[00] 2020-10-19 12:56:22 garbage block: 59392,12
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[00] 2020-10-19 12:56:22 >> log scanned up to (59392)
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The fix is simple: accept a log block with no payload bytes:
diff --git a/extra/mariabackup/xtrabackup.cc b/extra/mariabackup/xtrabackup.cc
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index 13c10c2921d..8ad9af9a9c9 100644
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--- a/extra/mariabackup/xtrabackup.cc
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+++ b/extra/mariabackup/xtrabackup.cc
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@@ -2650,7 +2650,7 @@ static lsn_t xtrabackup_copy_log(lsn_t start_lsn, lsn_t end_lsn, bool last)
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/* We got a full log block. */
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scanned_lsn += data_len;
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} else if (data_len >= log_sys.trailer_offset()
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- || data_len <= LOG_BLOCK_HDR_SIZE) {
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+ || data_len < LOG_BLOCK_HDR_SIZE) {
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/* We got a garbage block (abrupt end of the log). */
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msg(0,"garbage block: " LSN_PF ",%zu",scanned_lsn, data_len);
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break;
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