Mroonga hangs on invalid FULLTEXTMroonga index flag

Originally reported in Debian as #1110683

Creating a Mroonga table with an invalid FULLTEXT index flag can make mariadbd hang.

The reproducer is:

CREATE TABLE torrent (

  hash BINARY(20) PRIMARY KEY,

  mtime INT UNSIGNED NOT NULL,

  stime INT UNSIGNED NOT NULL,

  flags INT UNSIGNED NOT NULL,

  name TEXT NOT NULL,

  size BIGINT UNSIGNED NOT NULL,

  fcount INT NOT NULL,

  fnames LONGTEXT NOT NULL,

  FULLTEXT INDEX (name)

    COMMENT 'tokenizer "TokenBigramSplitSymbolAlphaDigit", flags "COMPRESS_ZSTD"',

  FULLTEXT INDEX (fnames)

    COMMENT 'tokenizer "TokenBigramSplitSymbolAlphaDigit", flags "COMPRESS_ZSTD"'

) ENGINE=Mroonga DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;

COMPRESS_ZSTD is a valid column flag, but it is not a valid index flag.

Mroonga detects the invalid flag and tries to report:

The index flag 'COMPRESS_ZSTD' is invalid. It is ignored

However, the CREATE TABLE statement never returns. In the reported case, mariadbd had to be killed forcefully after a normal restart did not complete.

After the forced termination, the SQL table was not visible as expected, but Groonga side objects had already been created. Re-running the same CREATE TABLE then failed because the Groonga database object already existed.

A gdb backtrace from the stuck server shows the thread repeatedly going through push_warning_printf() from mrn_parse_grn_index_column_flags(), called while creating the Mroonga indexes.

The issue seems to be in the invalid index flag handling. The DDL gets stuck and a forced kill can leave Groonga-side metadata behind.