InnoDB silently writes uncompressed data on compression error or if compressed length = 0

While working on MDEV-22895, I noticed that if the server is started without a compression library (say, lzma), and innodb_compression_algorithm = lzma, InnoDB can still create, read and write tables with page_compressed=1.

On further investigation, I found that InnoDB will silently write uncompressed data if the compression fails (returns non-zero status) or if the compressed length is 0.

storage/innobase/fil/fil0pagecompress.cc#L211-L220

	ulint write_size = fil_page_compress_low(

		buf, out_buf, header_len,

		fil_space_t::get_compression_algo(flags),

		static_cast<unsigned>(comp_level));

	if (write_size == 0) {

fail:

		srv_stats.pages_page_compression_error.inc();

		return 0;

	}

To reproduce:

• On branch feature-compression-service-lzma
• Start the server with --use-compression=
• Run:

  SET GLOBAL innodb_compression_algorithm = "lzma";

  CREATE TABLE t (a int, b char(20))

  engine = innodb

  page_compressed = 1;

  INSERT INTO t (a, b) VALUES (0, "abc");

  INSERT INTO t (a, b) VALUES (1, "def");

  INSERT INTO t (a, b) VALUES (2, "ghi");

  SELECT * FROM t;

• The is no error.
• Restart the server with --use-compression=lzma.
• Run:

  SELECT * FROM t;

• All rows are visible
• Again, there is no error
• Run:

  INSERT INTO t (a, b) VALUES (3, "jkl");

• Restart the server with --use-compression=
• Table is corrupted.