Try to use path from MySQL "secure_file_priv" system variable for batch inserting via load infile

[tsteur]

fixes #9528

Background:
See #9419 (comment) and #9419 (comment)

From https://dev.mysql.com/doc/refman/5.7/en/server-system-variables.html#sysvar_secure_file_priv

If set to the name of a directory, the server limits import and export operations to work only with files in that directory. ...

... Before MySQL 5.7.6, this variable is empty by default. As of 5.7.6, the default value is platform specific ...

So since 5.7.6 eg the default value is /var/lib/mysql-files instead of being empty. This means load data ... only works when the file was placed within that directory. If it is not in that directory the following error might occur

SQLSTATE[HY000]: General error: 1290 The MySQL server is running with the --secure-file-priv option so it cannot execute this statement

However, this applies to LOAD DATA INFILE. The fallback LOAD DATA LOCAL INFILE still seems to work even if we put the file not into the specified directory. Background for this: When using batch insert, by default we try to use the more secure way LOAD DATA INFILE and if this fails for some reason we try to use LOAD DATA LOCAL INFILE if possible. Using the LOCAL keyword means the client reads the file and sends it to the server. This fallback way using LOCAL is only used when there are no restrictions re open_basedir and safemode.

This means by default it should still work even when we don't put the CSV file into the secure_file_priv directory unless there were restrictions made to the mentioned methods in which it would always fail.

This patch introduces a change to trying to use the directory as specified in the secure_file_priv variable. If that directory actually exists, eg /var/lib/mysql-files, and if we are allowed to write into this directory, we will put the CSV file into the specified directory and read it from there. If the setting is not specified, or if we are not allowed to write into this directory, which should be the case by default, we will use the regular tmp/assets directory as it might still work fine via the fallback way with LOCAL.

Also to be considered is that secure_file_priv was introduced in MySQL 5.0.38 so for all previous versions the tmp/assets directory will be fine anyway.

[mattab]

maybe you should also check is_readable as a folder can be writable yet not readable?

[tsteur]

but only MySQL would need to be able to read from it or not?

[oraclerob]

This description looks good.

[nisdis]

According to the piwik docs
https://piwik.org/faq/troubleshooting/#faq_194

This is what worked for me:

update to the latest PHP version, or use the mysqli client (there are some known bugs with older PDO clients for mysql)
and/or switch the client to adapter=MYSQLI in the config/config.ini.php