Goal

the goal is to remove all these INSTR/REPLACE/SUBSTR/etc that replication tests are using to figure out the topology. The ultimate goal is: mysqltest should only send those commands to the server that constitute a test case, not anything it needs internally for itself.

Options

bash-like substitutions

• ${var}
• ${parameter:offset:length}
• ${#parameter}
• ${parameter/pattern/string/flags} — same as in replace_regex
• perhaps ${parameterˆ}, ${parameterˆˆ}, ${parameter,}, ${parameter,,}
• or ${parameter@function}, like ${a@U}, ${a@qs} or even ${a@uppercase}, ${a@shellquote}
• $( expression ) see MDEV-36107

also, repetition

• ${parameter*N}

and recursive expansion:

• ${${var}}

The problem — it's difficult to extend. There is a syntax for SUBSTR(), but if we'd like to add INSTR() we'd need to invent another combination of punctuation characters inside curly braces. E.g. ${parameter%substring} It's difficult to remember and there are only so many punctuation characters.

functions — this is currently the preferred approach

$( instr($a, "foo") ) or, more complex, $( substr($a, 1, instr($a, "/") )

this is more extendable that bash syntax. Function names should be the same as in SQL to not force users to learn a new language

cmake-like generator expressions

$<instr:$a:foo>, or, more complex, $<substr:$a:1:$<instr:$a:/>>

also more extendable than bash. Naturally string substitutions. Looks very weird.