[llvm] [symbolizer] Empty string is not an error (PR #92660)

[James Henderson via llvm-commits]

jh7370 wrote:

> If we were going to do something else, `ECHO\n` returning `ECHO\n` would work for me. It would also be backwards compatible with older versions of llvm-symbolizer.

Let's go with this then if there aren't any objections? To be clear, I'm happy with either a line starting with `ECHO` being completely echoed back to the client, or the `ECHO` being stripped off and the rest being echoed back. Whichever approach we go with, the feature should be documented in the llvm-symbolizer docs, which should include a use-case or two explaining why you might use this (e.g. to establish that llvm-symbolizer is ready, or to insert comments into the output stream).

Regarding printing to stderr, I can see why that might be a problem under some usage. Probably the simplest solution, if there's a need for it, is to add a new switch that does the "print stderr to stdout" aspect. That should be another PR, if there's a request for it though.

https://github.com/llvm/llvm-project/pull/92660