Fix query cache dbver issue with concurrent DDL

[fantix]

During a query compilation, if the schema is changed, the server was caching the compiled query under a wrong dbver, causing future queries to fail. See the test for issue reproduction.

@msullivan, I think you mentioned adding a similar compiler sleep hook for testing, but I can't find it. Please let me know if there's a better way than the sleep sentinel I added here in the query string.

[msullivan]

@msullivan, I think you mentioned adding a similar compiler sleep hook for testing, but I can't find it. Please let me know if there's a better way than the sleep sentinel I added here in the query string.

I don't know if there is really any good way to do such things. (I did a different bad way, and then didn't merge it.)

[msullivan]

Maybe a better (I guess?) way would be to add some testmode function, and then have a compiler function hook using simple_special_case that sleeps. Downside there though is that it is tough to pass in the sleep amount, since constants will be extracted. (You could have it wait for the the length of an array, or something silly like that)

[msullivan]

This makes sense to me.

It's probably worth thinking more about the race conditions against DDL, though, that can cause transient ISEs like this. That's longstanding, though, I think, so probably doesn't need to be fixed right now