Lighttpd

In this config:

$HTTP["host"] =~ "example\.com" {
  alias.url += (
    "/1" => "/tmp/1",
  )
}

alias.url += (
  "/2" => "/tmp/2",
)

Of these, only the first mapping will actually work. That's because the parser transforms this into:

    alias.url                      = (
        "/2" => "/tmp/2",
    )

    $SERVER["socket"] == "[::]:80" {
        # block 1

    } # end of $SERVER["socket"] == "[::]:80" 

    $HTTP["host"] =~ "example\.com" {
        # block 2
        alias.url = (
            "/1" => "/tmp/1",
        )

    } # end of $HTTP["host"] =~ "example\.com" 

So the "/2" mapping that was supposed to be unconditional, instead gets clobbered by the "/1", because it's moved to be the first, and then both alias.url directives are transformed from "+=" into "=".

Moving the "/2" in front of the conditional block avoids the issue.

However the expected behavior is that both mappings should work when the condition matches, and the order of these blocks should not matter. That it actually does, seems to be caused by some implementation detail, and leads to a nasty "gotcha", where just about the simplest and most clean-cut example of trying to using conditions and concatenations blows up right into your (my) face.

If such behavior is for some reason unfeasible to implement, then at the very least, some sort of a log warning should be printed, e.g. that some of the unconditional values actually get overwritten by a condition.