Detect 404 before headers are sent

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I’ve a piece of code that shows a banner the first time a user visits, then sets a cookie, which prevents it showing again.

However, if the page has a 404 error, the 404 page of the requested resource sets the cookie and the banner is not shown. Checking is_404() on the init hook does not avoid this situation.

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So how do I correct this problem and detect the 404 on the init hook?

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  1. Use a later action hook

    You cannot use Conditional Tags before the posts_selection hook has run (as per the codex), which happens right after pre_get_posts and quite a bit after init.

    Hence, would it not be easier to set the cookie at a later stage of the request, rather than attempting to detect the 404 earlier (which, off the bat, I would not know how)?

    The wp action, for instance, should be early enough.
    If I understand you correctly, you should basically be able to set the cookie at the very end of the request, since it becomes relevant on the following request only anyway.

    [Further Info] How do Conditional Tags work?

    Conditional Tags depend on the query object’s (instance of WP_Query) public “query type” boolean properties, which by default are set to false (see wp-includes/query.php, Lines 1008 ff. [as of 3.5.1]).
    These properties are populated with the correct values in WP_Query‘s parse_query method, which runs right before pre_get_posts (see wp-includes/query.php, Lines 1907 ff. [as of 3.5.1]).

    Hence, even though the codex suggests differently, Conditional Tags should (not vouching for it, though) actually already be usable in pre_get_posts callbacks.
    Definitely not earlier though – init is out of the question.

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