An output for exact cache hit

I think it would be useful to indicate somehow whether cache was restored using the exact key or one of the fallback restore-keys.

A use case for this is being able to skip some subsequent steps only in the case of an exact cache hit. For example, if a build tool uses incremental compilation, a partial cache hit is good, it saves some time, but if the cache hit was exact, we can skip the compilation step altogether which might save us much more time (e.g. build tool loading the project). A concrete case of such build tool it sbt: in a medium-sized project with several modules it might spend up to a minute loading the project.

So I’m suggesting to add another output value to the action which will indicate whether hit was exact or not.

[jsoref]

cache-hit indeed is only true if you’re hitting the primary key.

Unfortunately, you can’t distinguish between a cache-miss and a hit of a restore-key.

I would love for cache-hit to tell me the key it hit instead of just a boolean for the primary key.

[OndrejSpanel]

After reading the documentation again I think cache-hit is set to true only on the exact hit, not on the restore key hit. You are therefore not notified about a partial hit at all - it might speed up something or it might not, you are expected to proceed as if there is no hit.

A boolean value to indicate an exact match was found for the key

This is confirmed by looking at sources:

            const isExactKeyMatch = utils.isExactKeyMatch(primaryKey, cacheKey);
            utils.setCacheHitOutput(isExactKeyMatch);

The documentation could perhaps be changed to explicitly state this, like:

A boolean value to indicate an exact match was found for the key (not set when only a restore key matches)