TSHttpTxnCacheLookupStatusGet: handle cannot respond cases

[bneradt]

ATS initially experiences cache lookups as a HIT for resources that we
ultimately cannot use due to a need for authentication or if the methods
for the incoming request and that of the request for the cached response
don't match. Our TSHttpTxnCacheLookupStatusGet plugin interface returned
the initial HIT status instead of marking these as MISSES. This patch
changes the status to return a MISS in these circumstances.

Closes #8539

[bneradt]

@SolidWallOfCode volunteered to take a look at this.

[SolidWallOfCode]

Why have a plugin for checking the cache status rather than a custom log?

[bneradt]

Why have a plugin for checking the cache status rather than a custom log?

Thank you for taking a look at this PR.

The issue this addresses (see the link) is for the compress plugin. Namely, this code block:

trafficserver/plugins/compress/compress.cc

Lines 884 to 891 in d0a44f8

	case TS_EVENT_HTTP_CACHE_LOOKUP_COMPLETE: {
	int obj_status;
	if (TS_ERROR != TSHttpTxnCacheLookupStatusGet(txnp, &obj_status) && (TS_CACHE_LOOKUP_HIT_FRESH == obj_status)) {
	if (hc != nullptr) {
	info("handling compression of cached object");
	if (transformable(txnp, false, hc, &compress_type, &algorithms)) {
	compress_transform_add(txnp, hc, compress_type, algorithms);

The plugin is not using the status for logging. Rather, the plugin grabs the cache status via TSHttpTxnCacheLookupStatusGet and conditionally performs logic if ATS is serving the object out of cache. Without this patch, the TS_CACHE_LOOKUP_HIT_FRESH is provided to the plugin even when the object is deemed to be something it cannot in fact reply to the client with (such as the request is a POST method but the cached object is a response to a GET request). In this case, it's a bug to call such cached objects a hit for these requests and the plugin is negatively impacted by this. This patch corrects the status in such cases to a miss.

[zwoop]

Cherry-picked to v9.2.x

[bneradt]

Adding the 9.2.x project as this will be reverted in 9.2.x when this is cherry-picked back:
#8637