Function readExtent

fdbserver/VersionedBTree.actor.cpp:3328–3380

	}

	Future<Reference<ArenaPage>> readExtent(LogicalPageID pageID) override {
		debug_printf("DWALPager(%s) op=readExtent %s\n", filename.c_str(), toString(pageID).c_str());
		PageCacheEntry* pCacheEntry = extentCache.getIfExists(pageID);
		auto& eventReasons = g_redwoodMetrics.level(nonBtreeLevel).metrics.events;
		if (pCacheEntry != nullptr) {
			eventReasons.addEventReason(PagerEvents::CacheLookup, PagerEventReasons::MetaData);
			debug_printf("DWALPager(%s) Cache Entry exists for %s\n", filename.c_str(), toString(pageID).c_str());
			return pCacheEntry->readFuture;
		}
		eventReasons.addEventReason(PagerEvents::CacheLookup, PagerEventReasons::MetaData);

		LogicalPageID headPageID = header.remapQueue.headPageID;
		LogicalPageID tailPageID = header.remapQueue.tailPageID;
		int readSize = physicalExtentSize;
		bool headExt = false;
		bool tailExt = false;
		debug_printf("DWALPager(%s) #extentPages: %d, headPageID: %s, tailPageID: %s\n",
		             filename.c_str(),
		             pagesPerExtent,
		             toString(headPageID).c_str(),
		             toString(tailPageID).c_str());

		if (headPageID >= pageID && ((headPageID - pageID) < pagesPerExtent))
			headExt = true;
		if ((tailPageID - pageID) < pagesPerExtent)
			tailExt = true;
		if (headExt && tailExt) {
			readSize = (tailPageID - headPageID + 1) * physicalPageSize;
		} else if (headExt) {
			readSize = (pagesPerExtent - (headPageID - pageID)) * physicalPageSize;
		} else if (tailExt) {
			readSize = (tailPageID - pageID + 1) * physicalPageSize;
		}

		PageCacheEntry& cacheEntry = extentCache.get(pageID, 1);
		if (!cacheEntry.initialized()) {
			cacheEntry.writeFuture = Void();
			cacheEntry.readFuture =
			    forwardError(readPhysicalExtent(this, (PhysicalPageID)pageID, readSize), errorPromise);
			debug_printf("DWALPager(%s) Set the cacheEntry readFuture for page: %s\n",
			             filename.c_str(),
			             toString(pageID).c_str());

			++g_redwoodMetrics.metric.pagerCacheMiss;
			eventReasons.addEventReason(PagerEvents::CacheMiss, PagerEventReasons::MetaData);
			eventReasons.addEventReason(PagerEvents::CacheLookup, PagerEventReasons::MetaData);
		} else {
			++g_redwoodMetrics.metric.pagerCacheHit;
			eventReasons.addEventReason(PagerEvents::CacheHit, PagerEventReasons::MetaData);
			eventReasons.addEventReason(PagerEvents::CacheLookup, PagerEventReasons::MetaData);
		}
		return cacheEntry.readFuture;
	}

	// Get snapshot as of the most recent committed version of the pager
	Reference<IPagerSnapshot> getReadSnapshot(Version v) override;