cmd/dcrdata: add cache headers to improve page load on select pages (#2030)

* cmd/dcrdata: add cache headers to improve page load on select pages

This PR adds the ETag, Last-Modified and Cache-Control headers on select
explorer changes to improve user experience and save resources.

The ETag and Last-Modified are reset when *explorerUI receives a new block
or mempool data. In the future, they might be reset in other call sites where
update to backend cached data are made and API endpoints or pages depending on
cached data should then use the ETagAndLastModifiedIntercept middleware.

Signed-off-by: Philemon Ukane <ukanephilemon@gmail.com>

* format main.go

Signed-off-by: Philemon Ukane <ukanephilemon@gmail.com>

---------

Signed-off-by: Philemon Ukane <ukanephilemon@gmail.com>

Author: ukane-philemon

cmd/dcrdata/internal/explorer/explorer.go

@@ -8,8 +8,10 @@ package explorer
 
 import (
 	"context"
+	"crypto/rand"
 	"fmt"
 	"math"
+	"math/big"
 	"net/http"
 	"os"
 	"os/signal"
@@ -201,6 +203,8 @@ type pageData struct {
 	BlockInfo      *types.BlockInfo
 	BlockchainInfo *chainjson.GetBlockChainInfoResult
 	HomeInfo       *types.HomeInfo
+	eTag           string
+	lastModified   time.Time
 }
 
 type explorerUI struct {
@@ -359,6 +363,7 @@ func New(cfg *ExplorerConfig) *explorerUI {
 			},
 		},
 	}
+	exp.resetETagAndLastModified()
 
 	log.Infof("Mean Voting Blocks calculated: %d", exp.pageData.HomeInfo.Params.MeanVotingBlocks)
 
@@ -447,6 +452,9 @@ func (exp *explorerUI) StoreMPData(_ *mempool.StakeData, _ []types.MempoolTx, in
 	exp.invsMtx.Lock()
 	exp.invs = inv
 	exp.invsMtx.Unlock()
+
+	exp.resetETagAndLastModified()
+
 	log.Debugf("Updated mempool details for the explorerUI.")
 }
 
@@ -616,6 +624,8 @@ func (exp *explorerUI) Store(blockData *blockdata.BlockData, msgBlock *wire.MsgB
 		}()
 	}
 
+	exp.resetETagAndLastModified()
+
 	return nil
 }
 
@@ -629,6 +639,21 @@ func (exp *explorerUI) ChartsUpdated() {
 	exp.pageData.Unlock()
 }
 
+// resetETagAndLastModified resets the eTag and last modified time to new
+// values and is protected by the exp.pageData mutex.
+func (exp *explorerUI) resetETagAndLastModified() {
+	exp.pageData.Lock()
+	exp.pageData.eTag = generateRandomString()
+	exp.pageData.lastModified = time.Now()
+	exp.pageData.Unlock()
+}
+
+func (exp *explorerUI) eTagAndLastModified() (eTag string, lastModified time.Time) {
+	exp.pageData.RLock()
+	defer exp.pageData.RUnlock()
+	return exp.pageData.eTag, exp.pageData.lastModified
+}
+
 func (exp *explorerUI) updateDevFundBalance() {
 	// yield processor to other goroutines
 	runtime.Gosched()
@@ -879,3 +904,16 @@ func indexPrice(index exchanges.CurrencyPair, indices map[string]map[exchanges.C
 	}
 	return price / nSources
 }
+
+const letters = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"
+const nLetters = len(letters)
+
+// generateRandomString creates a random alphanumeric string of length 16.
+func generateRandomString() string {
+	bytes := make([]byte, 16)
+	for i := range bytes {
+		num, _ := rand.Int(rand.Reader, big.NewInt(int64(nLetters)))
+		bytes[i] = letters[num.Int64()]
+	}
+	return string(bytes)
+}

cmd/dcrdata/internal/explorer/explorermiddleware.go

@@ -189,6 +189,33 @@ func (exp *explorerUI) SyncStatusFileIntercept(next http.Handler) http.Handler {
 	})
 }
 
+// ETagAndLastModifiedIntercept handles ETag and Last-Modified headers for caching purposes.
+func (exp *explorerUI) ETagAndLastModifiedIntercept(next http.Handler) http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		eTag, lastModified := exp.eTagAndLastModified()
+		if match := r.Header.Get("If-None-Match"); match != "" {
+			if match == eTag {
+				w.WriteHeader(http.StatusNotModified)
+				return
+			}
+		} else if modifiedSince := r.Header.Get("If-Modified-Since"); modifiedSince != "" {
+			if t, err := time.Parse(http.TimeFormat, modifiedSince); err == nil {
+				if lastModified.Before(t.Add(1 * time.Second)) {
+					w.WriteHeader(http.StatusNotModified)
+					return
+				}
+			}
+		}
+
+		// Set ETag and Last-Modified headers.
+		w.Header().Set("ETag", eTag)
+		w.Header().Set("Last-Modified", lastModified.Format(http.TimeFormat))
+		w.Header().Set("Cache-Control", "private")
+
+		next.ServeHTTP(w, r)
+	})
+}
+
 func getBlockHashCtx(r *http.Request) string {
 	hash, ok := r.Context().Value(ctxBlockHash).(string)
 	if !ok {

cmd/dcrdata/main.go

@@ -761,23 +761,15 @@ func _main(ctx context.Context) error {
 		r.Get("/rejects", func(w http.ResponseWriter, r *http.Request) {
 			http.Redirect(w, r, "/disapproved", http.StatusPermanentRedirect)
 		})
-		r.Get("/disapproved", explore.DisapprovedBlocks)
-		r.Get("/mempool", explore.Mempool)
-		r.Get("/parameters", explore.ParametersPage)
 		r.With(explore.BlockHashPathOrIndexCtx).Get("/block/{blockhash}", explore.Block)
 		r.With(explorer.TransactionHashCtx).Get("/tx/{txid}", explore.TxPage)
 		r.With(explorer.TransactionHashCtx, explorer.TransactionIoIndexCtx).Get("/tx/{txid}/{inout}/{inoutid}", explore.TxPage)
 		r.With(explorer.AddressPathCtx).Get("/address/{address}", explore.AddressPage)
 		r.With(explorer.AddressPathCtx).Get("/addresstable/{address}", explore.AddressTable)
-		r.Get("/treasury", explore.TreasuryPage)
-		r.Get("/treasurytable", explore.TreasuryTable)
-		r.Get("/agendas", explore.AgendasPage)
-		r.With(explorer.AgendaPathCtx).Get("/agenda/{agendaid}", explore.AgendaPage)
 		r.Get("/proposals", explore.ProposalsPage)
 		r.With(explorer.ProposalPathCtx).Get("/proposal/{proposaltoken}", explore.ProposalPage)
 		r.Get("/decodetx", explore.DecodeTxPage)
 		r.Get("/search", explore.Search)
-		r.Get("/charts", explore.Charts)
 		r.Get("/ticketpool", explore.Ticketpool)
 		r.Get("/market", explore.MarketPage)
 		r.Get("/stats", func(w http.ResponseWriter, r *http.Request) {
@@ -786,9 +778,23 @@ func _main(ctx context.Context) error {
 		// MenuFormParser will typically redirect, but going to the homepage as a
 		// fallback.
 		r.With(explorer.MenuFormParser).Post("/set", explore.Home)
-		r.Get("/attack-cost", explore.AttackCost)
 		r.Get("/verify-message", explore.VerifyMessagePage)
 		r.With(mw.Tollbooth(limiter)).Post("/verify-message", explore.VerifyMessageHandler)
+
+		// Pages that can be cached because they depend on block and/or mempool data cached by
+		// *explorer.explorerUI. This middleware sets ETag and Last-Modified headers that are
+		// reset if a new block or mempool change is detected.
+		withCache := r.With(explore.ETagAndLastModifiedIntercept)
+		withCache.Get("/", explore.Home)
+		withCache.Get("/disapproved", explore.DisapprovedBlocks)
+		withCache.Get("/mempool", explore.Mempool)
+		withCache.Get("/charts", explore.Charts)
+		withCache.Get("/treasury", explore.TreasuryPage)
+		withCache.Get("/treasurytable", explore.TreasuryTable)
+		withCache.Get("/parameters", explore.ParametersPage)
+		withCache.Get("/agendas", explore.AgendasPage)
+		withCache.With(explorer.AgendaPathCtx).Get("/agenda/{agendaid}", explore.AgendaPage)
+		withCache.Get("/attack-cost", explore.AttackCost)
 	})
 
 	// Configure a page for the bare "/insight" path. This mounts the static