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

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