Merge: -(dnsforward): fixing an issue with the querylog speed

Squashed commit of the following:

commit 7db062d2ce40369fc6c7eaa89f678c660d531516
Author: Andrey Meshkov <am@adguard.com>
Date:   Thu Feb 27 12:08:58 2020 +0300

    *(dnsforward): added comment about oldest

commit dbec84d8a8c60ca040faa59f8aa818ed5959e92a
Author: Simon Zolin <s.zolin@adguard.com>
Date:   Thu Feb 27 11:16:58 2020 +0300

    fix

commit d6df1ee28b3918e8d1676ff6ab4b516d46fe4202
Author: Andrey Meshkov <am@adguard.com>
Date:   Wed Feb 26 20:42:11 2020 +0300

    -(dnsforward): fixing an issue with the querylog speed

querylog/qlog.go

@@ -231,6 +231,13 @@ func (l *queryLog) getData(params getDataParams) map[string]interface{} {
 		// remove extra records
 		entries = entries[(len(entries) - getDataLimit):]
 	}
+	if len(entries) == getDataLimit {
+		// change the "oldest" value here.
+		// we cannot use the "oldest" we got from "searchFiles" anymore
+		// because after adding in-memory records and removing extra records
+		// the situation has changed
+		oldest = entries[len(entries)-1].Time
+	}
 
 	// init the response object
 	var data = []map[string]interface{}{}
@@ -246,9 +253,6 @@ func (l *queryLog) getData(params getDataParams) map[string]interface{} {
 		len(entries), total, params.OlderThan, time.Since(now))
 
 	var result = map[string]interface{}{}
-	if len(entries) == getDataLimit {
-		oldest = entries[0].Time
-	}
 	result["oldest"] = ""
 	if !oldest.IsZero() {
 		result["oldest"] = oldest.Format(time.RFC3339Nano)

querylog/querylog_search.go

@@ -36,10 +36,16 @@ func (l *queryLog) searchFiles(params getDataParams) ([]*logEntry, time.Time, in
 		err = r.SeekStart()
 	} else {
 		err = r.Seek(params.OlderThan.UnixNano())
+		if err == nil {
+			// Read to the next record right away
+			// The one that was specified in the "oldest" param is not needed,
+			// we need only the one next to it
+			_, err = r.ReadNext()
+		}
 	}
 
 	if err != nil {
-		log.Error("Failed to Seek(): %v", err)
+		log.Debug("Cannot Seek() to %v: %v", params.OlderThan, err)
 		return entries, oldest, 0
 	}
 
@@ -54,12 +60,16 @@ func (l *queryLog) searchFiles(params getDataParams) ([]*logEntry, time.Time, in
 			break
 		}
 
-		if entry != nil {
-			entries = append(entries, entry)
-		}
-
 		oldestNano = ts
 		total++
+
+		if entry != nil {
+			entries = append(entries, entry)
+			if len(entries) == getDataLimit {
+				// Do not read more than "getDataLimit" records at once
+				break
+			}
+		}
 	}
 
 	oldest = time.Unix(0, oldestNano)