webseed improvements

internal/piecepicker/piecepicker.go

@@ -64,16 +64,12 @@ func (p *myPiece) StalledDownloads() int {
  return p.Snubbed.Len() + p.Choked.Len()
 }
 
-// AvailableForWebseed returns true if the piece can be downloaded from a webseed source.
-// If the piece is already requested from a peer, it does not become eligible for downloading from webseed until entering the endgame mode.
-func (p *myPiece) AvailableForWebseed(duplicate bool) bool {
- if p.Done || p.Writing || p.RequestedWebseed != nil {
+// AvailableForWebseed returns true if the piece is allowed to be downloaded from a webseed source.
+func (p *myPiece) AvailableForWebseed() bool {
+ if p.Done || p.Writing {
  return false
  }
- if !duplicate {
- return p.RequestedWebseed != nil
- }
- return true
+ return p.RequestedWebseed == nil
 }
 
 // New returns a new PiecePicker.

internal/piecepicker/webseed.go

@@ -1,6 +1,7 @@
 package piecepicker
 
 import (
+ "math/rand"
  "sort"
 
  "github.com/cenkalti/rain/internal/peer"
@@ -16,21 +17,21 @@ type WebseedDownloadSpec struct {
 
 // PickWebseed returns the next spec for downloading files from webseed sources.
 func (p *PiecePicker) PickWebseed(src *webseedsource.WebseedSource) *WebseedDownloadSpec {
- begin, end := p.findPieceRangeForWebseed()
- if begin == end {
+ r := p.findPieceRangeForWebseed()
+ if r == nil {
  return nil
  }
  // Mark selected range as being downloaded so we won't select it again.
- for i := begin; i < end; i++ {
+ for i := r.Begin; i < r.End; i++ {
  if p.pieces[i].RequestedWebseed != nil {
  panic("already downloading from webseed url")
  }
  p.pieces[i].RequestedWebseed = src
  }
  return &WebseedDownloadSpec{
  Source: src,
- Begin: begin,
- End: end,
+ Begin: r.Begin,
+ End: r.End,
  }
 }
 
@@ -43,14 +44,24 @@ func (p *PiecePicker) downloadingWebseed() bool {
  return false
 }
 
-func (p *PiecePicker) findPieceRangeForWebseed() (begin, end uint32) {
+func (p *PiecePicker) findPieceRangeForWebseed() *Range {
  gaps := p.findGaps()
  if len(gaps) == 0 {
- gap := p.webseedStealsFromAnotherWebseed()
- return gap.Begin, gap.End
+ return p.webseedStealsFromAnotherWebseed()
  }
+ gap := selectRandomLargestGap(gaps)
+ return &gap
+}
+
+func selectRandomLargestGap(gaps []Range) Range {
  sort.Slice(gaps, func(i, j int) bool { return gaps[i].Len() > gaps[j].Len() })
- return gaps[0].Begin, gaps[0].End
+ length := gaps[0].Len()
+ for i := range gaps {
+ if gaps[i].Len() != length {
+ return gaps[rand.Intn(i)]
+ }
+ }
+ return gaps[rand.Intn(len(gaps))]
 }
 
 func (p *PiecePicker) getDownloadingSources() []*webseedsource.WebseedSource {
@@ -63,17 +74,22 @@ func (p *PiecePicker) getDownloadingSources() []*webseedsource.WebseedSource {
  return ret
 }
 
-func (p *PiecePicker) webseedStealsFromAnotherWebseed() (r Range) {
+func (p *PiecePicker) webseedStealsFromAnotherWebseed() *Range {
  downloading := p.getDownloadingSources()
  if len(downloading) == 0 {
- return
+ return nil
  }
  sort.Slice(downloading, func(i, j int) bool { return downloading[i].Remaining() > downloading[j].Remaining() })
  src := downloading[0]
- r.End = src.Downloader.End
- r.Begin = (src.Downloader.ReadCurrent() + src.Downloader.End + 1) / 2
+ r := &Range{
+ Begin: (src.Downloader.ReadCurrent() + src.Downloader.End + 1) / 2,
+ End: src.Downloader.End,
+ }
+ if r.Begin >= r.End {
+ return nil
+ }
  p.WebseedStopAt(src, r.Begin)
- return
+ return r
 }
 
 func (p *PiecePicker) peerStealsFromWebseed(pe *peer.Peer) *myPiece {
@@ -101,31 +117,23 @@ func (p *PiecePicker) peerStealsFromWebseed(pe *peer.Peer) *myPiece {
 }
 
 func (p *PiecePicker) findGaps() []Range {
- gaps := p.findGapsWithDuplicate(false)
- if len(gaps) == 0 {
- gaps = p.findGapsWithDuplicate(true)
- }
- return gaps
-}
-
-func (p *PiecePicker) findGapsWithDuplicate(duplicate bool) []Range {
  a := make([]Range, 0, len(p.pieces)/2)
  var inGap bool // See BEP19 for definition of "gap".
  var begin uint32
  for _, pi := range p.pieces {
  if !inGap {
- if pi.AvailableForWebseed(duplicate) {
+ if pi.AvailableForWebseed() {
  begin = pi.Index
  inGap = true
  }
  } else {
  r := Range{Begin: begin, End: pi.Index}
- if r.Len() == p.maxWebseedPieces {
- a = append(a, r)
- begin = pi.Index
- } else if !pi.AvailableForWebseed(duplicate) {
+ if !pi.AvailableForWebseed() {
  a = append(a, r)
  inGap = false
+ } else if r.Len() == p.maxWebseedPieces {
+ a = append(a, r)
+ begin = pi.Index
  }
  }
  }