Revert "Cache file descriptors" (fixes #1096)

This reverts commit 992ad97ad5.

Causes issues on Windows which uses file locking.
Meaning we cannot archive or modify the file while it's open.

internal/model/puller.go

@@ -26,6 +26,8 @@ import (
 	"sync"
 	"time"
 
+	"github.com/AudriusButkevicius/lfu-go"
+
 	"github.com/syncthing/syncthing/internal/config"
 	"github.com/syncthing/syncthing/internal/events"
 	"github.com/syncthing/syncthing/internal/osutil"
@@ -600,6 +602,19 @@ nextFile:
 			p.progressEmitter.Register(state.sharedPullerState)
 		}
 
+		evictionChan := make(chan lfu.Eviction)
+
+		fdCache := lfu.New()
+		fdCache.UpperBound = 50
+		fdCache.LowerBound = 20
+		fdCache.EvictionChannel = evictionChan
+
+		go func() {
+			for item := range evictionChan {
+				item.Value.(*os.File).Close()
+			}
+		}()
+
 		folderRoots := make(map[string]string)
 		p.model.fmut.RLock()
 		for folder, cfg := range p.model.folderCfgs {
@@ -613,11 +628,18 @@ nextFile:
 			found := p.model.finder.Iterate(block.Hash, func(folder, file string, index uint32) bool {
 				path := filepath.Join(folderRoots[folder], file)
 
-				fd, err := p.model.cache.Open(path)
-				if err != nil {
-					return false
+				var fd *os.File
+
+				fdi := fdCache.Get(path)
+				if fdi != nil {
+					fd = fdi.(*os.File)
+				} else {
+					fd, err = os.Open(path)
+					if err != nil {
+						return false
+					}
+					fdCache.Set(path, fd)
 				}
-				defer fd.Close()
 
 				_, err = fd.ReadAt(buf, protocol.BlockSize*int64(index))
 				if err != nil {
@@ -666,6 +688,8 @@ nextFile:
 				state.copyDone()
 			}
 		}
+		fdCache.Evict(fdCache.Len())
+		close(evictionChan)
 		out <- state.sharedPullerState
 	}
 }