Control termshark's cache of pcap files on disk

When termshark is run in live capture mode, it saves the pcaps it
creates under $XDG_CACHE_HOME/termshark/pcaps/. Termshark doesn't yet
cap the growth of that directory. Ross reported his cache dir was 40GB!
This change adds a new config option to put a limit on this directory e.g.

[main]
  disk-cache-size-mb = 300

Termshark will check the disk cache once per session, after the first 5s
of inactivity.

Omit this config setting, or set the value to -1, to preserve the
current behavior and let the directory grow. The default is -1 because I
don't want to suddenly delete pcaps that existing users might expect to
remain there based on termshark's current behavior.

cmd/termshark/termshark.go

@@ -877,12 +877,17 @@ func cmain() int {
 	inactiveDuration := 30 * time.Second
 	inactivityTimer := time.NewTimer(inactiveDuration)
 
+	checkedPcapCache := false
+	checkPcapCacheDuration := 5 * time.Second
+	checkPcapCacheTimer := time.NewTimer(checkPcapCacheDuration)
+
 Loop:
 	for {
 		var finChan <-chan time.Time
 		var opsChan <-chan pcap.RunFn
 		var tickChan <-chan time.Time
 		var inactivityChan <-chan time.Time
+		var checkPcapCacheChan <-chan time.Time
 		var emptyStructViewChan <-chan time.Time
 		var emptyHexViewChan <-chan time.Time
 		var psmlFinChan <-chan struct{}
@@ -957,6 +962,10 @@ Loop:
 			inactivityChan = inactivityTimer.C
 		}
 
+		if !checkedPcapCache {
+			checkPcapCacheChan = checkPcapCacheTimer.C
+		}
+
 		// (User) operations are enabled by default (the test predicate is nil), or if the predicate returns true
 		// meaning the operation has reached its desired state. Only one operation can be in progress at a time.
 		if ui.PcapScheduler.IsEnabled() {
@@ -986,6 +995,9 @@ Loop:
 
 		select {
 
+		case <-checkPcapCacheChan:
+			termshark.PrunePcapCache()
+
 		case <-inactivityChan:
 			ui.Fin.Activate()
 			app.Redraw()
@@ -1191,6 +1203,7 @@ Loop:
 		case ev := <-tcellEvents:
 			app.HandleTCellEvent(ev, gowid.IgnoreUnhandledInput)
 			inactivityTimer.Reset(inactiveDuration)
+			checkPcapCacheTimer.Reset(checkPcapCacheDuration)
 
 		case ev, ok := <-afterRenderEvents:
 			// This means app.Quit() has been called, which closes the AfterRenderEvents

go.sum

@@ -21,8 +21,6 @@ github.com/fsnotify/fsnotify v1.4.7 h1:IXs+QLmnXW2CcXuY+8Mzv/fWEsPGWxqefPtCP5CnV
 github.com/fsnotify/fsnotify v1.4.7/go.mod h1:jwhsz4b93w/PPRr/qN1Yymfu8t87LnFCMoQvtojpjFo=
 github.com/gcla/deep v1.0.2 h1:qBOx6eepcOSRYnHJ+f2ih4hP4Vca1YnLtXxp73n5KWI=
 github.com/gcla/deep v1.0.2/go.mod h1:evE9pbpSGhItmFoBIk8hPOIC/keKTGYhFl6Le1Av+GE=
-github.com/gcla/gowid v1.1.1-0.20200129034750-719170c39287 h1:moW/sE8xooyhWU6GTO1XO9eiHESv+y15tarIM5eNDpI=
-github.com/gcla/gowid v1.1.1-0.20200129034750-719170c39287/go.mod h1:kwHYNePmuaNa60IAkHfd/OfPeZuoSuz7ww+CeA5q/aQ=
 github.com/gcla/gowid v1.1.1-0.20200202174024-45eed270dfd5 h1:e/Jk8oCfaWdXtXHcUC6XbxF26F+3Fqy36+LWfv8NZlI=
 github.com/gcla/gowid v1.1.1-0.20200202174024-45eed270dfd5/go.mod h1:kwHYNePmuaNa60IAkHfd/OfPeZuoSuz7ww+CeA5q/aQ=
 github.com/gcla/tail v1.0.1-0.20190505190527-650e90873359 h1:3xEhacR7pIJV8daurdBygptxhzTJeYFqJp1V6SDl+pE=
@@ -31,7 +29,6 @@ github.com/gcla/tcell v1.1.2-0.20200115035344-b90e69b9dbe0 h1:6fMu73gAbCSAYAGFhM
 github.com/gcla/tcell v1.1.2-0.20200115035344-b90e69b9dbe0/go.mod h1:vxEiSDZdW3L+Uhjii9c3375IlDmR05bzxY404ZVSMo0=
 github.com/gcla/term v0.0.0-20191015020247-31cba2f9f402 h1:d8cpYNgYjXDjvrdnSaFy4+o1D3BpPUKTBIcijrBKv4c=
 github.com/gcla/term v0.0.0-20191015020247-31cba2f9f402/go.mod h1:YCPU+G35BFc/575HWMl8oeqq+dTbunufWbWaV0Y2sqY=
-github.com/gcla/termshark v1.0.0 h1:3jDyqYHeGIfN2khlAfWmJzoJJTh6Iau2mz81nakLBPk=
 github.com/gdamore/encoding v1.0.0 h1:+7OoQ1Bc6eTm5niUzBa0Ctsh6JbMW6Ra+YNuAtDBdko=
 github.com/gdamore/encoding v1.0.0/go.mod h1:alR0ol34c49FCSBLjhosxzcPHQbf2trDkoo5dl+VrEg=
 github.com/go-test/deep v1.0.1 h1:UQhStjbkDClarlmv0am7OXXO4/GaPdCGiUiMTvi28sg=

utils.go

@@ -22,6 +22,7 @@ import (
 	"path/filepath"
 	"regexp"
 	"runtime"
+	"sort"
 	"strconv"
 	"strings"
 	"sync"
@@ -637,6 +638,69 @@ func SaveOffsetToConfig(name string, offsets2 []resizable.Offset) {
 
 //======================================================================
 
+func PrunePcapCache() error {
+	// This is a new option. Best to err on the side of caution and, if not, present
+	// assume the cache can grow indefinitely - in case users are now relying on this
+	// to keep old pcaps around. I don't want to delete any files without the user's
+	// explicit permission.
+	var diskCacheSize int64 = int64(ConfInt("main.disk-cache-size-mb", -1))
+
+	if diskCacheSize == -1 {
+		log.Infof("No pcap disk cache size set. Skipping cache pruning.")
+		return nil
+	}
+
+	// Let user use MB as the most sensible unit of disk size. Convert to
+	// bytes for comparing to file sizes.
+	diskCacheSize = diskCacheSize * 1024 * 1024
+
+	log.Infof("Pruning termshark's pcap disk cache at %s...", PcapDir())
+
+	var totalSize int64
+	var fileInfos []os.FileInfo
+	err := filepath.Walk(PcapDir(),
+		func(path string, info os.FileInfo, err error) error {
+			if err == nil {
+				totalSize += info.Size()
+				fileInfos = append(fileInfos, info)
+			}
+			return nil
+		},
+	)
+	if err != nil {
+		return err
+	}
+
+	sort.Slice(fileInfos, func(i, j int) bool {
+		return fileInfos[i].ModTime().Before(fileInfos[j].ModTime())
+	})
+
+	filesRemoved := 0
+	curCacheSize := totalSize
+	for len(fileInfos) > 0 && curCacheSize > diskCacheSize {
+		err = os.Remove(filepath.Join(PcapDir(), fileInfos[0].Name()))
+		if err != nil {
+			log.Warnf("Could not remove pcap cache file %s while pruning - %v", fileInfos[0].Name(), err)
+		} else {
+			curCacheSize = curCacheSize - fileInfos[0].Size()
+			filesRemoved++
+		}
+		fileInfos = fileInfos[1:]
+	}
+
+	if filesRemoved > 0 {
+		log.Infof("Pruning complete. Removed %d old pcaps. Cache size is now %d MB",
+			filesRemoved, curCacheSize/(1024*1024))
+	} else {
+		log.Infof("Pruning complete. No old pcaps removed. Cache size is %d MB",
+			curCacheSize/(1024*1024))
+	}
+
+	return nil
+}
+
+//======================================================================
+
 var cpuProfileRunning *abool.AtomicBool
 
 func init() {