Add a summary of stderr to the dialog displayed when tshark fails

If a tshark process - run by termshark - returns a non-zero exit code,
termshark will send that code and an error string to a UI dialog. But
often this is not enough to help you diagnose the problem, and you have
to dig through termshark's log file for more info. This change adds a
summary of stderr to the error dialog displayed, specifically the first
two and last two lines read from the stderr of the failed tshark
process.

CHANGELOG.md

@@ -12,6 +12,8 @@
 
 ### Changed
 
+- Added a summary of standard error to the error dialogs displayed when a tshark process fails to run
+  correctly (has a non-zero exit code).
 - Fixed a race condition that caused extcap captures (e.g. randpkt) to sporadically fail.
 - Dark-mode is now the default in the absence of a specific user-setting.
 - Fixed a bug that caused mouse-clicks within the hex view to not function correctly if the viewport was not

capinfo/loader.go

@@ -122,11 +122,7 @@ func (c *Loader) loadCapinfoAsync(pcapf string, app gowid.IApp, cb ICapinfoCallb
 				state = pcap.Terminated
 				if !c.SuppressErrors && err != nil {
 					if _, ok := err.(*exec.ExitError); ok {
-						cerr := gowid.WithKVs(termshark.BadCommand, map[string]interface{}{
-							"command": c.capinfoCmd.String(),
-							"error":   err,
-						})
-						pcap.HandleError(pcap.CapinfoCode, app, cerr, cb)
+						pcap.HandleError(pcap.CapinfoCode, app, pcap.MakeUsefulError(c.capinfoCmd, err), cb)
 					}
 				}
 

cmd/termshark/termshark.go

@@ -25,6 +25,7 @@ import (
 	"github.com/gcla/termshark/v2/configs/profiles"
 	"github.com/gcla/termshark/v2/convs"
 	"github.com/gcla/termshark/v2/pcap"
+	"github.com/gcla/termshark/v2/pkg/summary"
 	"github.com/gcla/termshark/v2/shark"
 	"github.com/gcla/termshark/v2/streams"
 	"github.com/gcla/termshark/v2/system"
@@ -62,6 +63,7 @@ func main() {
 	convs.Goroutinewg = &ensureGoroutinesStopWG
 	ui.Goroutinewg = &ensureGoroutinesStopWG
 	wormhole.Goroutinewg = &ensureGoroutinesStopWG
+	summary.Goroutinewg = &ensureGoroutinesStopWG
 
 	res := cmain()
 	ensureGoroutinesStopWG.Wait()

convs/loader.go

@@ -134,11 +134,7 @@ func (c *Loader) loadConvAsync(pcapf string, convs []string, filter string, abs
 				state = pcap.Terminated
 				if !c.SuppressErrors && err != nil {
 					if _, ok := err.(*exec.ExitError); ok {
-						cerr := gowid.WithKVs(termshark.BadCommand, map[string]interface{}{
-							"command": c.convsCmd.String(),
-							"error":   err,
-						})
-						pcap.HandleError(pcap.ConvCode, app, cerr, cb)
+						pcap.HandleError(pcap.ConvCode, app, pcap.MakeUsefulError(c.convsCmd, err), cb)
 					}
 				}
 

pcap/cmds.go

@@ -14,6 +14,7 @@ import (
 
 	"github.com/gcla/termshark/v2"
 	"github.com/gcla/termshark/v2/configs/profiles"
+	"github.com/gcla/termshark/v2/pkg/summary"
 	"github.com/gcla/termshark/v2/shark"
 	"github.com/kballard/go-shellquote"
 )
@@ -35,6 +36,8 @@ func (e ProcessNotStarted) Error() string {
 type Command struct {
 	sync.Mutex
 	*exec.Cmd
+	summaryReader *summary.Reader
+	summaryWriter io.Closer
 }
 
 func (c *Command) String() string {
@@ -46,14 +49,21 @@ func (c *Command) String() string {
 func (c *Command) Start() error {
 	c.Lock()
 	defer c.Unlock()
-	c.Cmd.Stderr = termshark.ErrLogger("cmd", c.Path)
+	pr, pw := io.Pipe()
+	c.summaryWriter = pw
+	c.summaryReader = summary.New(pr)
+	c.Cmd.Stderr = io.MultiWriter(pw, termshark.ErrLogger("cmd", c.Path))
 	c.PutInNewGroupOnUnix()
 	res := c.Cmd.Start()
 	return res
 }
 
 func (c *Command) Wait() error {
-	return c.Cmd.Wait()
+	err := c.Cmd.Wait()
+	c.Lock()
+	c.summaryWriter.Close()
+	c.Unlock()
+	return err
 }
 
 func (c *Command) StdoutReader() (io.ReadCloser, error) {
@@ -62,6 +72,13 @@ func (c *Command) StdoutReader() (io.ReadCloser, error) {
 	return c.Cmd.StdoutPipe()
 }
 
+func (c *Command) StderrSummary() []string {
+	c.Lock()
+	defer c.Unlock()
+
+	return c.summaryReader.Summary()
+}
+
 func (c *Command) SetStdout(w io.Writer) {
 	c.Lock()
 	defer c.Unlock()

pcap/loader.go

@@ -93,6 +93,15 @@ type IBasicCommand interface {
 	Wait() error
 	Pid() int
 	Kill() error
+	StderrSummary() []string
+}
+
+func MakeUsefulError(cmd IBasicCommand, err error) gowid.KeyValueError {
+	return gowid.WithKVs(termshark.BadCommand, map[string]interface{}{
+		"command": cmd.String(),
+		"error":   err,
+		"stderr":  strings.Join(cmd.StderrSummary(), "\n"),
+	})
 }
 
 type ITailCommand interface {
@@ -1548,11 +1557,7 @@ func (p *PsmlLoader) loadPsmlSync(iloader *InterfaceLoader, e iPsmlLoaderEnv, cb
 					if !p.psmlStoppedDeliberately_ {
 						if err != nil {
 							if _, ok := err.(*exec.ExitError); ok {
-								cerr := gowid.WithKVs(termshark.BadCommand, map[string]interface{}{
-									"command": psmlCmd.String(),
-									"error":   err,
-								})
-								HandleError(PsmlCode, app, cerr, cb)
+								HandleError(PsmlCode, app, MakeUsefulError(psmlCmd, err), cb)
 							}
 						}
 					}
@@ -1676,11 +1681,7 @@ func (p *PsmlLoader) loadPsmlSync(iloader *InterfaceLoader, e iPsmlLoaderEnv, cb
 						if !p.psmlStoppedDeliberately_ && !e.TailStoppedDeliberately() {
 							if err != nil {
 								if _, ok := err.(*exec.ExitError); ok {
-									cerr := gowid.WithKVs(termshark.BadCommand, map[string]interface{}{
-										"command": tailCmd.String(),
-										"error":   err,
-									})
-									HandleError(PsmlCode, app, cerr, cb)
+									HandleError(PsmlCode, app, MakeUsefulError(tailCmd, err), cb)
 								}
 							}
 						}
@@ -2144,11 +2145,7 @@ func (i *InterfaceLoader) loadIfacesSync(e iIfaceLoaderEnv, cb interface{}, app
 						// This could be if termshark is started like this: cat nosuchfile.pcap | termshark -i -
 						// Then dumpcap will be started with /dev/fd/3 as its stdin, but will fail with EOF and
 						// exit status 1.
-						cerr := gowid.WithKVs(termshark.BadCommand, map[string]interface{}{
-							"command": ifaceCmd.String(),
-							"error":   err,
-						})
-						HandleError(IfaceCode, app, cerr, cb)
+						HandleError(IfaceCode, app, MakeUsefulError(ifaceCmd, err), cb)
 					}
 				}
 

pkg/summary/summary.go

@@ -0,0 +1,95 @@
+// Copyright 2019-2022 Graham Clark. All rights reserved.  Use of this source
+// code is governed by the MIT license that can be found in the LICENSE
+// file.
+
+package summary
+
+import (
+	"bufio"
+	"io"
+	"sync"
+
+	"github.com/gcla/termshark/v2"
+)
+
+//======================================================================
+
+// Reader maintains the first two and last two lines read from a source io.Reader.
+// At any point, the Summary() function can be called to extract a summary of
+// what's been read so far. I'm using this to create a summary of the stderr of
+// a termshark command.
+type Reader struct {
+	source      io.Reader
+	first       *string
+	second      *string
+	penultimate *string
+	last        *string
+	num         int
+	lock        sync.Mutex
+}
+
+func New(source io.Reader) *Reader {
+	res := &Reader{
+		source: source,
+	}
+
+	termshark.TrackedGo(func() {
+		res.start()
+	}, Goroutinewg)
+
+	return res
+}
+
+func (h *Reader) Summary() []string {
+	h.lock.Lock()
+	defer h.lock.Unlock()
+
+	res := make([]string, 0, 5)
+	if h.num >= 1 {
+		res = append(res, *h.first)
+	}
+	if h.num >= 2 {
+		res = append(res, *h.second)
+	}
+	if h.num >= 5 {
+		res = append(res, "...")
+	}
+	if h.num >= 4 {
+		res = append(res, *h.penultimate)
+	}
+	if h.num >= 3 {
+		res = append(res, *h.last)
+	}
+
+	return res
+}
+
+func (h *Reader) start() {
+	scanner := bufio.NewScanner(h.source)
+	for scanner.Scan() {
+		line := scanner.Text()
+		h.lock.Lock()
+		h.num += 1
+		if h.first == nil {
+			h.first = &line
+		} else if h.second == nil {
+			h.second = &line
+		}
+
+		h.penultimate = h.last
+		h.last = &line
+		h.lock.Unlock()
+	}
+}
+
+//======================================================================
+
+// This is a debugging aid - I use it to ensure goroutines stop as expected. If they don't
+// the main program will hang at termination.
+var Goroutinewg *sync.WaitGroup
+
+//======================================================================
+// Local Variables:
+// mode: Go
+// fill-column: 78
+// End:

streams/loader.go

@@ -155,11 +155,7 @@ func (c *Loader) loadStreamReassemblyAsync(pcapf string, proto string, idx int,
 				state = pcap.Terminated
 				if !c.SuppressErrors && err != nil {
 					if _, ok := err.(*exec.ExitError); ok {
-						cerr := gowid.WithKVs(termshark.BadCommand, map[string]interface{}{
-							"command": c.streamCmd.String(),
-							"error":   err,
-						})
-						pcap.HandleError(pcap.StreamCode, app, cerr, cb)
+						pcap.HandleError(pcap.StreamCode, app, pcap.MakeUsefulError(c.streamCmd, err), cb)
 					}
 				}
 
@@ -274,11 +270,7 @@ func (c *Loader) startStreamIndexerAsync(pcapf string, proto string, idx int, ap
 				state = pcap.Terminated
 				if !c.SuppressErrors && err != nil {
 					if _, ok := err.(*exec.ExitError); ok {
-						cerr := gowid.WithKVs(termshark.BadCommand, map[string]interface{}{
-							"command": c.indexerCmd.String(),
-							"error":   err,
-						})
-						pcap.HandleError(pcap.StreamCode, app, cerr, cb)
+						pcap.HandleError(pcap.StreamCode, app, pcap.MakeUsefulError(c.indexerCmd, err), cb)
 					}
 				}
 				streamOut.Close()

ui/prochandlers.go

@@ -8,6 +8,7 @@ package ui
 import (
 	"fmt"
 	"os"
+	"sort"
 	"strings"
 	"time"
 
@@ -126,10 +127,15 @@ func (t updatePacketViews) OnError(code pcap.HandlerCode, app gowid.IApp, err er
 			if kverr, ok := err.(gowid.KeyValueError); ok {
 				errstr = fmt.Sprintf("%v\n\n", kverr.Cause())
 				kvs := make([]string, 0, len(kverr.KeyVals))
-				for k, v := range kverr.KeyVals {
-					kvs = append(kvs, fmt.Sprintf("%v: %v", k, v))
+				ks := make([]string, 0, len(kverr.KeyVals))
+				for k := range kverr.KeyVals {
+					ks = append(ks, k)
 				}
-				errstr = errstr + strings.Join(kvs, "\n")
+				sort.Sort(sort.StringSlice(ks))
+				for _, k := range ks {
+					kvs = append(kvs, fmt.Sprintf("%v: %v", k, kverr.KeyVals[k]))
+				}
+				errstr = errstr + strings.Join(kvs, "\n\n")
 			} else {
 				errstr = fmt.Sprintf("%v", err)
 			}