Attempt to fix sporadic ctrl-c bug

Issue #120 reports that if you hit ctrl-c shortly after startup,
occasionally termshark will not quit correctly, and subsequent ctrl-c
invocations make no difference.

This occurs only if reading from an interface. In that case, the
PSML-reading process starts with stdin set to the read-end of a
pipe. That pipe is supplied with pcap data via a tail command, reading
from the gradually accumulating pcap on disk.

The goroutine that tracks changes to PSML process state needs to ensure
that if the controlling context is cancelled, this pipe is
closed. Failing to do this can lead to Go's cmd.Exec never responding to
Wait() because internally, the Wait() calls itself waits for a private
goroutine - that goroutine is running io.Copy() from the user's process
IO reader (our pipe) to the real process's stdin. Closing the pipe will
cause io.Copy() to terminate, then cmd.Exec's private goroutine to
terminate, and so allows Wait() to return a value.

pcap/loader.go

@@ -1359,6 +1359,15 @@ func (p *PsmlLoader) loadPsmlSync(iloader *InterfaceLoader, e iPsmlLoaderEnv, cb
 
 		//======================================================================
 
+		closedPipe := false
+		closePipe := func() {
+			if !closedPipe {
+				fifoPipeWriter.Close()
+				fifoPipeReader.Close()
+				closedPipe = true
+			}
+		}
+
 		if p.ReadingFromFifo() {
 			// PcapPsml will be nil if here
 
@@ -1378,8 +1387,7 @@ func (p *PsmlLoader) loadPsmlSync(iloader *InterfaceLoader, e iPsmlLoaderEnv, cb
 			// is used as stdin for the psml command, which also runs in this
 			// goroutine.
 			defer func() {
-				fifoPipeWriter.Close()
-				fifoPipeReader.Close()
+				closePipe()
 			}()
 
 			// wrap the read end of the pipe with a Read() function that counts
@@ -1425,6 +1433,10 @@ func (p *PsmlLoader) loadPsmlSync(iloader *InterfaceLoader, e iPsmlLoaderEnv, cb
 				if err != nil {
 					log.Infof("Did not kill tshark psml process: %v", err)
 				}
+
+				if p.ReadingFromFifo() {
+					closePipe()
+				}
 			}
 
 		loop: