console: clean up Interactive

There is an unavoidable goroutine leak in Interactive: when we
receive a signal, the line reader goroutine cannot be unblocked
and will get stuck. The previous patch tried to fix up the situation
with a stop channel, but there is really nothing we can do to fix it.

This change tries to avoid the leak by adding a one-element buffer to
the output channels of the line reader. Should the reader eventually
return from its blocked state, at least it won't get stuck trying to
send to the interpreter loop which has quit long ago.

console/console.go

@@ -340,72 +340,62 @@ func (c *Console) Evaluate(statement string) {
 // the configured user prompter.
 func (c *Console) Interactive() {
 	var (
-		prompt    = c.prompt            // Current prompt line (used for multi-line inputs)
-		indents   = 0                   // Current number of input indents (used for multi-line inputs)
-		input     = ""                  // Current user input
-		scheduler = make(chan string)   // Channel to send the next prompt on and receive the input
-		stop      = make(chan struct{}) // Channel to signal the return of Interactive
+		prompt      = c.prompt             // the current prompt line (used for multi-line inputs)
+		indents     = 0                    // the current number of input indents (used for multi-line inputs)
+		input       = ""                   // the current user input
+		inputLine   = make(chan string, 1) // receives user input
+		inputErr    = make(chan error, 1)  // receives liner errors
+		requestLine = make(chan string)    // requests a line of input
+		interrupt   = make(chan os.Signal, 1)
 	)
-	defer close(stop)
-	// Start a goroutine to listen for prompt requests and send back inputs
-	go func() {
-		for {
-			// Read the next user input
-			line, err := c.prompter.PromptInput(<-scheduler)
-			if err != nil {
-				// In case of an error, either clear the prompt or fail
-				if err == liner.ErrPromptAborted { // ctrl-C
-					prompt, indents, input = c.prompt, 0, ""
-					select {
-					case <-stop:
-						return
-					case scheduler <- "":
-					}
-					continue
-				}
-				close(scheduler)
-				return
-			}
-			// User input retrieved, send for interpretation and loop
-			select {
-			case <-stop:
-				return
-			case scheduler <- line:
-			}
-		}
-	}()
-	// Monitor Ctrl-C too in case the input is empty and we need to bail
-	abort := make(chan os.Signal, 1)
-	signal.Notify(abort, syscall.SIGINT, syscall.SIGTERM)
 
-	// Start sending prompts to the user and reading back inputs
+	// Monitor Ctrl-C. While liner does turn on the relevant terminal mode bits to avoid
+	// the signal, a signal can still be received for unsupported terminals. Unfortunately
+	// there is no way to cancel the line reader when this happens. The readLines
+	// goroutine will be leaked in this case.
+	signal.Notify(interrupt, syscall.SIGINT, syscall.SIGTERM)
+	defer signal.Stop(interrupt)
+
+	// The line reader runs in a separate goroutine.
+	go c.readLines(inputLine, inputErr, requestLine)
+	defer close(requestLine)
+
 	for {
-		// Send the next prompt, triggering an input read and process the result
-		scheduler <- prompt
+		// Send the next prompt, triggering an input read.
+		requestLine <- prompt
+
 		select {
-		case <-abort:
-			// User forcefully quite the console
+		case <-interrupt:
 			fmt.Fprintln(c.printer, "caught interrupt, exiting")
 			return
 
-		case line, ok := <-scheduler:
-			// User input was returned by the prompter, handle special cases
-			if !ok || (indents <= 0 && exit.MatchString(line)) {
+		case err := <-inputErr:
+			fmt.Printf("input: %q, err: %q\n", input, err)
+			if err == liner.ErrPromptAborted && indents > 0 {
+				// When prompting for multi-line input, the first Ctrl-C resets
+				// the multi-line state.
+				prompt, indents, input = c.prompt, 0, ""
+				continue
+			}
+			return
+
+		case line := <-inputLine:
+			// User input was returned by the prompter, handle special cases.
+			if indents <= 0 && exit.MatchString(line) {
 				return
 			}
 			if onlyWhitespace.MatchString(line) {
 				continue
 			}
-			// Append the line to the input and check for multi-line interpretation
+			// Append the line to the input and check for multi-line interpretation.
 			input += line + "\n"
-
 			indents = countIndents(input)
 			if indents <= 0 {
 				prompt = c.prompt
 			} else {
 				prompt = strings.Repeat(".", indents*3) + " "
 			}
-			// If all the needed lines are present, save the command and run
+			// If all the needed lines are present, save the command and run it.
 			if indents <= 0 {
 				if len(input) > 0 && input[0] != ' ' && !passwordRegexp.MatchString(input) {
 					if command := strings.TrimSpace(input); len(c.history) == 0 || command != c.history[len(c.history)-1] {
@@ -422,6 +412,18 @@ func (c *Console) Interactive() {
 	}
 }
 
+// readLines runs in its own goroutine, prompting for input.
+func (c *Console) readLines(input chan<- string, errc chan<- error, prompt <-chan string) {
+	for p := range prompt {
+		line, err := c.prompter.PromptInput(p)
+		if err != nil {
+			errc <- err
+		} else {
+			input <- line
+		}
+	}
+}
+
 // countIndents returns the number of identations for the given input.
 // In case of invalid input such as var a = } the result can be negative.
 func countIndents(input string) int {