go/oasis-node/cmd/common: Add Isatty and use it

Having to use a command line flag when not running in an interactive
terminal is somewhat silly, and recursively calling the Fisher Price
Baby's First Interactive Prompt routine till the command exhauses the
heap and crashes when not run in a tty is even sillier.

go/oasis-node/cmd/common/common.go

@@ -278,24 +278,34 @@ func ExportEntity(signerBackend, entityDir string) error {
 	return err
 }
 
-// GetUserConfirmation scans the input for user's confirmation.
+// GetUserConfirmation displays the prompt, and scans the input for
+// the user's confirmation, until the user either explicitly confirms
+// or rejects the prompt.
 //
-// If the user's response is not recognized, it prompts the user again.
-func GetUserConfirmation() bool {
-	var response string
-
-	_, err := fmt.Scanln(&response)
-	if err != nil && err.Error() != "unexpected newline" {
-		rootLog.Error("Error reading from line", "err", err)
+// Note: If standard input is not a tty, this will omit displaying
+// the prompt, and assume the user entered yes.
+func GetUserConfirmation(prompt string) bool {
+	if !Isatty(os.Stdin.Fd()) {
+		return true
 	}
 
-	switch strings.ToLower(response) {
-	case "y", "yes":
-		return true
-	case "n", "no":
-		return false
-	default:
-		fmt.Printf("Unrecognized response: '%s'. Please, type (y)es or (n)o: ", response)
-		return GetUserConfirmation()
+	fmt.Printf("%s", prompt)
+
+	var response string
+	for {
+		_, err := fmt.Scanln(&response)
+		if err != nil && err.Error() != "unexpected newline" {
+			rootLog.Error("Error reading from line", "err", err)
+			continue
+		}
+
+		switch strings.ToLower(response) {
+		case "y", "yes":
+			return true
+		case "n", "no":
+			return false
+		default:
+			fmt.Printf("Unrecognized response: '%s'. Please, type (y)es or (n)o: ", response)
+		}
 	}
 }

go/oasis-node/cmd/common/consensus/consensus.go

@@ -124,8 +124,7 @@ func SignAndSaveTx(ctx context.Context, tx *transaction.Transaction) {
 	tx.PrettyPrint(ctx, "  ", os.Stdout)
 
 	if !cmdFlags.AssumeYes() && cmdSigner.Backend() == signerFile.SignerName {
-		fmt.Printf("\nAre you sure you want to continue? (y)es/(n)o ")
-		if !cmdCommon.GetUserConfirmation() {
+		if !cmdCommon.GetUserConfirmation("\nAre you sure you want to continue? (y)es/(n)o: ") {
 			os.Exit(1)
 		}
 	}

go/oasis-node/cmd/common/isatty.go

@@ -0,0 +1,31 @@
+package common
+
+import (
+	"syscall"
+	"unsafe"
+)
+
+// Isatty returns true iff the provided file descriptor is a terminal.
+func Isatty(fd uintptr) bool {
+	var attrs syscall.Termios
+
+	// This could examine the error more specifically to see if
+	// something really strange is going on since we expect it
+	// to return 0 or ENOTTY all the time, but, "the messed up
+	// thing the user passed in that makes it complain" also
+	// is not a tty.
+	//
+	// And yes, this is the standard way of doing this, see your
+	// libc implementation of choice.
+	_, _, errno := syscall.Syscall6(
+		syscall.SYS_IOCTL,
+		fd,
+		syscall.TCGETS,
+		uintptr(unsafe.Pointer(&attrs)),
+		0,
+		0,
+		0,
+	)
+
+	return errno == 0
+}