btcctl: Support - argument to read from stdin.

This commit modifies the argument handling for btcctl to treat a
parameter that is a single dash as an indicator to read that paramter from
stdin instead.

This change allows commands, such as the submitblock, to accept data piped
from stdin for any parameter.  This, in turn, allows large arguments, such
as blocks, which can often be too big for a single argument due to
Operating System limitations to be submitted by putting them into a file
and redirecting stdin.

For example:

btcctl submitblock - <block.hex
cat block.hex | btcctl submitblock -

btcctl sendrawtransaction - <tx.hex
cat tx.hex | btcctl sendrawtransaction -

cmd/btcctl/btcctl.go

@@ -1,9 +1,11 @@
 package main
 
 import (
+	"bufio"
 	"bytes"
 	"encoding/json"
 	"fmt"
+	"io"
 	"os"
 	"path/filepath"
 	"strings"
@@ -72,8 +74,31 @@ func main() {
 
 	// Convert remaining command line args to a slice of interface values
 	// to be passed along as parameters to new command creation function.
+	//
+	// Since some commands, such as submitblock, can involve data which is
+	// too large for the Operating System to allow as a normal command line
+	// parameter, support using '-' as an argument to allow the argument
+	// to be read from a stdin pipe.
+	bio := bufio.NewReader(os.Stdin)
 	params := make([]interface{}, 0, len(args[1:]))
 	for _, arg := range args[1:] {
+		if arg == "-" {
+			param, err := bio.ReadString('\n')
+			if err != nil && err != io.EOF {
+				fmt.Fprintf(os.Stderr, "Failed to read data "+
+					"from stdin: %v\n", err)
+				os.Exit(1)
+			}
+			if err == io.EOF && len(param) == 0 {
+				fmt.Fprintln(os.Stderr, "Not enough lines "+
+					"provided on stdin")
+				os.Exit(1)
+			}
+			param = strings.TrimRight(param, "\r\n")
+			params = append(params, param)
+			continue
+		}
+
 		params = append(params, arg)
 	}
 

cmd/btcctl/config.go

@@ -190,6 +190,11 @@ func loadConfig() (*config, []string, error) {
 	if err != nil {
 		if e, ok := err.(*flags.Error); ok && e.Type == flags.ErrHelp {
 			fmt.Fprintln(os.Stderr, err)
+			fmt.Fprintln(os.Stderr, "")
+			fmt.Fprintln(os.Stderr, "The special parameter `-` "+
+				"indicates that a parameter should be read "+
+				"from the\nnext unread line from standard "+
+				"input.")
 			return nil, nil, err
 		}
 	}