env var substitution prototype

infile.toml

[hekad]
maxprocs = 4
base_dir = "/home/rob/go/heka/var/cache/hekad"
#max_pack_idle = 2
#cpuprof = "cpu.prof"
memprof = "mem.prof"

[TcpInput]
address = "127.0.0.1:5565"
parser_type = "message.proto"
decoder = "ProtobufDecoder"
something = "%ENV[GOPATH]"

[CounterFilter]
message_matcher = "Type != 'heka.counter-output'"
ticker_interval = 1
bogus = "%this is a test%"
double_bogus = "%ENjustkidding%ENV"
triple_bogus = "%EN%ENV[GOPATH]"

[PayloadEncoder]
prefix_ts = false
path = "%ENV[PATH]"

[LogOutput]
open_only = "%ENV[no_ending"
message_matcher = "Type == 'heka.counter-output'"
encoder = "PayloadEncoder"

subber.go

package main

import (
	"bufio"
	"bytes"
	"io"
	"io/ioutil"
	"log"
	"os"
)

func main() {
	f, err := os.Open("infile.toml")
	bufIn := bufio.NewReader(f)
	bufOut := new(bytes.Buffer)
	for {
		chunk, err := bufIn.ReadBytes(byte('%'))
		if err != nil {
			if err == io.EOF {
				// We're done.
				bufOut.Write(chunk)
				break
			}
			log.Fatalln("ReadBytes error: ", err)
		}
		bufOut.Write(chunk[:len(chunk)-1])

		tmp := make([]byte, 4)
		tmp, err = bufIn.Peek(4)
		if err != nil {
			if err == io.EOF {
				// End of file, write the last few bytes out and exit.
				bufOut.WriteRune('%')
				bufOut.Write(tmp)
				break
			}
			log.Fatalln("Peek error: ", err)
		}

		if string(tmp) == "ENV[" {
			// Found opening delimiter, advance the read cursor and look for
			// closing delimiter.
			tmp, err = bufIn.ReadBytes(byte('['))
			if err != nil {
				// This shouldn't happen, since the Peek succeeded.
				log.Fatalln("Read error: ", err)
			}
			chunk, err = bufIn.ReadBytes(byte(']'))
			if err != nil {
				if err == io.EOF {
					// No closing delimiter, write everything we've collected
					// and be done.
					bufOut.WriteRune('%')
					bufOut.Write(tmp)
					bufOut.Write(chunk)
					break
				}
				log.Fatalln("ReadBytes error (2): ", err)
			}
			// `chunk` is now holding var name + closing delimiter.
			varName := string(chunk[:len(chunk)-1])
			varVal := os.Getenv(varName)
			bufOut.WriteString(varVal)
		} else {
			// Just a random '%', not an opening delimiter, write it out and
			// keep going.
			bufOut.WriteRune('%')
		}
	}

	// All substituted.
	err = ioutil.WriteFile("outfile.toml", bufOut.Bytes(), 0664)
	if err != nil {
		log.Fatalln("WriteFile error: ", err)
	}
}