log: support for user defined log levels

This PR brings support for better log level handling. Terraform logs
are already written in the form which can be understood by the module
https://github.com/hashicorp/logutils .

The TF_LOG environment variable now accepts a log level. Users can pass
levels in the form of "info", "Info" or "INFO" If an invalid log level is
passed, we print a warning but still continue, for backward compatibility
with the old TF_LOG=1 style.

log.go

@@ -2,28 +2,65 @@ package main
 
 import (
 	"io"
+	"log"
 	"os"
+	"strings"
+
+	"github.com/hashicorp/logutils"
 )
 
 // These are the environmental variables that determine if we log, and if
 // we log whether or not the log should go to a file.
-const EnvLog = "TF_LOG"          //Set to True
-const EnvLogFile = "TF_LOG_PATH" //Set to a file
+const (
+	EnvLog     = "TF_LOG"      // Set to True
+	EnvLogFile = "TF_LOG_PATH" // Set to a file
+)
 
-// logOutput determines where we should send logs (if anywhere).
+var validLevels = []logutils.LogLevel{"TRACE", "DEBUG", "INFO", "WARN", "ERROR"}
+
+// logOutput determines where we should send logs (if anywhere) and the log level.
 func logOutput() (logOutput io.Writer, err error) {
 	logOutput = nil
-	if os.Getenv(EnvLog) != "" {
-		logOutput = os.Stderr
-
-		if logPath := os.Getenv(EnvLogFile); logPath != "" {
-			var err error
-			logOutput, err = os.Create(logPath)
-			if err != nil {
-				return nil, err
-			}
+	envLevel := os.Getenv(EnvLog)
+	if envLevel == "" {
+		return
+	}
+
+	logOutput = os.Stderr
+	if logPath := os.Getenv(EnvLogFile); logPath != "" {
+		var err error
+		logOutput, err = os.Create(logPath)
+		if err != nil {
+			return nil, err
 		}
 	}
 
+	// This was the default since the beginning
+	logLevel := logutils.LogLevel("TRACE")
+
+	if isValidLogLevel(envLevel) {
+		// allow following for better ux: info, Info or INFO
+		logLevel = logutils.LogLevel(strings.ToUpper(envLevel))
+	} else {
+		log.Printf("[WARN] Invalid log level: %q. Defaulting to level: TRACE. Valid levels are: %+v",
+			envLevel, validLevels)
+	}
+
+	logOutput = &logutils.LevelFilter{
+		Levels:   validLevels,
+		MinLevel: logLevel,
+		Writer:   logOutput,
+	}
+
 	return
 }
+
+func isValidLogLevel(level string) bool {
+	for _, l := range validLevels {
+		if strings.ToUpper(level) == string(l) {
+			return true
+		}
+	}
+
+	return false
+}