Handle config precedence properly (#64)

* Implement override precedence between config files

* Fix precedence between config files and environment variables

* Document configuration precedence

* Fix logic of readConfigFile

* Fix merge logic

* Update go.mod

* Update comment

* Simplify main

* Update comments

README.md

@@ -219,6 +219,13 @@ instance = myinstance
 prompt = "[\\p:\\i:\\d]\\t> "
 ```
 
+## Configuration Precedence
+
+1. Command line flags(highest)
+2. Environment variables
+3. `.spanner_cli.cnf` in current directory
+4. `.spanner_cli.cnf` in home directory(lowest)
+
 ## How to develop
 
 Run unit tests.

main.go

@@ -45,15 +45,14 @@ type spannerOptions struct {
 
 func main() {
 	var gopts globalOptions
-	parser := flags.NewParser(&gopts, flags.Default)
-
-	// check config file at first
-	if err := readConfigFile(parser); err != nil {
+	// process config files at first
+	if err := readConfigFile(flags.NewParser(&gopts, flags.Default)); err != nil {
 		exitf("Invalid config file format\n")
 	}
 
-	// then, parse command line options
-	if _, err := parser.Parse(); err != nil {
+	// then, process environment variables and command line options
+	// use another parser to process environment variable
+	if _, err := flags.NewParser(&gopts, flags.Default).Parse(); err != nil {
 		exitf("Invalid options\n")
 	}
 
@@ -117,30 +116,24 @@ func exitf(format string, a ...interface{}) {
 const cnfFileName = ".spanner_cli.cnf"
 
 func readConfigFile(parser *flags.Parser) error {
-	var cnfFile string
+	var cnfFiles []string
+	if currentUser, err := user.Current(); err == nil {
+		cnfFiles = append(cnfFiles, filepath.Join(currentUser.HomeDir, cnfFileName))
+	}
 
 	cwd, _ := os.Getwd() // ignore err
-	if _, err := os.Stat(filepath.Join(cwd, cnfFileName)); err == nil {
-		cnfFile = filepath.Join(cwd, cnfFileName)
-	} else {
-		currentUser, err := user.Current()
-		if err != nil {
-			// ignore error
-			return nil
-		}
-
-		// TODO: customize config path
-		cnfFile = filepath.Join(currentUser.HomeDir, cnfFileName)
+	cwdCnfFile := filepath.Join(cwd, cnfFileName)
+	cnfFiles = append(cnfFiles, cwdCnfFile)
 
-		// check config file existence
+	iniParser := flags.NewIniParser(parser)
+	for _, cnfFile := range cnfFiles {
+		// skip if missing
 		if _, err := os.Stat(cnfFile); err != nil {
-			return nil
+			continue
+		}
+		if err := iniParser.ParseFile(cnfFile); err != nil {
+			return err
 		}
-	}
-
-	iniParser := flags.NewIniParser(parser)
-	if err := iniParser.ParseFile(cnfFile); err != nil {
-		return err
 	}
 
 	return nil