Use os.LookupEnv instead of os.Getenv (#151)

* Use os.LookupEnv instead of os.Getenv

* Make linter happy

* ffenv: support 'KEY=' with no val

Author: peterbourgon

ffenv/ffenv.go

@@ -46,10 +46,6 @@ func Parse(r io.Reader, set func(name, value string) error) error {
 			return fmt.Errorf("%w: %s", ErrInvalidLine, line)
 		}
 
-		if len(value) <= 0 {
-			return fmt.Errorf("%w: %s", ErrInvalidLine, line)
-		}
-
 		if unquoted, err := strconv.Unquote(value); err == nil {
 			value = unquoted
 		}

ffenv/ffenv_test.go

@@ -6,7 +6,6 @@ import (
 	"time"
 
 	"github.com/peterbourgon/ff/v4"
-	"github.com/peterbourgon/ff/v4/ffenv"
 	"github.com/peterbourgon/ff/v4/fftest"
 )
 
@@ -38,7 +37,7 @@ func TestEnvFileParser(t *testing.T) {
 		},
 		{
 			ConfigFile: "testdata/no-value.env",
-			Want:       fftest.Vars{WantParseErrorIs: ffenv.ErrInvalidLine},
+			Want:       fftest.Vars{WantParseErrorString: "D: parse error"},
 		},
 		{
 			ConfigFile: "testdata/spaces.env",

fftest/test_case.go

@@ -1,7 +1,6 @@
 package fftest
 
 import (
-	"os"
 	"path/filepath"
 	"strings"
 	"testing"
@@ -76,8 +75,7 @@ func (tc *ParseTest) Run(t *testing.T) {
 	// be run in parallel.
 	if len(tc.Environment) > 0 {
 		for k, v := range tc.Environment {
-			defer os.Setenv(k, os.Getenv(k))
-			os.Setenv(k, v)
+			t.Setenv(k, v)
 		}
 	}
 

parse.go

@@ -91,8 +91,8 @@ func parse(fs Flags, args []string, options ...Option) error {
 					key := getEnvVarKey(name, pc.envVarPrefix)
 
 					// Look up the value from the environment.
-					val := os.Getenv(key)
-					if val == "" {
+					val, ok := os.LookupEnv(key)
+					if !ok {
 						continue
 					}
 

parse_test.go

@@ -16,7 +16,7 @@ import (
 var testdataConfigFS embed.FS
 
 func TestParse(t *testing.T) {
-	t.Parallel()
+	// t.Parallel() not possible
 
 	testcases := fftest.TestCases{
 		{
@@ -134,13 +134,20 @@ func TestParse(t *testing.T) {
 			Options:     []ff.Option{ff.WithEnvVarPrefix("TEST_PARSE"), ff.WithEnvVarSplit("xx")},
 			Want:        fftest.Vars{S: `axxb`, X: []string{`one`, `twoxxthree`}},
 		},
+		{
+			Name:        "env var provided but empty",
+			Default:     fftest.Vars{S: "non-empty-default", I: 42, F: 1.23},
+			Environment: map[string]string{"TEST_PARSE_S": ""},
+			Options:     []ff.Option{ff.WithEnvVarPrefix("TEST_PARSE")},
+			Want:        fftest.Vars{S: "", I: 42, F: 1.23},
+		},
 	}
 
 	testcases.Run(t)
 }
 
 func TestParse_FlagSet(t *testing.T) {
-	t.Parallel()
+	// t.Parallel() not possible
 
 	testcases := fftest.TestCases{
 		{
@@ -271,7 +278,7 @@ func TestParse_FlagSet(t *testing.T) {
 }
 
 func TestParse_StdFlagSetAdapter(t *testing.T) {
-	t.Parallel()
+	// t.Parallel() not possible
 
 	testcases := fftest.TestCases{
 		{
@@ -312,7 +319,7 @@ func TestParse_StdFlagSetAdapter(t *testing.T) {
 }
 
 func TestParse_PlainParser(t *testing.T) {
-	t.Parallel()
+	// t.Parallel() not possible
 
 	testcases := fftest.TestCases{
 		{