tests: add unit tests for parser errors

- Declares errors and wraps them for error checking
by clients
- Adds two unit tests for
  - terminated quote error
  - unexpected char

godotenv_test.go

@@ -2,6 +2,7 @@ package godotenv
 
 import (
 	"bytes"
+	"errors"
 	"fmt"
 	"os"
 	"reflect"
@@ -582,42 +583,42 @@ func TestWhitespace(t *testing.T) {
 	}{
 		"Leading whitespace": {
 			input: " A=a\n",
-			key: "A",
+			key:   "A",
 			value: "a",
 		},
 		"Leading tab": {
 			input: "\tA=a\n",
-			key: "A",
+			key:   "A",
 			value: "a",
 		},
 		"Leading mixed whitespace": {
 			input: " \t \t\n\t \t A=a\n",
-			key: "A",
+			key:   "A",
 			value: "a",
 		},
 		"Leading whitespace before export": {
 			input: " \t\t export    A=a\n",
-			key: "A",
+			key:   "A",
 			value: "a",
 		},
 		"Trailing whitespace": {
 			input: "A=a \t \t\n",
-			key: "A",
+			key:   "A",
 			value: "a",
 		},
 		"Trailing whitespace with export": {
 			input: "export A=a\t \t \n",
-			key: "A",
+			key:   "A",
 			value: "a",
 		},
 		"No EOL": {
 			input: "A=a",
-			key: "A",
+			key:   "A",
 			value: "a",
 		},
 		"Trailing whitespace with no EOL": {
 			input: "A=a ",
-			key: "A",
+			key:   "A",
 			value: "a",
 		},
 	}
@@ -634,3 +635,28 @@ func TestWhitespace(t *testing.T) {
 		})
 	}
 }
+
+func TestParserErrors(t *testing.T) {
+	cases := map[string]struct {
+		input string
+		err   error
+	}{
+		"Invalid char": {
+			input: "foo-1=bar",
+			err:   ErrUnexpectedChar,
+		},
+		"UnterminatedQuote": {
+			input: "foo=\"bar",
+			err:   ErrUnterminatedQuote,
+		},
+	}
+
+	for n, c := range cases {
+		t.Run(n, func(t *testing.T) {
+			v, err := Unmarshal(c.input)
+			if !errors.Is(err, c.err) {
+				t.Errorf("Input: %q Expected:\t %q\nGot:\t %q Val: %v", c.input, c.err, err, v)
+			}
+		})
+	}
+}

parser.go

@@ -17,6 +17,12 @@ const (
 	exportPrefix = "export"
 )
 
+var (
+	ErrZeroLengthString  = errors.New("zero length string")
+	ErrUnexpectedChar    = errors.New("unexpected character")
+	ErrUnterminatedQuote = errors.New("unterminated quoted value")
+)
+
 func parseBytes(src []byte, out map[string]string) error {
 	src = bytes.Replace(src, []byte("\r\n"), []byte("\n"), -1)
 	cutset := src
@@ -101,13 +107,14 @@ loop:
 			}
 
 			return "", nil, fmt.Errorf(
-				`unexpected character %q in variable name near %q`,
+				`%w %q in variable name near %q`,
+				ErrUnexpectedChar,
 				string(char), string(src))
 		}
 	}
 
 	if len(src) == 0 {
-		return "", nil, errors.New("zero length string")
+		return "", nil, ErrZeroLengthString
 	}
 
 	// trim whitespace
@@ -186,7 +193,7 @@ func extractVarValue(src []byte, vars map[string]string) (value string, rest []b
 		valEndIndex = len(src)
 	}
 
-	return "", nil, fmt.Errorf("unterminated quoted value %s", src[:valEndIndex])
+	return "", nil, fmt.Errorf("%w %s", ErrUnterminatedQuote, src[:valEndIndex])
 }
 
 func expandEscapes(str string) string {