Adds Err() method to scanner and checks error in Scan()

Stores the first error encountered by Scan() and checks it to make sure
scanning stops unrecoverably. The Err() method can use used to fetch the
last non-EOF error.

scanner.go

@@ -58,6 +58,7 @@ type Scanner struct {
 	tokenDepth         int
 	repeateSpecialChar bool //  only '}' can be repeated
 	prev               string
+	err                error
 }
 
 // NewScanner returns a new Scanner to read from r.
@@ -75,6 +76,20 @@ func NewScanner(r io.Reader) *Scanner {
 	return s
 }
 
+// Err returns the first non-EOF error that was encountered by the Scanner.
+func (s *Scanner) Err() error {
+	if s.err == io.EOF {
+		return nil
+	}
+	return s.err
+}
+
+func (s *Scanner) setErr(err error) {
+	if s.err == nil || s.err != io.EOF {
+		s.err = err
+	}
+}
+
 // Scan reads the next token from source and returns it.. It returns io.EOF at the end of the source. Scanner errors are
 // returned when encountered.
 func (s *Scanner) Scan() (Token, error) { //nolint: funlen, gocognit, gocyclo
@@ -88,21 +103,26 @@ func (s *Scanner) Scan() (Token, error) { //nolint: funlen, gocognit, gocyclo
 	var r, quote string
 
 	for {
+		if s.err != nil {
+			return Token{}, s.err
+		}
+
 		switch {
 		case s.prev != "":
-			r = s.prev
-			s.prev = ""
+			r, s.prev = s.prev, ""
 		case readNext:
 			if !s.scanner.Scan() {
 				if tok.Len() > 0 {
 					return Token{Text: tok.String(), Line: s.tokenStartLine, IsQuoted: lexState == inQuote}, nil
 				}
 
 				if s.tokenDepth > 0 {
-					return Token{}, &scannerError{line: s.tokenStartLine, msg: "unexpected end of file, expecting }"}
+					s.setErr(&scannerError{line: s.tokenStartLine, msg: "unexpected end of file, expecting }"})
+					return Token{}, s.err
 				}
 
-				return Token{}, io.EOF
+				s.setErr(io.EOF)
+				return Token{}, s.err
 			}
 
 			nextRune := s.scanner.Text()
@@ -185,7 +205,8 @@ func (s *Scanner) Scan() (Token, error) { //nolint: funlen, gocognit, gocyclo
 
 				// only } can be repeated
 				if s.repeateSpecialChar && r != "}" {
-					return Token{}, newScannerErrf(s.tokenStartLine, "unxpected %q", r)
+					s.setErr(newScannerErrf(s.tokenStartLine, "unxpected %q", r))
+					return Token{}, s.err
 				}
 
 				s.repeateSpecialChar = true
@@ -196,7 +217,8 @@ func (s *Scanner) Scan() (Token, error) { //nolint: funlen, gocognit, gocyclo
 				if r == "}" {
 					s.tokenDepth--
 					if s.tokenDepth < 0 {
-						return Token{}, &scannerError{line: s.tokenStartLine, msg: `unexpected "}"`}
+						s.setErr(&scannerError{line: s.tokenStartLine, msg: `unexpected "}"`})
+						return Token{}, s.err
 					}
 				}
 

scanner_test.go

@@ -1,6 +1,7 @@
 package crossplane
 
 import (
+	"errors"
 	"io"
 	"os"
 	"strings"
@@ -66,7 +67,16 @@ func TestScanner_unhappy(t *testing.T) {
 
 				if err != nil {
 					t.Logf("got error: %v", err)
-					return
+
+					if gotErr := s.Err(); !errors.Is(gotErr, err) {
+						t.Fatalf("error do not match: have=%+v, want=%+v", gotErr, err)
+					}
+
+					if _, gotErr := s.Scan(); !errors.Is(gotErr, err) {
+						t.Fatalf("error after scan does not match: have=%+v, want=%+v", gotErr, err)
+					}
+
+					break
 				}
 			}
 		})