fix: overflow-safe span checks and goroutine recover in buildResult

Author: Kinnouts

apps/api/services/text/spellcheck/service.go

@@ -92,6 +92,11 @@ func (s *Service) CheckBatch(texts []string) []Result {
 		go func(i int, text string) {
 			defer wg.Done()
 			defer func() { <-sem }()
+			defer func() {
+				if rec := recover(); rec != nil {
+					results[i] = Result{}
+				}
+			}()
 
 			r, err := s.Check(text)
 			if err != nil {
@@ -106,14 +111,24 @@ func (s *Service) CheckBatch(texts []string) []Result {
 	return results
 }
 
+// isValidSpan reports whether m describes a non-empty, non-overflowing span
+// within a slice of runeCount runes.
+func isValidSpan(m ltMatch, runeCount int) bool {
+	return len(m.Replacements) > 0 &&
+		m.Offset >= 0 &&
+		m.Length >= 0 &&
+		m.Offset < runeCount &&
+		m.Length <= runeCount-m.Offset
+}
+
 // buildResult converts LanguageTool matches into our Result type.
 // Replacements are applied in reverse order to keep offsets valid.
 func buildResult(text string, matches []ltMatch) Result {
 	runes := []rune(text)
 	corrections := make([]Correction, 0, len(matches))
 
 	for _, m := range matches {
-		if len(m.Replacements) == 0 || m.Offset < 0 || m.Length < 0 || m.Offset+m.Length > len(runes) {
+		if !isValidSpan(m, len(runes)) {
 			continue
 		}
 		// Collect up to 3 suggestions so callers can present a picker.
@@ -137,7 +152,7 @@ func buildResult(text string, matches []ltMatch) Result {
 	copy(corrected, runes)
 	for i := len(matches) - 1; i >= 0; i-- {
 		m := matches[i]
-		if len(m.Replacements) == 0 || m.Offset < 0 || m.Length < 0 || m.Offset+m.Length > len(corrected) {
+		if !isValidSpan(m, len(corrected)) {
 			continue
 		}
 		repl := []rune(m.Replacements[0].Value)

docs/apis/text/spell-check.md

@@ -133,19 +133,19 @@ Each text in the batch counts as **1 unit** of quota. A request with 10 texts co
 
 ### Partial failure
 
-This endpoint always returns `200` with a result for every input text. Up to 10
-texts are checked concurrently. If LanguageTool is unreachable for an individual
-text, that slot returns a zero-value result (`corrected: ""`, `corrections: null`)
-and the batch continues — the caller receives a full-length `results` array
-regardless of individual failures.
+Once validation passes, this endpoint returns `200` with a result for every input
+text. Up to 10 texts are checked concurrently. If LanguageTool is unreachable for
+an individual text, that slot returns a zero-value result
+(`corrected: ""`, `corrections: null`) and the batch continues — the caller
+receives a full-length `results` array regardless of individual failures.
 
 ### Batch Error Codes
 
-| Code                | Status | When                                                       |
-| ------------------- | ------ | ---------------------------------------------------------- |
-| `validation_failed` | 422    | `texts` is missing, empty, or contains more than 50 items  |
-| `bad_request`       | 400    | Request body is missing or not JSON                        |
-| `internal_error`    | 500    | Unexpected failure                                         |
+| Code                | Status | When                                                                         |
+| ------------------- | ------ | ---------------------------------------------------------------------------- |
+| `validation_failed` | 422    | `texts` is missing, empty, or contains more than 50 items                    |
+| `bad_request`       | 400    | Request body is missing or not JSON                                          |
+| `internal_error`    | 500    | Infrastructure-level failure before processing begins (e.g. handler error)  |
 
 ---