Ensure exit code is within valid range

The exitHandler function now properly validates the exit code provided
in the query string. It checks for conversion errors and ensures the
code is within the valid range of 0 to 125. If the validation fails,
it responds with an HTTP 400 Bad Request error. This prevents potential
misuse of the exit endpoint by restricting the exit codes to expected
values.

internal/api/api.go

@@ -233,7 +233,14 @@ func exitHandler(w http.ResponseWriter, r *http.Request) {
 	}
 
 	s := r.URL.Query().Get("code")
-	code, _ := strconv.Atoi(s)
+	code, err := strconv.Atoi(s)
+
+	// https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_08_02
+	if err != nil || code < 0 || code > 125 {
+		http.Error(w, "Code must be in the range [0, 125]", http.StatusBadRequest)
+		return
+	}
+
 	os.Exit(code)
 }