Add fuzz testing for production Tell parsers

.github/workflows/modernization.yml

@@ -30,6 +30,11 @@ jobs:
         run: |
           go test ./...
           go vet ./...
+      - name: Fuzz production Tell parsers
+        working-directory: native
+        run: |
+          go test -run=^$ -fuzz=FuzzTellSourceParserNoPanic -fuzztime=10s -parallel=2 ./internal/tell
+          go test -run=^$ -fuzz=FuzzTellExpressionParserNoPanic -fuzztime=10s -parallel=2 ./internal/tell
       - name: Run Python tests
         run: |
           python -m compileall -q tell tests scripts

native/internal/tell/parser_fuzz_test.go

@@ -0,0 +1,60 @@
+package tell
+
+import "testing"
+
+// FuzzTellSourceParserNoPanic checks that arbitrary UTF-8 or arbitrary-byte
+// source is rejected with an error rather than crashing the production parser.
+func FuzzTellSourceParserNoPanic(f *testing.F) {
+	seeds := []string{
+		"",
+		"Say 1.",
+		"Let answer be 2 plus 3.\nSay answer.",
+		"If true:\n    Say \"ok\".\nEnd if.",
+		"To f with x:\n    Give back f(x).\nEnd function.",
+		"Create a class called Box:\n    When creating a Box:\n        Remember 1 as this.value.\n    End constructor.\nEnd class.",
+		"Tell object to open with \"wide\".",
+		"Call add with 1 and 2.",
+		"Use the module \"./helper.tell\" as helper.",
+		"Set items[-1] to 9.",
+		"Set the key \"name\" in data to \"Ada\".",
+		"Say \"unterminated",
+		"Say \xff\xfe\x00.",
+		"\r\n\t# comments\r\n",
+		"Otherwise if (",
+	}
+	for _, seed := range seeds {
+		f.Add(seed)
+	}
+	f.Fuzz(func(t *testing.T, source string) {
+		_, _ = parse(source, "fuzz.tell")
+	})
+}
+
+// FuzzTellExpressionParserNoPanic exercises the production expression lexer,
+// Pratt parser, and AST construction independently of statement parsing.
+func FuzzTellExpressionParserNoPanic(f *testing.F) {
+	seeds := []string{
+		"",
+		"1 + 2 * 3",
+		"not true or false",
+		"2 ** 3 ** 2",
+		"values[1:3].length",
+		"mapping[\"name\"]",
+		"f(1, right=2)",
+		"1 if condition else 0",
+		"\"escaped\\ntext\"",
+		"{1, 2, 2}",
+		"{\"name\": \"Ada\"}",
+		"1 / 0",
+		"(((((((((",
+		"\"unterminated",
+		"\xff\xfe\x00",
+		"λ + 1",
+	}
+	for _, seed := range seeds {
+		f.Add(seed)
+	}
+	f.Fuzz(func(t *testing.T, expression string) {
+		_, _ = parseTellExpressionAt(expression, 1, 1)
+	})
+}