fix(compiler): reduce empty array without init val (#911)

Add bounds check before accessing first element when reduce is called
without an initial value. Previously this caused a panic on empty arrays.
Now returns a clear error: "reduce of empty array with no initial value".

Signed-off-by: Ville Vesilehto <ville@vesilehto.fi>

Author: thevilledev

builtin/builtin_test.go

@@ -150,6 +150,8 @@ func TestBuiltin(t *testing.T) {
 		{`reduce(1..9, # + #acc)`, 45},
 		{`reduce([.5, 1.5, 2.5], # + #acc, 0)`, 4.5},
 		{`reduce([], 5, 0)`, 0},
+		{`reduce(10..1, # + #acc, 100)`, 100},
+		{`reduce([], # + #acc, 42)`, 42},
 		{`concat(ArrayOfString, ArrayOfInt)`, []any{"foo", "bar", "baz", 1, 2, 3}},
 		{`concat(PtrArrayWithNil, [nil])`, []any{42, nil}},
 		{`flatten([["a", "b"], [1, 2]])`, []any{"a", "b", 1, 2}},

compiler/compiler.go

@@ -1124,9 +1124,18 @@ func (c *compiler) BuiltinNode(node *ast.BuiltinNode) {
 			c.derefInNeeded(node.Arguments[2])
 			c.emit(OpSetAcc)
 		} else {
+			// When no initial value is provided, we use the first element as the
+			// accumulator. But first we must check if the array is empty to avoid
+			// an index out of range panic.
+			empty := c.emit(OpJumpIfEnd, placeholder)
 			c.emit(OpPointer)
 			c.emit(OpIncrementIndex)
 			c.emit(OpSetAcc)
+			jumpPastError := c.emit(OpJump, placeholder)
+			c.patchJump(empty)
+			c.emit(OpPush, c.addConstant(fmt.Errorf("reduce of empty array with no initial value")))
+			c.emit(OpThrow)
+			c.patchJump(jumpPastError)
 		}
 		c.emitLoop(func() {
 			c.compile(node.Arguments[1])

expr_test.go

@@ -1485,6 +1485,12 @@ func TestExpr_error(t *testing.T) {
  | ArrayOfAny[-7]
  | ..........^`,
 		},
+		{
+			`reduce(10..1, # + #acc)`,
+			`reduce of empty array with no initial value (1:1)
+ | reduce(10..1, # + #acc)
+ | ^`,
+		},
 	}
 
 	for _, tt := range tests {

test/fuzz/fuzz_test.go

@@ -54,6 +54,7 @@ func FuzzExpr(f *testing.F) {
 		regexp.MustCompile(`operator "in" not defined on .*`),
 		regexp.MustCompile(`cannot sum .*`),
 		regexp.MustCompile(`index out of range: .* \(array length is .*\)`),
+		regexp.MustCompile(`reduce of empty array with no initial value`),
 		regexp.MustCompile(`cannot use <nil> as argument \(type .*\) to call .*`),
 		regexp.MustCompile(`illegal base64 data at input byte .*`),
 	}