Merge 39a9e49233 into 7c9032dff6

accounts/abi/abi.go

@@ -78,8 +78,12 @@ func (abi ABI) Pack(name string, args ...interface{}) ([]byte, error) {
 	if err != nil {
 		return nil, err
 	}
-	// Pack up the method ID too if not a constructor and return
-	return append(method.ID, arguments...), nil
+	// Pack up the method ID too if not a constructor and return. The method ID
+	// is copied into a fresh slice instead of appended to directly: method.ID is
+	// derived from keccak256(sig)[:4], so it has length 4 but capacity 32, and a
+	// no-arg pack would otherwise return that backing array itself, aliasing the
+	// cached selector.
+	return append(append([]byte{}, method.ID...), arguments...), nil
 }
 
 func (abi ABI) getArguments(name string, data []byte) (Arguments, error) {

accounts/abi/pack_selector_alias_test.go

@@ -0,0 +1,68 @@
+// Copyright 2026 The go-ethereum Authors
+// This file is part of the go-ethereum library.
+//
+// The go-ethereum library is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Lesser General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// The go-ethereum library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU Lesser General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public License
+// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
+
+package abi
+
+import (
+	"bytes"
+	"strings"
+	"testing"
+)
+
+// TestPackNoArgMethodDoesNotAliasSelector is a regression test for a bug where
+// ABI.Pack of a zero-argument method returned a slice that aliased the cached
+// method.ID. method.ID is keccak256(sig)[:4]: length 4 but capacity 32, so
+// `append(method.ID, arguments...)` with no arguments returned the selector's
+// own backing array. Mutating the returned calldata then corrupted the cached
+// selector for every subsequent encode of that method.
+func TestPackNoArgMethodDoesNotAliasSelector(t *testing.T) {
+	const def = `[{ "type" : "function", "name" : "balance", "stateMutability" : "view" }]`
+	abi, err := JSON(strings.NewReader(def))
+	if err != nil {
+		t.Fatal(err)
+	}
+	// selector aliases the cached method.ID; want is an independent copy of the
+	// expected value, so it stays correct even if the bug corrupts the cache.
+	selector := abi.Methods["balance"].ID
+	want := bytes.Clone(selector)
+
+	data1, err := abi.Pack("balance")
+	if err != nil {
+		t.Fatal(err)
+	}
+	// The returned calldata must be exactly the 4-byte selector...
+	if !bytes.Equal(data1, want) {
+		t.Fatalf("unexpected calldata: got %x, want %x", data1, want)
+	}
+	// ...but it must not share storage with the cached selector.
+	if len(data1) > 0 && len(selector) > 0 && &data1[0] == &selector[0] {
+		t.Fatal("Pack returned a slice aliasing the cached method.ID backing array")
+	}
+
+	// Mutating the first result must not affect the selector or later packs.
+	data1[0] ^= 0xff
+
+	data2, err := abi.Pack("balance")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !bytes.Equal(data2, want) {
+		t.Fatalf("second pack corrupted: got %x, want %x", data2, want)
+	}
+	if !bytes.Equal(abi.Methods["balance"].ID, want) {
+		t.Fatalf("cached method.ID corrupted: got %x, want %x", abi.Methods["balance"].ID, want)
+	}
+}