trie, tests/fuzzers: implement a stacktrie fuzzer + stacktrie fixes (#21799)

* trie: fix error in stacktrie not committing small roots

* fuzzers: make trie-fuzzer use correct returnvalues

* trie: improved tests

* tests/fuzzers: fuzzer for stacktrie vs regular trie

* test/fuzzers: make stacktrie fuzzer use 32-byte keys

* trie: fix error in stacktrie with small nodes

* trie: add (skipped) testcase for stacktrie

* tests/fuzzers: address review comments for stacktrie fuzzer

* trie: fix docs in stacktrie

trie/stacktrie_test.go

@@ -240,3 +240,52 @@ func TestDerivableList(t *testing.T) {
 		}
 	}
 }
+
+// TestUpdateSmallNodes tests a case where the leaves are small (both key and value),
+// which causes a lot of node-within-node. This case was found via fuzzing.
+func TestUpdateSmallNodes(t *testing.T) {
+	st := NewStackTrie(nil)
+	nt, _ := New(common.Hash{}, NewDatabase(memorydb.New()))
+	kvs := []struct {
+		K string
+		V string
+	}{
+		{"63303030", "3041"}, // stacktrie.Update
+		{"65", "3000"},       // stacktrie.Update
+	}
+	for _, kv := range kvs {
+		nt.TryUpdate(common.FromHex(kv.K), common.FromHex(kv.V))
+		st.TryUpdate(common.FromHex(kv.K), common.FromHex(kv.V))
+	}
+	if nt.Hash() != st.Hash() {
+		t.Fatalf("error %x != %x", st.Hash(), nt.Hash())
+	}
+}
+
+// TestUpdateVariableKeys contains a case which stacktrie fails: when keys of different
+// sizes are used, and the second one has the same prefix as the first, then the
+// stacktrie fails, since it's unable to 'expand' on an already added leaf.
+// For all practical purposes, this is fine, since keys are fixed-size length
+// in account and storage tries.
+//
+// The test is marked as 'skipped', and exists just to have the behaviour documented.
+// This case was found via fuzzing.
+func TestUpdateVariableKeys(t *testing.T) {
+	t.SkipNow()
+	st := NewStackTrie(nil)
+	nt, _ := New(common.Hash{}, NewDatabase(memorydb.New()))
+	kvs := []struct {
+		K string
+		V string
+	}{
+		{"0x33303534636532393561313031676174", "303030"},
+		{"0x3330353463653239356131303167617430", "313131"},
+	}
+	for _, kv := range kvs {
+		nt.TryUpdate(common.FromHex(kv.K), common.FromHex(kv.V))
+		st.TryUpdate(common.FromHex(kv.K), common.FromHex(kv.V))
+	}
+	if nt.Hash() != st.Hash() {
+		t.Fatalf("error %x != %x", st.Hash(), nt.Hash())
+	}
+}