From 4acfd078159de3a102f47897f0698ee3da28a119 Mon Sep 17 00:00:00 2001
From: Brad Fitzpatrick <bradfitz@tailscale.com>
Date: Wed, 27 Jul 2022 21:06:00 -0700
Subject: [PATCH] WIP

Change-Id: Ibc7de639b31ed85c1270f5f9a889b4e6b4228be7
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
---
 util/deephash/deephash.go      | 49 +++++++++++++++++++++++++++++++--
 util/deephash/deephash_test.go | 50 ++++++++++++++++++++++++++++++++++
 2 files changed, 97 insertions(+), 2 deletions(-)

diff --git a/util/deephash/deephash.go b/util/deephash/deephash.go
index 93ab1abc5..a5036028d 100644
--- a/util/deephash/deephash.go
+++ b/util/deephash/deephash.go
@@ -424,11 +424,56 @@ func genHashPtrToMemoryRange(eleType reflect.Type) typeHasherFunc {
 	}
 }
 
-const debug = false
+const debug = true
+
+// canMaybeFastPath reports whether t is a type that we might be able to
+// hash quickly at runtime with a typeHasherFunc.
+//
+// If it returns false, the slow path should be used directly.
+func canMaybeFastPath(t reflect.Type) (ret bool) {
+	defer func() {
+		if !ret {
+			log.Printf("Can't on %v", t)
+		}
+	}()
+	ti := getTypeInfo(t)
+	if ti.isRecursive {
+		return false
+	}
+	switch t.Kind() {
+	case reflect.Map, reflect.Interface, reflect.Func, reflect.UnsafePointer, reflect.Chan:
+		return false
+	case reflect.Array:
+		return t.Size() == 0 || canMaybeFastPath(t.Elem())
+	case reflect.Slice, reflect.Pointer:
+		return canMaybeFastPath(t.Elem())
+	case reflect.Struct:
+		if t == timeTimeType || t.Implements(appenderToType) {
+			return true
+		}
+		for i, n := 0, t.NumField(); i < n; i++ {
+			sf := t.Field(i)
+			if sf.Type.Size() == 0 {
+				continue
+			}
+			if !canMaybeFastPath(sf.Type) {
+				return false
+			}
+		}
+		return true
+	}
+	return true
+}
 
 func genTypeHasher(t reflect.Type) typeHasherFunc {
+	fastable := canMaybeFastPath(t)
 	if debug {
-		log.Printf("generating func for %v", t)
+		log.Printf("generating func for %v; fastable=%v", t, fastable)
+	}
+	if !fastable {
+		return func(h *hasher, v reflect.Value) bool {
+			return false
+		}
 	}
 
 	switch t.Kind() {
diff --git a/util/deephash/deephash_test.go b/util/deephash/deephash_test.go
index 3f20b8e61..0b8211997 100644
--- a/util/deephash/deephash_test.go
+++ b/util/deephash/deephash_test.go
@@ -317,6 +317,45 @@ type RecursiveStruct struct {
 	}
 }
 
+func TestCanMaybeFastPath(t *testing.T) {
+	type RecursiveStruct struct {
+		v *RecursiveStruct
+	}
+	type RecursiveChan chan *RecursiveChan
+	type FastStruct struct{ _, _ string }
+
+	tests := []struct {
+		val  any
+		want bool
+	}{
+		{val: 42, want: true},
+		{val: "string", want: true},
+		{val: 1 + 2i, want: true},
+		{val: struct{}{}, want: true},
+		{val: (*RecursiveStruct)(nil), want: false},
+		{val: RecursiveStruct{}, want: false},
+		{val: FastStruct{}, want: true},
+		{val: time.Unix(0, 0), want: true},
+		{val: structs.Incomparable{}, want: true}, // ignore its [0]func()
+		{val: tailcfg.NetPortRange{}, want: true}, // uses structs.Incomparable
+		{val: (*tailcfg.Node)(nil), want: true},
+		{val: map[string]bool{}, want: false},
+		{val: func() {}, want: false},
+		{val: make(chan int), want: false},
+		{val: unsafe.Pointer(nil), want: false},
+		{val: make(RecursiveChan), want: false},
+		{val: make(chan int), want: false},
+		{val: tailcfg.SSHRule{}, want: false}, // contains a map
+		{val: (*tailcfg.SSHRule)(nil), want: false},
+	}
+	for _, tt := range tests {
+		got := canMaybeFastPath(reflect.TypeOf(tt.val))
+		if got != tt.want {
+			t.Errorf("for type %T: got %v, want %v", tt.val, got, tt.want)
+		}
+	}
+}
+
 type IntThenByte struct {
 	i int
 	b byte
@@ -611,6 +650,17 @@ func TestGetTypeHasher(t *testing.T) {
 			val:  &tailcfg.Node{},
 			out:  "\x01\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x140001-01-01T00:00:00Z\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x140001-01-01T00:00:00Z\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00",
 		},
+		{
+			name: "no_hashing_for_mixed_fast_slow_type",
+			val: struct {
+				A, B string
+				M    map[string]string
+			}{
+				"foo", "bar", map[string]string{"alice": "bob"},
+			},
+			want: false,
+			out:  "", // no "foo" or "bar" included
+		},
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {