util/deephash: include type as part of hash for interfaces (#2476)

A Go interface may hold any number of different concrete types.
Just because two underlying values hash to the same thing
does not mean the two values are identical if they have different
concrete types. As such, include the type in the hash.

util/deephash/deephash_test.go

@@ -5,6 +5,7 @@
 package deephash
 
 import (
+	"archive/tar"
 	"bufio"
 	"bytes"
 	"fmt"
@@ -21,6 +22,33 @@ import (
 	"tailscale.com/wgengine/wgcfg"
 )
 
+func TestHash(t *testing.T) {
+	type tuple [2]interface{}
+	type iface struct{ X interface{} }
+	type MyBool bool
+	type MyHeader tar.Header
+	tests := []struct {
+		in     tuple
+		wantEq bool
+	}{
+		{in: tuple{iface{MyBool(true)}, iface{MyBool(true)}}, wantEq: true},
+		{in: tuple{iface{true}, iface{MyBool(true)}}, wantEq: false},
+		{in: tuple{iface{MyHeader{}}, iface{MyHeader{}}}, wantEq: true},
+		{in: tuple{iface{MyHeader{}}, iface{tar.Header{}}}, wantEq: false},
+		{in: tuple{iface{&MyHeader{}}, iface{&MyHeader{}}}, wantEq: true},
+		{in: tuple{iface{&MyHeader{}}, iface{&tar.Header{}}}, wantEq: false},
+		{in: tuple{iface{[]map[string]MyBool{}}, iface{[]map[string]MyBool{}}}, wantEq: true},
+		{in: tuple{iface{[]map[string]bool{}}, iface{[]map[string]MyBool{}}}, wantEq: false},
+	}
+
+	for _, tt := range tests {
+		gotEq := Hash(tt.in[0]) == Hash(tt.in[1])
+		if gotEq != tt.wantEq {
+			t.Errorf("(Hash(%v) == Hash(%v)) = %v, want %v", tt.in[0], tt.in[1], gotEq, tt.wantEq)
+		}
+	}
+}
+
 func TestDeepHash(t *testing.T) {
 	// v contains the types of values we care about for our current callers.
 	// Mostly we're just testing that we don't panic on handled types.