Fix user visible check (#21210)

Fixes #21206

If user and viewer are equal the method should return true.
Also the common organization check was wrong as `count` can never be
less then 0.

Co-authored-by: 6543 <6543@obermui.de>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

models/user/user_test.go

@@ -400,3 +400,56 @@ func TestUnfollowUser(t *testing.T) {
 
 	unittest.CheckConsistencyFor(t, &user_model.User{})
 }
+
+func TestIsUserVisibleToViewer(t *testing.T) {
+	assert.NoError(t, unittest.PrepareTestDatabase())
+
+	user1 := unittest.AssertExistsAndLoadBean(t, &user_model.User{ID: 1})   // admin, public
+	user4 := unittest.AssertExistsAndLoadBean(t, &user_model.User{ID: 4})   // normal, public
+	user20 := unittest.AssertExistsAndLoadBean(t, &user_model.User{ID: 20}) // public, same team as user31
+	user29 := unittest.AssertExistsAndLoadBean(t, &user_model.User{ID: 29}) // public, is restricted
+	user31 := unittest.AssertExistsAndLoadBean(t, &user_model.User{ID: 31}) // private, same team as user20
+	user33 := unittest.AssertExistsAndLoadBean(t, &user_model.User{ID: 33}) // limited, follows 31
+
+	test := func(u, viewer *user_model.User, expected bool) {
+		name := func(u *user_model.User) string {
+			if u == nil {
+				return "<nil>"
+			}
+			return u.Name
+		}
+		assert.Equal(t, expected, user_model.IsUserVisibleToViewer(db.DefaultContext, u, viewer), "user %v should be visible to viewer %v: %v", name(u), name(viewer), expected)
+	}
+
+	// admin viewer
+	test(user1, user1, true)
+	test(user20, user1, true)
+	test(user31, user1, true)
+	test(user33, user1, true)
+
+	// non admin viewer
+	test(user4, user4, true)
+	test(user20, user4, true)
+	test(user31, user4, false)
+	test(user33, user4, true)
+	test(user4, nil, true)
+
+	// public user
+	test(user4, user20, true)
+	test(user4, user31, true)
+	test(user4, user33, true)
+
+	// limited user
+	test(user33, user33, true)
+	test(user33, user4, true)
+	test(user33, user29, false)
+	test(user33, nil, false)
+
+	// private user
+	test(user31, user31, true)
+	test(user31, user4, false)
+	test(user31, user20, true)
+	test(user31, user29, false)
+	test(user31, user33, true)
+	test(user31, nil, false)
+}