Add and populate mod time for BaseModel (#4065)

Get the last time a model was modified and return it in BaseModel. This will help with discovering what items can be garbage collected during incomplete backup cleanup as we don't want to accidentally delete in-flight backups.

---

#### Does this PR need a docs update or release note?

- [ ] ✅ Yes, it's included
- [ ] 🕐 Yes, but in a later PR
- [x] ⛔ No

#### Type of change

- [x] 🌻 Feature
- [ ] 🐛 Bugfix
- [ ] 🗺️ Documentation
- [ ] 🤖 Supportability/Tests
- [ ] 💻 CI/Deployment
- [ ] 🧹 Tech Debt/Cleanup

#### Issue(s)

* #3217

#### Test Plan

- [ ] 💪 Manual
- [x] ⚡ Unit test
- [ ] 💚 E2E

src/internal/kopia/model_store.go

@@ -210,6 +210,7 @@ func (ms ModelStore) populateBaseModelFromMetadata(
 	base.ID = model.StableID(id)
 	base.ModelVersion = v
 	base.Tags = m.Labels
+	base.ModTime = m.ModTime
 
 	stripHiddenTags(base.Tags)
 

src/internal/kopia/model_store_test.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"sync"
 	"testing"
+	"time"
 
 	"github.com/alcionai/clues"
 	"github.com/google/uuid"
@@ -34,6 +35,18 @@ func getModelStore(t *testing.T, ctx context.Context) *ModelStore {
 	return &ModelStore{c: c, modelVersion: globalModelVersion}
 }
 
+func assertEqualNoModTime(t *testing.T, expected, got *fooModel) {
+	t.Helper()
+
+	expectedClean := *expected
+	gotClean := *got
+
+	expectedClean.ModTime = time.Time{}
+	gotClean.ModTime = time.Time{}
+
+	assert.Equal(t, expectedClean, gotClean)
+}
+
 // ---------------
 // unit tests
 // ---------------
@@ -259,6 +272,8 @@ func (suite *ModelStoreIntegrationSuite) TestPutGet() {
 			// Avoid some silly test errors from comparing nil to empty map.
 			foo.Tags = map[string]string{}
 
+			startTime := time.Now()
+
 			err := suite.m.Put(suite.ctx, test.s, foo)
 			test.check(t, err, clues.ToCore(err))
 
@@ -273,11 +288,17 @@ func (suite *ModelStoreIntegrationSuite) TestPutGet() {
 			returned := &fooModel{}
 			err = suite.m.Get(suite.ctx, test.s, foo.ID, returned)
 			require.NoError(t, err, clues.ToCore(err))
-			assert.Equal(t, foo, returned)
+
+			assertEqualNoModTime(t, foo, returned)
+			assert.WithinDuration(t, startTime, returned.ModTime, 5*time.Second)
+
+			returned = &fooModel{}
 
 			err = suite.m.GetWithModelStoreID(suite.ctx, test.s, foo.ModelStoreID, returned)
 			require.NoError(t, err, clues.ToCore(err))
-			assert.Equal(t, foo, returned)
+
+			assertEqualNoModTime(t, foo, returned)
+			assert.WithinDuration(t, startTime, returned.ModTime, 5*time.Second)
 		})
 	}
 }
@@ -324,11 +345,11 @@ func (suite *ModelStoreIntegrationSuite) TestPutGet_PreSetID() {
 
 			err = suite.m.Get(suite.ctx, mdl, foo.ID, returned)
 			require.NoError(t, err, clues.ToCore(err))
-			assert.Equal(t, foo, returned)
+			assertEqualNoModTime(t, foo, returned)
 
 			err = suite.m.GetWithModelStoreID(suite.ctx, mdl, foo.ModelStoreID, returned)
 			require.NoError(t, err, clues.ToCore(err))
-			assert.Equal(t, foo, returned)
+			assertEqualNoModTime(t, foo, returned)
 		})
 	}
 }
@@ -350,11 +371,11 @@ func (suite *ModelStoreIntegrationSuite) TestPutGet_WithTags() {
 	returned := &fooModel{}
 	err = suite.m.Get(suite.ctx, theModelType, foo.ID, returned)
 	require.NoError(t, err, clues.ToCore(err))
-	assert.Equal(t, foo, returned)
+	assertEqualNoModTime(t, foo, returned)
 
 	err = suite.m.GetWithModelStoreID(suite.ctx, theModelType, foo.ModelStoreID, returned)
 	require.NoError(t, err, clues.ToCore(err))
-	assert.Equal(t, foo, returned)
+	assertEqualNoModTime(t, foo, returned)
 }
 
 func (suite *ModelStoreIntegrationSuite) TestGet_NotFoundErrors() {
@@ -559,7 +580,16 @@ func (suite *ModelStoreIntegrationSuite) TestGetOfTypeWithTags() {
 			ids, err := suite.m.GetIDsForType(suite.ctx, test.s, test.tags)
 			require.NoError(t, err, clues.ToCore(err))
 
-			assert.ElementsMatch(t, expected, ids)
+			cleanIDs := make([]*model.BaseModel, 0, len(ids))
+
+			for _, id := range ids {
+				id2 := *id
+				id2.ModTime = time.Time{}
+
+				cleanIDs = append(cleanIDs, &id2)
+			}
+
+			assert.ElementsMatch(t, expected, cleanIDs)
 		})
 	}
 }
@@ -627,7 +657,7 @@ func (suite *ModelStoreIntegrationSuite) TestPutUpdate() {
 
 			err = m.GetWithModelStoreID(ctx, theModelType, foo.ModelStoreID, returned)
 			require.NoError(t, err, clues.ToCore(err))
-			assert.Equal(t, foo, returned)
+			assertEqualNoModTime(t, foo, returned)
 
 			ids, err := m.GetIDsForType(ctx, theModelType, nil)
 			require.NoError(t, err, clues.ToCore(err))
@@ -822,7 +852,7 @@ func (suite *ModelStoreRegressionSuite) TestFailDuringWriteSessionHasNoVisibleEf
 
 	err = m.GetWithModelStoreID(ctx, theModelType, foo.ModelStoreID, returned)
 	require.NoError(t, err, clues.ToCore(err))
-	assert.Equal(t, foo, returned)
+	assertEqualNoModTime(t, foo, returned)
 }
 
 func openConnAndModelStore(

src/internal/model/model.go

@@ -1,6 +1,8 @@
 package model
 
 import (
+	"time"
+
 	"github.com/kopia/kopia/repo/manifest"
 )
 
@@ -69,6 +71,7 @@ type BaseModel struct {
 	// the struct are not serialized directly into the stored model, but are part
 	// of the metadata for the model.
 	Tags    map[string]string `json:"-"`
+	ModTime time.Time         `json:"-"`
 }
 
 func (bm *BaseModel) Base() *BaseModel {