feat(save): support custom timestamps on commit

setting the input dataset commit.Timestamp value will now be preserved instead
of being overwritten with the current time. Input timestamp will ALWAYS be
converted to UTC time zone.

This change makes it much easier to generate dataset histories with commit
timestamps that aren't in chronological order, which is perfectly fine
according to spec, but will cause bugs for any code that relies on such an
assumption.

base/dsfs/compute_fields.go

@@ -168,8 +168,12 @@ func (cff *computeFieldsFile) handleRows(ctx context.Context) {
 	}
 
 	cff.Lock()
-	// assign timestamp early. saving process on large files can take many minutes
-	cff.ds.Commit.Timestamp = Timestamp()
+	if cff.ds.Commit.Timestamp.IsZero() {
+		// assign timestamp early. saving process on large files can take many minutes
+		cff.ds.Commit.Timestamp = Timestamp()
+	} else {
+		cff.ds.Commit.Timestamp = cff.ds.Commit.Timestamp.In(time.UTC)
+	}
 	cff.acc = dsstats.NewAccumulator(st)
 	cff.Unlock()
 

base/dsfs/dataset.go

@@ -111,6 +111,8 @@ func DerefDataset(ctx context.Context, store qfs.Filesystem, ds *dataset.Dataset
 
 // SaveSwitches represents options for saving a dataset
 type SaveSwitches struct {
+	// Use a custom timestamp, defaults to time.Now if unset
+	Time time.Time
 	// Replace is whether the save is a full replacement or a set of patches to previous
 	Replace bool
 	// Pin is whether the dataset should be pinned

base/dsfs/dataset_test.go

@@ -112,7 +112,6 @@ func TestLoadDataset(t *testing.T) {
 			continue
 		}
 	}
-
 }
 
 func TestCreateDataset(t *testing.T) {
@@ -297,6 +296,34 @@ func TestCreateDataset(t *testing.T) {
 	// case: previous dataset isn't valid
 }
 
+func TestDatasetSaveCustomTimestamp(t *testing.T) {
+	ctx := context.Background()
+	fs := qfs.NewMemFS()
+	privKey := testPeers.GetTestPeerInfo(10).PrivKey
+
+	// use a custom timestamp in local zone. should be converted to UTC for saving
+	ts := time.Date(2100, 1, 2, 3, 4, 5, 6, time.Local)
+
+	ds := &dataset.Dataset{
+		Commit: &dataset.Commit{
+			Timestamp: ts,
+		},
+		Structure: &dataset.Structure{Format: "json", Schema: dataset.BaseSchemaArray},
+	}
+	ds.SetBodyFile(qfs.NewMemfileBytes("/body.json", []byte(`[]`)))
+
+	path, err := CreateDataset(ctx, fs, fs, ds, nil, privKey, SaveSwitches{})
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	got, err := LoadDataset(ctx, fs, path)
+
+	if !ts.In(time.UTC).Equal(got.Commit.Timestamp) {
+		t.Errorf("result timestamp mismatch.\nwant: %q\ngot:  %q", ts.In(time.UTC), got.Commit.Timestamp)
+	}
+}
+
 // BaseTabularSchema is the base schema for tabular data
 // NOTE: Do not use if possible, prefer github.com/qri-io/dataset/tabular
 // TODO(dustmop): Possibly move this to tabular package