Switch DataCollection to return a channel (#244)

* Change DataCollection to return channel directly

Precursor to restoring multiple items from kopia. Allows one to keep a
DataCollection open until all items are processed without blocking
consumers of the DataCollection (they can use a select-block if needed).

* Update tests for new DataCollection interface

* Handle context cancellation with DataCollection

src/internal/connector/exchange_data_collection.go

@@ -14,9 +14,11 @@ const (
 // A DataCollection represents a collection of data of the
 // same type (e.g. mail)
 type DataCollection interface {
-	// Returns either the next item in the collection or an error if one occurred.
-	// If not more items are available in the collection, returns (nil, nil).
-	NextItem() (DataStream, error)
+	// Items returns a channel from which items in the collection can be read.
+	// Each returned struct contains the next item in the collection
+	// The channel is closed when there are no more items in the collection or if
+	// an unrecoverable error caused an early termination in the sender.
+	Items() <-chan DataStream
 	// FullPath returns a slice of strings that act as metadata tags for this
 	// DataCollection. Returned items should be ordered from most generic to least
 	// generic. For example, a DataCollection for emails from a specific user
@@ -27,9 +29,9 @@ type DataCollection interface {
 // DataStream represents a single item within a DataCollection
 // that can be consumed as a stream (it embeds io.Reader)
 type DataStream interface {
-	// Returns an io.Reader for the DataStream
+	// ToReader returns an io.Reader for the DataStream
 	ToReader() io.ReadCloser
-	// Provides a unique identifier for this data
+	// UUID provides a unique identifier for this data
 	UUID() string
 }
 
@@ -40,12 +42,8 @@ type DataStream interface {
 type ExchangeDataCollection struct {
 	// M365 user
 	user string
-	// TODO: We would want to replace this with a channel so that we
-	// don't need to wait for all data to be retrieved before reading it out
-
-	data chan ExchangeData
+	data chan DataStream
 	// FullPath is the slice representation of the action context passed down through the hierarchy.
-
 	//The original request can be gleaned from the slice. (e.g. {<tenant ID>, <user ID>, "emails"})
 	fullPath []string
 }
@@ -54,13 +52,13 @@ type ExchangeDataCollection struct {
 func NewExchangeDataCollection(aUser string, pathRepresentation []string) ExchangeDataCollection {
 	collection := ExchangeDataCollection{
 		user:     aUser,
-		data:     make(chan ExchangeData, collectionChannelBufferSize),
+		data:     make(chan DataStream, collectionChannelBufferSize),
 		fullPath: pathRepresentation,
 	}
 	return collection
 }
 
-func (edc *ExchangeDataCollection) PopulateCollection(newData ExchangeData) {
+func (edc *ExchangeDataCollection) PopulateCollection(newData *ExchangeData) {
 	edc.data <- newData
 }
 
@@ -76,14 +74,8 @@ func (edc *ExchangeDataCollection) Length() int {
 	return len(edc.data)
 }
 
-// NextItem returns either the next item in the collection or an error if one occurred.
-// If not more items are available in the collection, returns (nil, nil).
-func (edc *ExchangeDataCollection) NextItem() (DataStream, error) {
-	item, ok := <-edc.data
-	if !ok {
-		return nil, io.EOF
-	}
-	return &item, nil
+func (edc *ExchangeDataCollection) Items() <-chan DataStream {
+	return edc.data
 }
 
 func (edc *ExchangeDataCollection) FullPath() []string {

src/internal/connector/exchange_data_collection_test.go

@@ -61,27 +61,26 @@ func (suite *ExchangeDataCollectionSuite) TestExchangeDataCollection_PopulateCol
 	expected := len(inputStrings) / 2 // We are using pairs
 	edc := NewExchangeDataCollection("Fletcher", []string{"sugar", "horses", "painted red"})
 	for i := 0; i < expected; i++ {
-		edc.PopulateCollection(ExchangeData{id: inputStrings[i*2], message: []byte(inputStrings[i*2+1])})
+		edc.PopulateCollection(&ExchangeData{id: inputStrings[i*2], message: []byte(inputStrings[i*2+1])})
 	}
 	suite.Equal(expected, edc.Length())
 }
+
 func (suite *ExchangeDataCollectionSuite) TestExchangeDataCollection_NextItem() {
 	inputStrings := []string{"Jack", "and", "Jill", "went", "up", "the", "hill to",
 		"fetch", "a", "pale", "of", "water"}
 	expected := len(inputStrings) / 2 // We are using pairs
 	edc := NewExchangeDataCollection("Fletcher", []string{"sugar", "horses", "painted red"})
 	for i := 0; i < expected; i++ {
-		edc.PopulateCollection(ExchangeData{id: inputStrings[i*2], message: []byte(inputStrings[i*2+1])})
+		edc.PopulateCollection(&ExchangeData{id: inputStrings[i*2], message: []byte(inputStrings[i*2+1])})
 	}
 	edc.FinishPopulation() // finished writing
-	for i := 0; i < 6; i++ {
-		data, err := edc.NextItem()
-		assert.Nil(suite.T(), err)
-		assert.NotNil(suite.T(), data)
-	}
-	// Need that EOF
-	data, err := edc.NextItem()
-	assert.Nil(suite.T(), data)
-	assert.NotNil(suite.T(), err)
 
+	count := 0
+	for data := range edc.Items() {
+		assert.NotNil(suite.T(), data)
+		count++
+	}
+
+	assert.Equal(suite.T(), expected, count)
 }

src/internal/connector/graph_connector.go

@@ -6,7 +6,6 @@ import (
 	"bytes"
 	"context"
 	"fmt"
-	"io"
 
 	az "github.com/Azure/azure-sdk-for-go/sdk/azidentity"
 	"github.com/alcionai/corso/internal/connector/support"
@@ -168,14 +167,19 @@ func (gc *GraphConnector) RestoreMessages(ctx context.Context, dc DataCollection
 	var errs error
 	// must be user.GetId(), PrimaryName no longer works 6-15-2022
 	user := dc.FullPath()[1]
+	items := dc.Items()
+
 	for {
-		data, err := dc.NextItem()
-		if err == io.EOF {
-			break
+		select {
+		case <-ctx.Done():
+			return support.WrapAndAppend("context cancelled", ctx.Err(), errs)
+		case data, ok := <-items:
+			if !ok {
+				return errs
 			}
 
 			buf := &bytes.Buffer{}
-		_, err = buf.ReadFrom(data.ToReader())
+			_, err := buf.ReadFrom(data.ToReader())
 			if err != nil {
 				errs = support.WrapAndAppend(data.UUID(), err, errs)
 				continue
@@ -210,7 +214,7 @@ func (gc *GraphConnector) RestoreMessages(ctx context.Context, dc DataCollection
 			}
 			// This completes the restore loop for a message..
 		}
-	return errs
+	}
 }
 
 // serializeMessages: Temp Function as place Holder until Collections have been added
@@ -286,7 +290,7 @@ func (gc *GraphConnector) serializeMessages(ctx context.Context, user string) ([
 				return true
 			}
 			if byteArray != nil {
-				edc.PopulateCollection(ExchangeData{id: *message.GetId(), message: byteArray})
+				edc.PopulateCollection(&ExchangeData{id: *message.GetId(), message: byteArray})
 			}
 			return true
 		}

src/internal/connector/graph_connector_test.go

@@ -86,7 +86,7 @@ func (suite *GraphConnectorIntegrationSuite) TestGraphConnector_restoreMessages(
 	}
 	ds := ExchangeData{id: "test", message: bytes}
 	edc := NewExchangeDataCollection("tenant", []string{"tenantId", evs[user], mailCategory, "Inbox"})
-	edc.PopulateCollection(ds)
+	edc.PopulateCollection(&ds)
 	edc.FinishPopulation()
 	err = suite.connector.RestoreMessages(context.Background(), &edc)
 	assert.NoError(suite.T(), err)

src/internal/connector/mockconnector/mock_data_collection.go

@@ -13,7 +13,6 @@ import (
 type MockExchangeDataCollection struct {
 	fullPath     []string
 	messageCount int
-	messagesRead int
 }
 
 var (
@@ -27,7 +26,6 @@ func NewMockExchangeDataCollection(pathRepresentation []string, numMessagesToRet
 	collection := &MockExchangeDataCollection{
 		fullPath:     pathRepresentation,
 		messageCount: numMessagesToReturn,
-		messagesRead: 0,
 	}
 	return collection
 }
@@ -36,16 +34,22 @@ func (medc *MockExchangeDataCollection) FullPath() []string {
 	return append([]string{}, medc.fullPath...)
 }
 
-// NextItem returns either the next item in the collection or an error if one occurred.
-// If not more items are available in the collection, returns (nil, nil).
-func (medc *MockExchangeDataCollection) NextItem() (connector.DataStream, error) {
-	if medc.messagesRead < medc.messageCount {
-		medc.messagesRead++
+// Items returns a channel that has the next items in the collection. The
+// channel is closed when there are no more items available.
+func (medc *MockExchangeDataCollection) Items() <-chan connector.DataStream {
+	res := make(chan connector.DataStream)
+
+	go func() {
+		defer close(res)
+
+		for i := 0; i < medc.messageCount; i++ {
 			// We can plug in whatever data we want here (can be an io.Reader to a test data file if needed)
 			m := []byte("test message")
-		return &MockExchangeData{uuid.NewString(), io.NopCloser(bytes.NewReader(m))}, nil
+			res <- &MockExchangeData{uuid.NewString(), io.NopCloser(bytes.NewReader(m))}
 		}
-	return nil, io.EOF
+	}()
+
+	return res
 }
 
 // ExchangeData represents a single item retrieved from exchange

src/internal/connector/mockconnector/mock_data_collection_test.go

@@ -1,7 +1,6 @@
 package mockconnector_test
 
 import (
-	"io"
 	"io/ioutil"
 	"testing"
 
@@ -24,7 +23,7 @@ func (suite *MockExchangeDataCollectionSuite) TestMockExchangeDataCollection() {
 
 	messagesRead := 0
 
-	for item, err := mdc.NextItem(); err != io.EOF; item, err = mdc.NextItem() {
+	for item := range mdc.Items() {
 		_, err := ioutil.ReadAll(item.ToReader())
 		assert.NoError(suite.T(), err)
 		messagesRead++

src/internal/kopia/data_collection.go

@@ -17,13 +17,16 @@ type singleItemCollection struct {
 	used   bool
 }
 
-func (sic *singleItemCollection) NextItem() (connector.DataStream, error) {
+func (sic *singleItemCollection) Items() <-chan connector.DataStream {
 	if sic.used {
-		return nil, io.EOF
+		return nil
 	}
 
 	sic.used = true
-	return sic.stream, nil
+	res := make(chan connector.DataStream, 1)
+	res <- sic.stream
+	close(res)
+	return res
 }
 
 func (sic singleItemCollection) FullPath() []string {

src/internal/kopia/data_collection_test.go

@@ -50,15 +50,16 @@ func (suite *SingleItemCollectionUnitSuite) TestReturnsOnlyOneItem() {
 		path:   []string{},
 	}
 
-	returnedStream, err := c.NextItem()
-	require.NoError(t, err)
-
+	count := 0
+	for returnedStream := range c.Items() {
 		assert.Equal(t, returnedStream.UUID(), uuid)
 
-	_, err = c.NextItem()
-	assert.ErrorIs(t, err, io.EOF)
-
 		buf, err := ioutil.ReadAll(returnedStream.ToReader())
 		require.NoError(t, err)
 		assert.Equal(t, buf, data)
+
+		count++
+	}
+
+	assert.Equal(t, 1, count)
 }

src/internal/kopia/kopia.go

@@ -2,7 +2,6 @@ package kopia
 
 import (
 	"context"
-	"io"
 
 	"github.com/kopia/kopia/fs"
 	"github.com/kopia/kopia/fs/virtualfs"
@@ -173,22 +172,23 @@ func getStreamItemFunc(
 	collection connector.DataCollection,
 ) func(context.Context, func(context.Context, fs.Entry) error) error {
 	return func(ctx context.Context, cb func(context.Context, fs.Entry) error) error {
+		items := collection.Items()
 		for {
-			e, err := collection.NextItem()
-			if err != nil {
-				if err == io.EOF {
+			select {
+			case <-ctx.Done():
+				return ctx.Err()
+			case e, ok := <-items:
+				if !ok {
 					return nil
 				}
 
-				return errors.Wrap(err, "materializing directory entry")
-			}
-
 				entry := virtualfs.StreamingFileFromReader(e.UUID(), e.ToReader())
-			if err = cb(ctx, entry); err != nil {
+				if err := cb(ctx, entry); err != nil {
 					return errors.Wrap(err, "executing callback")
 				}
 			}
 		}
+	}
 }
 
 // buildKopiaDirs recursively builds a directory hierarchy from the roots up.

src/internal/kopia/kopia_test.go

@@ -335,15 +335,16 @@ func (suite *KopiaIntegrationSuite) TestBackupAndRestoreSingleItem() {
 
 	assert.Equal(t, c.FullPath(), testPath)
 
-	resultStream, err := c.NextItem()
-	require.NoError(t, err)
-
-	_, err = c.NextItem()
-	assert.ErrorIs(t, err, io.EOF)
-
+	count := 0
+	for resultStream := range c.Items() {
 		buf, err := ioutil.ReadAll(resultStream.ToReader())
 		require.NoError(t, err)
 		assert.Equal(t, buf, testFileData)
+
+		count++
+	}
+
+	assert.Equal(t, 1, count)
 }
 
 // TestBackupAndRestoreSingleItem_Errors exercises the public RestoreSingleItem