Drain sync event channel before returning (#253)

Not waiting means the last few events may or may not be printed.
This is relevant in the mode where sync runs once and then terminates.

cmd/sync/output.go

@@ -1,10 +1,10 @@
 package sync
 
 import (
+	"bufio"
 	"context"
 	"encoding/json"
 	"io"
-	"log"
 
 	"github.com/databricks/bricks/libs/sync"
 )
@@ -30,8 +30,10 @@ func jsonOutput(ctx context.Context, ch <-chan sync.Event, w io.Writer) {
 	}
 }
 
-// Read synchronization events and log them at the INFO level.
+// Read synchronization events and write them as text to the specified writer (typically stdout).
-func logOutput(ctx context.Context, ch <-chan sync.Event) {
+func textOutput(ctx context.Context, ch <-chan sync.Event, w io.Writer) {
+	bw := bufio.NewWriter(w)
+
 	for {
 		select {
 		case <-ctx.Done():
@@ -43,7 +45,9 @@ func logOutput(ctx context.Context, ch <-chan sync.Event) {
 			// Log only if something actually happened.
 			// Sync events produce an empty string if nothing happened.
 			if str := e.String(); str != "" {
-				log.Printf("[INFO] %s", e.String())
+				bw.WriteString(str)
+				bw.WriteString("\r\n")
+				bw.Flush()
 			}
 		}
 	}

cmd/sync/sync.go

@@ -1,9 +1,12 @@
 package sync
 
 import (
+	"context"
 	"flag"
 	"fmt"
+	"io"
 	"path/filepath"
+	stdsync "sync"
 	"time"
 
 	"github.com/databricks/bricks/bundle"
@@ -95,18 +98,32 @@ var syncCmd = &cobra.Command{
 			return err
 		}
 
+		var outputFunc func(context.Context, <-chan sync.Event, io.Writer)
 		switch output {
 		case flags.OutputText:
-			go logOutput(ctx, s.Events())
+			outputFunc = textOutput
 		case flags.OutputJSON:
-			go jsonOutput(ctx, s.Events(), cmd.OutOrStdout())
+			outputFunc = jsonOutput
+		}
+
+		var wg stdsync.WaitGroup
+		if outputFunc != nil {
+			wg.Add(1)
+			go func() {
+				defer wg.Done()
+				outputFunc(ctx, s.Events(), cmd.OutOrStdout())
+			}()
 		}
 
 		if watch {
-			return s.RunContinuous(ctx)
+			err = s.RunContinuous(ctx)
+		} else {
+			err = s.RunOnce(ctx)
 		}
 
-		return s.RunOnce(ctx)
+		s.Close()
+		wg.Wait()
+		return err
 	},
 
 	ValidArgsFunction: func(cmd *cobra.Command, args []string, toComplete string) ([]string, cobra.ShellCompDirective) {

libs/sync/event.go

@@ -142,6 +142,7 @@ func newEventComplete(seq int, put []string, delete []string) Event {
 
 type EventNotifier interface {
 	Notify(ctx context.Context, event Event)
+	Close()
 }
 
 // ChannelNotifier implements [EventNotifier] and sends events to its channel.
@@ -156,9 +157,17 @@ func (n *ChannelNotifier) Notify(ctx context.Context, e Event) {
 	}
 }
 
+func (n *ChannelNotifier) Close() {
+	close(n.ch)
+}
+
 // NopNotifier implements [EventNotifier] and does nothing.
 type NopNotifier struct{}
 
 func (n *NopNotifier) Notify(ctx context.Context, e Event) {
 	// Discard
 }
+
+func (n *NopNotifier) Close() {
+	// Nothing to do
+}

libs/sync/sync.go

@@ -96,6 +96,14 @@ func (s *Sync) Events() <-chan Event {
 	return ch
 }
 
+func (s *Sync) Close() {
+	if s.notifier == nil {
+		return
+	}
+	s.notifier.Close()
+	s.notifier = nil
+}
+
 func (s *Sync) notifyStart(ctx context.Context, d diff) {
 	// If this is not the initial iteration we can ignore no-ops.
 	if s.seq > 0 && d.IsEmpty() {