lib/model, lib/protocol: Index sending/receiving debugging (#9657)

This adds guardrails to the index sending and receiving, to verify that
what we thinks is happening is what actually happens.

lib/db/util.go

@@ -20,10 +20,14 @@ type FileInfoBatch struct {
 	infos   []protocol.FileInfo
 	size    int
 	flushFn func([]protocol.FileInfo) error
+	error   error
 }
 
 // NewFileInfoBatch returns a new FileInfoBatch that calls fn when it's time
-// to flush.
+// to flush. Errors from the flush function are considered non-recoverable;
+// once an error is returned the flush function wil not be called again, and
+// any further calls to Flush will return the same error (unless Reset is
+// called).
 func NewFileInfoBatch(fn func([]protocol.FileInfo) error) *FileInfoBatch {
 	return &FileInfoBatch{flushFn: fn}
 }
@@ -33,6 +37,9 @@ func (b *FileInfoBatch) SetFlushFunc(fn func([]protocol.FileInfo) error) {
 }
 
 func (b *FileInfoBatch) Append(f protocol.FileInfo) {
+	if b.error != nil {
+		panic("bug: calling append on a failed batch")
+	}
 	if b.infos == nil {
 		b.infos = make([]protocol.FileInfo, 0, MaxBatchSizeFiles)
 	}
@@ -45,6 +52,9 @@ func (b *FileInfoBatch) Full() bool {
 }
 
 func (b *FileInfoBatch) FlushIfFull() error {
+	if b.error != nil {
+		return b.error
+	}
 	if b.Full() {
 		return b.Flush()
 	}
@@ -52,10 +62,14 @@ func (b *FileInfoBatch) FlushIfFull() error {
 }
 
 func (b *FileInfoBatch) Flush() error {
+	if b.error != nil {
+		return b.error
+	}
 	if len(b.infos) == 0 {
 		return nil
 	}
 	if err := b.flushFn(b.infos); err != nil {
+		b.error = err
 		return err
 	}
 	b.Reset()
@@ -64,6 +78,7 @@ func (b *FileInfoBatch) Flush() error {
 
 func (b *FileInfoBatch) Reset() {
 	b.infos = nil
+	b.error = nil
 	b.size = 0
 }
 

lib/db/util_test.go

@@ -8,12 +8,14 @@ package db
 
 import (
 	"encoding/json"
+	"errors"
 	"io"
 	"os"
 	"testing"
 
 	"github.com/syncthing/syncthing/lib/db/backend"
 	"github.com/syncthing/syncthing/lib/events"
+	"github.com/syncthing/syncthing/lib/protocol"
 )
 
 // writeJSONS serializes the database to a JSON stream that can be checked
@@ -178,3 +180,53 @@ func snapshot(t testing.TB, fset *FileSet) *Snapshot {
 // 	defer fd.Close()
 // 	writeJSONS(fd, db)
 // }
+
+func TestFileInfoBatchError(t *testing.T) {
+	// Verify behaviour of the flush function returning an error.
+
+	var errReturn error
+	var called int
+	b := NewFileInfoBatch(func([]protocol.FileInfo) error {
+		called += 1
+		return errReturn
+	})
+
+	// Flush should work when the flush function error is nil
+	b.Append(protocol.FileInfo{Name: "test"})
+	if err := b.Flush(); err != nil {
+		t.Fatalf("expected nil, got %v", err)
+	}
+	if called != 1 {
+		t.Fatalf("expected 1, got %d", called)
+	}
+
+	// Flush should fail with an error retur
+	errReturn = errors.New("problem")
+	b.Append(protocol.FileInfo{Name: "test"})
+	if err := b.Flush(); err != errReturn {
+		t.Fatalf("expected %v, got %v", errReturn, err)
+	}
+	if called != 2 {
+		t.Fatalf("expected 2, got %d", called)
+	}
+
+	// Flush function should not be called again when it's already errored,
+	// same error should be returned by Flush()
+	if err := b.Flush(); err != errReturn {
+		t.Fatalf("expected %v, got %v", errReturn, err)
+	}
+	if called != 2 {
+		t.Fatalf("expected 2, got %d", called)
+	}
+
+	// Reset should clear the error (and the file list)
+	errReturn = nil
+	b.Reset()
+	b.Append(protocol.FileInfo{Name: "test"})
+	if err := b.Flush(); err != nil {
+		t.Fatalf("expected nil, got %v", err)
+	}
+	if called != 3 {
+		t.Fatalf("expected 3, got %d", called)
+	}
+}