block_types: add a NewEmptier func for block-making function

So the prefetcher can get a new block function without holding a
reference to the block that supplied it.

Issue: #1958

libkbfs/block_types.go

@@ -148,6 +148,11 @@ func (cb *CommonBlock) NewEmpty() Block {
 	return NewCommonBlock()
 }
 
+// NewEmptier implements the Block interface for CommonBlock.
+func (cb *CommonBlock) NewEmptier() func() Block {
+	return NewCommonBlock
+}
+
 // ToCommonBlock implements the Block interface for CommonBlock.
 func (cb *CommonBlock) ToCommonBlock() *CommonBlock {
 	return cb
@@ -226,6 +231,11 @@ func (db *DirBlock) NewEmpty() Block {
 	return NewDirBlock()
 }
 
+// NewEmptier implements the Block interface for DirBlock.
+func (db *DirBlock) NewEmptier() func() Block {
+	return NewDirBlock
+}
+
 // IsTail implements the Block interface for DirBlock.
 func (db *DirBlock) IsTail() bool {
 	if db.IsInd {
@@ -449,6 +459,11 @@ func (fb *FileBlock) NewEmpty() Block {
 	return &FileBlock{}
 }
 
+// NewEmptier implements the Block interface for FileBlock.
+func (fb *FileBlock) NewEmptier() func() Block {
+	return NewFileBlock
+}
+
 // IsTail implements the Block interface for FileBlock.
 func (fb *FileBlock) IsTail() bool {
 	if fb.IsInd {

libkbfs/crypto_common_test.go

@@ -94,6 +94,10 @@ func (tb TestBlock) NewEmpty() Block {
 	return &TestBlock{}
 }
 
+func (tb TestBlock) NewEmptier() func() Block {
+	return tb.NewEmpty
+}
+
 func (tb *TestBlock) Set(other Block) {
 	otherTb := other.(*TestBlock)
 	tb.A = otherTb.A
@@ -396,6 +400,10 @@ func (tba testBlockArray) NewEmpty() Block {
 	return &testBlockArray{}
 }
 
+func (tba testBlockArray) NewEmptier() func() Block {
+	return tba.NewEmpty
+}
+
 func (tba testBlockArray) ToCommonBlock() *CommonBlock {
 	return nil
 }

libkbfs/interfaces.go

@@ -142,6 +142,9 @@ type Block interface {
 	SetEncodedSize(size uint32)
 	// NewEmpty returns a new block of the same type as this block
 	NewEmpty() Block
+	// NewEmptier returns a function that creates a new block of the
+	// same type as this block.
+	NewEmptier() func() Block
 	// Set sets this block to the same value as the passed-in block
 	Set(other Block)
 	// ToCommonBlock retrieves this block as a *CommonBlock.

libkbfs/prefetcher.go

@@ -360,8 +360,8 @@ func (p *blockPrefetcher) request(ctx context.Context, priority int,
 	if !isPrefetchWaiting {
 		// If the block isn't in the tree, we add it with a block count of 1 (a
 		// later TriggerPrefetch will come in and decrement it).
-		req := &prefetchRequest{ptr, block.NewEmpty, kmd, priority, lifetime,
-			NoPrefetch, action, nil}
+		req := &prefetchRequest{ptr, block.NewEmptier(), kmd, priority,
+			lifetime, NoPrefetch, action, nil}
 		pre = p.newPrefetch(1, false, req)
 		p.prefetches[ptr.ID] = pre
 	}
@@ -971,7 +971,7 @@ func (p *blockPrefetcher) ProcessBlockForPrefetch(ctx context.Context,
 	ptr BlockPointer, block Block, kmd KeyMetadata, priority int,
 	lifetime BlockCacheLifetime, prefetchStatus PrefetchStatus,
 	action BlockRequestAction) {
-	req := &prefetchRequest{ptr, block.NewEmpty, kmd, priority, lifetime,
+	req := &prefetchRequest{ptr, block.NewEmptier(), kmd, priority, lifetime,
 		prefetchStatus, action, nil}
 	if prefetchStatus == FinishedPrefetch {
 		// Finished prefetches can always be short circuited.