chore(model): slightly improve handling of pulling empty blocks (#10679)

In the common case (sparse files enabled, not reusing old data) we'd
optimise away pulling & writing zero blocks. However in the corner cases
we'd go through the whole processing of pulling the block over the
network which is of course entirely unnecessary.

Now, instead, always take an optimised path for all-zeroes blocks. In
the clean case we do nothing, otherwise we materialise a block of zeroes
and write it directly.

---------

Signed-off-by: Jakob Borg <jakob@kastelo.net>

lib/model/folder_sendrecv.go

@@ -1558,10 +1558,21 @@ func (f *sendReceiveFolder) pullBlock(ctx context.Context, state pullBlockState,
 		return
 	}
 
-	if !f.DisableSparseFiles && state.reused == 0 && state.block.IsEmpty() {
+	if state.block.IsEmpty() {
 		// There is no need to request a block of all zeroes. Pretend we
 		// requested it and handled it correctly.
-		state.pullDone(state.block)
+		if state.reused != 0 || f.DisableSparseFiles {
+			// We are reusing a file (contents apparently weren't all-zeroes
+			// previously), or sparse files are disabled, so we need to
+			// actually write the block.
+			zeroes := make([]byte, state.block.Size)
+			err = f.limitedWriteAt(ctx, fd, zeroes, state.block.Offset)
+		}
+		if err != nil {
+			state.fail(fmt.Errorf("save: %w", err))
+		} else {
+			state.pullDone(state.block)
+		}
 		out <- state.sharedPullerState
 		return
 	}
@@ -1607,11 +1618,10 @@ loop:
 		}
 
 		// Verify that the received block matches the desired hash, if not
-		// try pulling it from another device.
-		// For receive-only folders, the hash is not SHA256 as it's an
-		// encrypted hash token. In that case we can't verify the block
-		// integrity so we'll take it on trust. (The other side can and
-		// will verify.)
+		// try pulling it from another device. For receive-encrypted
+		// folders, the hash is not SHA256 as it's an encrypted hash token.
+		// In that case we can't verify the block integrity so we'll take it
+		// on trust. (The other side can and will verify.)
 		if f.Type != config.FolderTypeReceiveEncrypted {
 			lastError = f.verifyBuffer(buf, state.block)
 		}