Make sync mode of "Geth for mobile" configurable (ethereum#77)

* Make Geth for mobile's sync mode configurable

1. Default to LightSync to maintain backward compatibility.
2. Gomobile, the tool for generating Android bindings does not handle
enums, so, a new set of integer constants have to be created.

mobile/geth.go

@@ -38,6 +38,14 @@ import (
  whisper "github.com/ethereum/go-ethereum/whisper/whisperv6"
 )
 
+// I am intentionally duplicating these constants different from downloader.SyncMode integer values, to ensure the
+// backwards compatibility where the mobile node defaults to LightSync.
+const SyncModeUnset = 0 // will be treated as SyncModeLightSync
+const SyncModeFullSync = 1
+const SyncModeFastSync = 2
+const SyncModeLightSync = 3
+const SyncModeCeloLatestSync = 4
+
 // NodeConfig represents the collection of configuration values to fine tune the Geth
 // node embedded into a mobile process. The available values are a subset of the
 // entire API provided by go-ethereum to reduce the maintenance surface and dev
@@ -76,6 +84,11 @@ type NodeConfig struct {
 
  // Listening address of pprof server.
  PprofAddress string
+
+ // Sync mode for the node (eth/downloader/modes.go)
+ // This has to be integer since Enum exports to Java are not supported by "gomobile"
+ // See getSyncMode(syncMode int)
+ SyncMode int
 }
 
 // defaultNodeConfig contains the default node configuration values to use if all
@@ -162,7 +175,7 @@ func NewNode(datadir string, config *NodeConfig) (stack *Node, _ error) {
  // Once https:/celo-org/geth/pull/62 is landed,
  // change the ethConf.SyncMode to downloader.CeloLatestSync.
  // It might be better to make this configurable.
- ethConf.SyncMode = downloader.LightSync
+ ethConf.SyncMode = getSyncMode(config.SyncMode)
  ethConf.NetworkId = uint64(config.EthereumNetworkID)
  ethConf.DatabaseCache = config.EthereumDatabaseCache
  if err := rawStack.Register(func(ctx *node.ServiceContext) (node.Service, error) {
@@ -193,6 +206,25 @@ func NewNode(datadir string, config *NodeConfig) (stack *Node, _ error) {
  return &Node{rawStack}, nil
 }
 
+func getSyncMode(syncMode int) downloader.SyncMode {
+ switch syncMode {
+ case SyncModeFullSync:
+ return downloader.FullSync
+ case SyncModeFastSync:
+ return downloader.FastSync
+ case SyncModeUnset:
+ fallthrough
+ // If unset, default to light sync.
+ // This maintains backward compatibility.
+ case SyncModeLightSync:
+ return downloader.LightSync
+ case SyncModeCeloLatestSync:
+ return downloader.CeloLatestSync
+ default:
+ panic(fmt.Sprintf("Unexpected sync mode value: %d", syncMode))
+ }
+}
+
 // Start creates a live P2P node and starts running it.
 func (n *Node) Start() error {
  return n.node.Start()