all, state: unexpose GenesisDoc, ChainID fields make them accessor methods

Fixes #671

Unexpose GenesisDoc and ChainID fields to avoid them being
serialized to the DB on every block write/state.Save()

A GenesisDoc can now be alternatively written to the state's
database, by serializing its JSON as a value of key "genesis-doc".

There are now accessors and a setter for these attributes:
- state.GenesisDoc() (*types.GenesisDoc, error)
- state.ChainID() (string, error)
- state.SetGenesisDoc(*types.GenesisDoc)

This is a breaking change since it changes how the state's
serialization and requires that if loading the GenesisDoc entirely
from the database, you'll need to set its value in the database
as the GenesisDoc's JSON marshaled bytes.

state/state.go

@@ -2,6 +2,7 @@ package state
 
 import (
 	"bytes"
+	"errors"
 	"fmt"
 	"io/ioutil"
 	"sync"
@@ -38,9 +39,11 @@ type State struct {
 	mtx sync.Mutex
 	db  dbm.DB
 
-	// should not change
-	GenesisDoc *types.GenesisDoc
-	ChainID    string
+	// genesisDoc is the memoized genesisDoc to cut down
+	// the number of unnecessary DB lookups since we no longer
+	// directly serialize the GenesisDoc in state.
+	// See https://github.com/tendermint/tendermint/issues/671.
+	genesisDoc *types.GenesisDoc
 
 	// These fields are updated by SetBlockAndValidators.
 	// LastBlockHeight=0 at genesis (ie. block(H=0) does not exist)
@@ -100,7 +103,6 @@ func loadState(db dbm.DB, key []byte) *State {
 		}
 		// TODO: ensure that buf is completely read.
 	}
-
 	return s
 }
 
@@ -113,8 +115,6 @@ func (s *State) SetLogger(l log.Logger) {
 func (s *State) Copy() *State {
 	return &State{
 		db:                          s.db,
-		GenesisDoc:                  s.GenesisDoc,
-		ChainID:                     s.ChainID,
 		LastBlockHeight:             s.LastBlockHeight,
 		LastBlockID:                 s.LastBlockID,
 		LastBlockTime:               s.LastBlockTime,
@@ -127,12 +127,57 @@ func (s *State) Copy() *State {
 	}
 }
 
+var (
+	errNilGenesisDoc = errors.New("no genesisDoc was found")
+
+	genesisDBKey = []byte("genesis-doc")
+)
+
+// GenesisDoc is the accessor to retrieve the genesisDoc associated
+// with a state. If the state has no set GenesisDoc, it fetches from
+// its database the JSON marshaled bytes keyed by "genesis-doc", and
+// parses the GenesisDoc from that memoizing it for later use.
+// If you'd like to change the value of the GenesisDoc, invoke SetGenesisDoc.
+func (s *State) GenesisDoc() (*types.GenesisDoc, error) {
+	s.mtx.Lock()
+	defer s.mtx.Unlock()
+
+	if s.genesisDoc == nil {
+		retrGenesisDocBytes := s.db.Get(genesisDBKey)
+		if len(retrGenesisDocBytes) == 0 {
+			return nil, errNilGenesisDoc
+		}
+		genDoc, err := types.GenesisDocFromJSON(retrGenesisDocBytes)
+		if err != nil {
+			return nil, err
+		}
+		s.genesisDoc = genDoc
+	}
+	return s.genesisDoc, nil
+}
+
+func (s *State) ChainID() (string, error) {
+	genDoc, err := s.GenesisDoc()
+	if err != nil {
+		return "", err
+	}
+	return genDoc.ChainID, nil
+}
+
 // Save persists the State to the database.
 func (s *State) Save() {
 	s.mtx.Lock()
-	defer s.mtx.Unlock()
 	s.saveValidatorsInfo()
 	s.db.SetSync(stateKey, s.Bytes())
+	s.mtx.Unlock()
+}
+
+// SetGenesisDoc sets the internal genesisDoc, but doesn't
+// serialize it to the database, until Save is invoked.
+func (s *State) SetGenesisDoc(genDoc *types.GenesisDoc) {
+	s.mtx.Lock()
+	s.genesisDoc = genDoc
+	s.mtx.Unlock()
 }
 
 // SaveABCIResponses persists the ABCIResponses to the database.
@@ -264,12 +309,18 @@ func (s *State) GetValidators() (*types.ValidatorSet, *types.ValidatorSet) {
 	return s.LastValidators, s.Validators
 }
 
+var blankConsensusParams = types.ConsensusParams{}
+
 // Params returns the consensus parameters used for
 // validating blocks
 func (s *State) Params() types.ConsensusParams {
 	// TODO: this should move into the State proper
 	// when we allow the app to change it
-	return *s.GenesisDoc.ConsensusParams
+	genDoc, err := s.GenesisDoc()
+	if err != nil || genDoc == nil {
+		return blankConsensusParams
+	}
+	return *genDoc.ConsensusParams
 }
 
 //------------------------------------------------------------------------
@@ -364,9 +415,9 @@ func MakeGenesisState(db dbm.DB, genDoc *types.GenesisDoc) (*State, error) {
 	}
 
 	return &State{
-		db:                          db,
-		GenesisDoc:                  genDoc,
-		ChainID:                     genDoc.ChainID,
+		db: db,
+
+		genesisDoc:                  genDoc,
 		LastBlockHeight:             0,
 		LastBlockID:                 types.BlockID{},
 		LastBlockTime:               genDoc.GenesisTime,