libs/db: conditional compilation (#3628)

* libs/db: conditional compilation

For cleveldb: go build -tags cleveldb
For boltdb: go build -tags boltdb

Fixes #3611

* document db_backend param better

* remove deprecated LevelDBBackend

* update changelog

* add missing lines

* add new line

* fix TestRemoteDB

* add a line about boltdb tag

* Revert "remove deprecated LevelDBBackend"

This reverts commit 1aa85453f76605e0c4d967601bbe26240e668d51.

* make PR non breaking

* change DEPRECATED label format

https://stackoverflow.com/a/36360323/820520

libs/db/backend_test.go

@@ -9,6 +9,7 @@ import (
 
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
+
 	cmn "github.com/tendermint/tendermint/libs/common"
 )
 

libs/db/boltdb.go

@@ -1,3 +1,5 @@
+// +build boltdb
+
 package db
 
 import (

libs/db/boltdb_test.go

@@ -1,3 +1,5 @@
+// +build boltdb
+
 package db
 
 import (

libs/db/c_level_db.go

@@ -1,4 +1,4 @@
-// +build gcc
+// +build cleveldb
 
 package db
 

libs/db/c_level_db_test.go

@@ -1,4 +1,4 @@
-// +build gcc
+// +build cleveldb
 
 package db
 
@@ -93,7 +93,7 @@ func TestCLevelDBBackend(t *testing.T) {
 	// Can't use "" (current directory) or "./" here because levigo.Open returns:
 	// "Error initializing DB: IO error: test_XXX.db: Invalid argument"
 	dir := os.TempDir()
-	db := NewDB(name, LevelDBBackend, dir)
+	db := NewDB(name, CLevelDBBackend, dir)
 	defer cleanupDBDir(dir, name)
 
 	_, ok := db.(*CLevelDB)
@@ -103,7 +103,7 @@ func TestCLevelDBBackend(t *testing.T) {
 func TestCLevelDBStats(t *testing.T) {
 	name := fmt.Sprintf("test_%x", cmn.RandStr(12))
 	dir := os.TempDir()
-	db := NewDB(name, LevelDBBackend, dir)
+	db := NewDB(name, CLevelDBBackend, dir)
 	defer cleanupDBDir(dir, name)
 
 	assert.NotEmpty(t, db.Stats())

libs/db/db.go

@@ -5,18 +5,37 @@ import (
 	"strings"
 )
 
-//----------------------------------------
-// Main entry
-
 type DBBackendType string
 
+// These are valid backend types.
 const (
-	LevelDBBackend   DBBackendType = "leveldb" // legacy, defaults to goleveldb unless +gcc
-	CLevelDBBackend  DBBackendType = "cleveldb"
+	// LevelDBBackend is a legacy type. Defaults to goleveldb unless cleveldb
+	// build tag was used, in which it becomes cleveldb.
+	// Deprecated: Use concrete types (golevedb, cleveldb, etc.)
+	LevelDBBackend DBBackendType = "leveldb"
+	// GoLevelDBBackend represents goleveldb (github.com/syndtr/goleveldb - most
+	// popular implementation)
+	//   - pure go
+	//   - stable
 	GoLevelDBBackend DBBackendType = "goleveldb"
-	MemDBBackend     DBBackendType = "memdb"
-	FSDBBackend      DBBackendType = "fsdb" // using the filesystem naively
-	BoltDBBackend    DBBackendType = "boltdb"
+	// CLevelDBBackend represents cleveldb (uses levigo wrapper)
+	//   - fast
+	//   - requires gcc
+	//   - use cleveldb build tag (go build -tags cleveldb)
+	CLevelDBBackend DBBackendType = "cleveldb"
+	// MemDBBackend represents in-memoty key value store, which is mostly used
+	// for testing.
+	MemDBBackend DBBackendType = "memdb"
+	// FSDBBackend represents filesystem database
+	//	 - EXPERIMENTAL
+	//   - slow
+	FSDBBackend DBBackendType = "fsdb"
+	// BoltDBBackend represents bolt (uses etcd's fork of bolt -
+	// github.com/etcd-io/bbolt)
+	//   - EXPERIMENTAL
+	//   - may be faster is some use-cases (random reads - indexer)
+	//   - use boltdb build tag (go build -tags boltdb)
+	BoltDBBackend DBBackendType = "boltdb"
 )
 
 type dbCreator func(name string, dir string) (DB, error)

libs/db/remotedb/remotedb_test.go

@@ -28,7 +28,7 @@ func TestRemoteDB(t *testing.T) {
 	client, err := remotedb.NewRemoteDB(ln.Addr().String(), cert)
 	require.Nil(t, err, "expecting a successful client creation")
 	dbName := "test-remote-db"
-	require.Nil(t, client.InitRemote(&remotedb.Init{Name: dbName, Type: "leveldb"}))
+	require.Nil(t, client.InitRemote(&remotedb.Init{Name: dbName, Type: "goleveldb"}))
 	defer func() {
 		err := os.RemoveAll(dbName + ".db")
 		if err != nil {