Add a tool to extract genesis.ssz from existing database (#3827)

* Add a tool to extract genesis.ssz from existing database

* close db

* panic on nil

beacon-chain/db/BUILD.bazel

@@ -7,7 +7,10 @@ go_library(
         "http_backup_handler.go",
     ],
     importpath = "github.com/prysmaticlabs/prysm/beacon-chain/db",
-    visibility = ["//beacon-chain:__subpackages__"],
+    visibility = [
+        "//beacon-chain:__subpackages__",
+        "//tools:__subpackages__",
+    ],
     deps = [
         "//beacon-chain/db/filters:go_default_library",
         "//beacon-chain/db/kv:go_default_library",

tools/interop/export-genesis/BUILD.bazel

@@ -0,0 +1,18 @@
+load("@io_bazel_rules_go//go:def.bzl", "go_binary", "go_library")
+
+go_library(
+    name = "go_default_library",
+    srcs = ["main.go"],
+    importpath = "github.com/prysmaticlabs/prysm/tools/interop/export-genesis",
+    visibility = ["//visibility:private"],
+    deps = [
+        "//beacon-chain/db:go_default_library",
+        "@com_github_prysmaticlabs_go_ssz//:go_default_library",
+    ],
+)
+
+go_binary(
+    name = "export-genesis",
+    embed = [":go_default_library"],
+    visibility = ["//visibility:public"],
+)

tools/interop/export-genesis/main.go

@@ -0,0 +1,45 @@
+package main
+
+import (
+	"context"
+	"fmt"
+	"io/ioutil"
+	"os"
+
+	"github.com/prysmaticlabs/go-ssz"
+	"github.com/prysmaticlabs/prysm/beacon-chain/db"
+)
+
+
+// A basic tool to extract genesis.ssz from existing beaconchain.db.
+// ex:
+//   bazel run //tools/interop/export-genesis:export-genesis -- /tmp/data/beaconchaindata /tmp/genesis.ssz
+func main() {
+	if len(os.Args) < 3 {
+		fmt.Println("Usage: ./main /path/to/datadir /path/to/output/genesis.ssz")
+		os.Exit(1)
+	}
+
+	fmt.Printf("Reading db at %s and writing ssz output to %s.\n", os.Args[1], os.Args[2])
+
+	d, err := db.NewDB(os.Args[1])
+	if err != nil {
+		panic(err)
+	}
+	defer d.Close()
+	gs, err := d.GenesisState(context.Background())
+	if err != nil {
+		panic(err)
+	}
+	if gs == nil {
+		panic("nil genesis state")
+	}
+	b, err := ssz.Marshal(gs)
+	if err != nil {
+		panic(err)
+	}
+	if err := ioutil.WriteFile(os.Args[2], b, 0644); err != nil {
+		panic(err)
+	}
+	fmt.Println("done")
+}