A tool to update genesis time of an existing state (#5664)

* Add a tool to update genesis time
* Minor touchups
* Merge branch 'master' into update-genesis-time
* Added a readme
* Merge refs/heads/master into update-genesis-time
* Merge branch 'update-genesis-time' of github.com:prysmaticlabs/prysm into update-genesis-time
* Merge refs/heads/master into update-genesis-time
* Merge refs/heads/master into update-genesis-time

tools/update-genesis-time/BUILD.bazel

@@ -0,0 +1,19 @@
+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/update-genesis-time",
+    visibility = ["//visibility:public"],
+    deps = [
+        "//proto/beacon/p2p/v1:go_default_library",
+        "//shared/roughtime:go_default_library",
+        "@com_github_prysmaticlabs_go_ssz//:go_default_library",
+    ],
+)
+
+go_binary(
+    name = "update-genesis-time",
+    embed = [":go_default_library"],
+    visibility = ["//visibility:public"],
+)

tools/update-genesis-time/README.md

@@ -0,0 +1,35 @@
+## Utility to Update Beacon State Genesis Time
+
+This is a utility to help users update genesis time of an input beacon state
+
+### Usage
+
+_Name:_
+**update-genesis-time** - this is a utility to update genesis time of a beacon state
+
+_Usage:_
+update-genesis-time [global options]
+
+_Flags:_
+
+- --input-ssz-state: Input filename of the SSZ marshaling of the genesis state
+- --genesis-time: Unix timestamp used as the genesis time in the generated genesis state (defaults to now)
+
+### Example
+
+To use private key with default RPC:
+
+```
+bazel run //tools/update-genesis-time -- --input-ssz-state=/tmp/genesis.ssz
+```
+
+### Output
+
+```
+INFO: Elapsed time: 5.887s, Critical Path: 4.99s
+INFO: 41 processes: 41 darwin-sandbox.
+INFO: Build completed successfully, 44 total actions
+INFO: Build completed successfully, 44 total actions
+2020/04/28 11:55:21 No --genesis-time specified, defaulting to now
+2020/04/28 11:55:21 Done writing to /tmp/genesis.ssz
+```

tools/update-genesis-time/main.go

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+package main
+
+import (
+	"flag"
+	"io/ioutil"
+	"log"
+
+	"github.com/prysmaticlabs/go-ssz"
+	pb "github.com/prysmaticlabs/prysm/proto/beacon/p2p/v1"
+	"github.com/prysmaticlabs/prysm/shared/roughtime"
+)
+
+var (
+	genesisTime   = flag.Uint64("genesis-time", 0, "Unix timestamp used as the genesis time in the generated genesis state (defaults to now)")
+	inputSSZState = flag.String("input-ssz-state", "", "Input filename of the SSZ marshaling of the genesis state")
+)
+
+func main() {
+	flag.Parse()
+	if *inputSSZState == "" {
+		log.Fatal("Expected --input-ssz-state")
+	}
+
+	beaconState := &pb.BeaconState{}
+	if err := unmarshalFile(*inputSSZState, beaconState); err != nil {
+		log.Fatal(err)
+	}
+	if *genesisTime == 0 {
+		log.Print("No --genesis-time specified, defaulting to now")
+		beaconState.GenesisTime = uint64(roughtime.Now().Unix())
+	} else {
+		beaconState.GenesisTime = *genesisTime
+	}
+
+	encodedState, err := ssz.Marshal(beaconState)
+	if err != nil {
+		log.Fatalf("Could not ssz marshal the beacon state: %v", err)
+	}
+	if err := ioutil.WriteFile(*inputSSZState, encodedState, 0644); err != nil {
+		log.Fatalf("Could not write encoded beacon state to file: %v", err)
+	}
+	log.Printf("Done writing to %s", *inputSSZState)
+}
+
+func unmarshalFile(fPath string, data interface{}) error {
+	rawFile, err := ioutil.ReadFile(fPath)
+	if err != nil {
+		return err
+	}
+	return ssz.Unmarshal(rawFile, data)
+}