Weak subjectivity start from genesis (#1675)

## Issue Addressed
Solution 2 proposed here: https://github.com/sigp/lighthouse/issues/1435#issuecomment-692317639

## Proposed Changes
- Adds an optional `--wss-checkpoint` flag that takes a string `root:epoch`
- Verify that the given checkpoint exists in the chain, or that the the chain syncs through this checkpoint. If not, shutdown and prompt the user to purge state before restarting.

## Additional Info


Co-authored-by: Paul Hauner <paul@paulhauner.com>

beacon_node/src/cli.rs

@@ -281,4 +281,14 @@ pub fn cli_app<'a, 'b>() -> App<'a, 'b> {
                 .takes_value(true)
                 .default_value("700")
         )
+        .arg(
+            Arg::with_name("wss-checkpoint")
+                .long("wss-checkpoint")
+                .help(
+                    "Used to input a Weak Subjectivity State Checkpoint in `block_root:epoch_number` format,\
+                     where block_root is an '0x' prefixed 32-byte hex string and epoch_number is an integer."
+                )
+                .value_name("WSS_CHECKPOINT")
+                .takes_value(true)
+        )
 }

beacon_node/src/config.rs

@@ -11,7 +11,7 @@ use std::fs;
 use std::net::{IpAddr, Ipv4Addr, ToSocketAddrs};
 use std::net::{TcpListener, UdpSocket};
 use std::path::PathBuf;
-use types::{ChainSpec, EthSpec, GRAFFITI_BYTES_LEN};
+use types::{ChainSpec, Checkpoint, Epoch, EthSpec, Hash256, GRAFFITI_BYTES_LEN};
 
 pub const BEACON_NODE_DIR: &str = "beacon";
 pub const NETWORK_DIR: &str = "network";
@@ -270,6 +270,41 @@ pub fn get_config<E: EthSpec>(
     client_config.graffiti[..trimmed_graffiti_len]
         .copy_from_slice(&raw_graffiti[..trimmed_graffiti_len]);
 
+    if let Some(wss_checkpoint) = cli_args.value_of("wss-checkpoint") {
+        let mut split = wss_checkpoint.split(':');
+        let root_str = split
+            .next()
+            .ok_or_else(|| "Improperly formatted weak subjectivity checkpoint".to_string())?;
+        let epoch_str = split
+            .next()
+            .ok_or_else(|| "Improperly formatted weak subjectivity checkpoint".to_string())?;
+
+        if !root_str.starts_with("0x") {
+            return Err(
+                "Unable to parse weak subjectivity checkpoint root, must have 0x prefix"
+                    .to_string(),
+            );
+        }
+
+        if !root_str.chars().count() == 66 {
+            return Err(
+                "Unable to parse weak subjectivity checkpoint root, must have 32 bytes".to_string(),
+            );
+        }
+
+        let root =
+            Hash256::from_slice(&hex::decode(&root_str[2..]).map_err(|e| {
+                format!("Unable to parse weak subjectivity checkpoint root: {:?}", e)
+            })?);
+        let epoch = Epoch::new(
+            epoch_str
+                .parse()
+                .map_err(|_| "Invalid weak subjectivity checkpoint epoch".to_string())?,
+        );
+
+        client_config.chain.weak_subjectivity_checkpoint = Some(Checkpoint { epoch, root })
+    }
+
     if let Some(max_skip_slots) = cli_args.value_of("max-skip-slots") {
         client_config.chain.import_max_skip_slots = match max_skip_slots {
             "none" => None,