Only use head if it's compatible with target (#15965)

* Only use head if it's compatible with target

* Allow blocks from the previous epoch to be viable for checkpoints

* Add feature flag to make it configurable

* fix tests

* @satushh's review

* Manu's nit

* Use fields in logs

config/features/config.go

@@ -69,6 +69,7 @@ type Flags struct {
 
 	DisableResourceManager     bool // Disables running the node with libp2p's resource manager.
 	DisableStakinContractCheck bool // Disables check for deposit contract when proposing blocks
+	DisableLastEpochTargets    bool // Disables processing of states for attestations to old blocks.
 
 	EnableVerboseSigVerification bool // EnableVerboseSigVerification specifies whether to verify individual signature if batch verification fails
 
@@ -274,11 +275,14 @@ func ConfigureBeaconChain(ctx *cli.Context) error {
 		logEnabled(forceHeadFlag)
 		cfg.ForceHead = ctx.String(forceHeadFlag.Name)
 	}
-
 	if ctx.IsSet(blacklistRoots.Name) {
 		logEnabled(blacklistRoots)
 		cfg.BlacklistedRoots = parseBlacklistedRoots(ctx.StringSlice(blacklistRoots.Name))
 	}
+	if ctx.IsSet(disableLastEpochTargets.Name) {
+		logEnabled(disableLastEpochTargets)
+		cfg.DisableLastEpochTargets = true
+	}
 
 	cfg.AggregateIntervals = [3]time.Duration{aggregateFirstInterval.Value, aggregateSecondInterval.Value, aggregateThirdInterval.Value}
 	Init(cfg)

config/features/flags.go

@@ -197,6 +197,11 @@ var (
 		Usage: "(Work in progress): Enables the web portal for the validator client.",
 		Value: false,
 	}
+	// disableLastEpochTargets is a flag to disable processing of attestations for old blocks.
+	disableLastEpochTargets = &cli.BoolFlag{
+		Name:  "disable-last-epoch-targets",
+		Usage: "Disables processing of last epoch targets.",
+	}
 )
 
 // devModeFlags holds list of flags that are set when development mode is on.
@@ -257,6 +262,7 @@ var BeaconChainFlags = combinedFlags([]cli.Flag{
 	enableExperimentalAttestationPool,
 	forceHeadFlag,
 	blacklistRoots,
+	disableLastEpochTargets,
 }, deprecatedBeaconFlags, deprecatedFlags, upcomingDeprecation)
 
 func combinedFlags(flags ...[]cli.Flag) []cli.Flag {