From 8696dd39cbb5664a4a6900993e113713e56f3def Mon Sep 17 00:00:00 2001
From: Vinod Damle <vdamle@users.noreply.github.com>
Date: Wed, 16 Sep 2020 02:39:35 -0400
Subject: [PATCH] params: allow setting Petersburg block before chain head
 (#21473)

* Allow setting PetersburgBlock before chainhead

if it is at the same block as ConstantinopleBlock

* Add a negative test
---
 params/config.go      |  6 +++++-
 params/config_test.go | 17 +++++++++++++++++
 2 files changed, 22 insertions(+), 1 deletion(-)

diff --git a/params/config.go b/params/config.go
index e5ec64b2bf..2cd66acc2f 100644
--- a/params/config.go
+++ b/params/config.go
@@ -528,7 +528,11 @@ func (c *ChainConfig) checkCompatible(newcfg *ChainConfig, head *big.Int) *Confi
 		return newCompatError("Constantinople fork block", c.ConstantinopleBlock, newcfg.ConstantinopleBlock)
 	}
 	if isForkIncompatible(c.PetersburgBlock, newcfg.PetersburgBlock, head) {
-		return newCompatError("Petersburg fork block", c.PetersburgBlock, newcfg.PetersburgBlock)
+		// the only case where we allow Petersburg to be set in the past is if it is equal to Constantinople
+		// mainly to satisfy fork ordering requirements which state that Petersburg fork be set if Constantinople fork is set
+		if isForkIncompatible(c.ConstantinopleBlock, newcfg.PetersburgBlock, head) {
+			return newCompatError("Petersburg fork block", c.PetersburgBlock, newcfg.PetersburgBlock)
+		}
 	}
 	if isForkIncompatible(c.IstanbulBlock, newcfg.IstanbulBlock, head) {
 		return newCompatError("Istanbul fork block", c.IstanbulBlock, newcfg.IstanbulBlock)
diff --git a/params/config_test.go b/params/config_test.go
index 02c5fe2917..3c8ebaf4a5 100644
--- a/params/config_test.go
+++ b/params/config_test.go
@@ -70,6 +70,23 @@ func TestCheckCompatible(t *testing.T) {
 				RewindTo:     9,
 			},
 		},
+		{
+			stored:  &ChainConfig{ConstantinopleBlock: big.NewInt(30)},
+			new:     &ChainConfig{ConstantinopleBlock: big.NewInt(30), PetersburgBlock: big.NewInt(30)},
+			head:    40,
+			wantErr: nil,
+		},
+		{
+			stored: &ChainConfig{ConstantinopleBlock: big.NewInt(30)},
+			new:    &ChainConfig{ConstantinopleBlock: big.NewInt(30), PetersburgBlock: big.NewInt(31)},
+			head:   40,
+			wantErr: &ConfigCompatError{
+				What:         "Petersburg fork block",
+				StoredConfig: nil,
+				NewConfig:    big.NewInt(31),
+				RewindTo:     30,
+			},
+		},
 	}
 
 	for _, test := range tests {