consensus/misc/eip4844: implement EIP-7918 (#31965)

https://eips.ethereum.org/EIPS/eip-7918

---------

Co-authored-by: Felix Lange <fjl@twurst.com>

consensus/misc/eip4844/eip4844.go

@@ -71,11 +71,30 @@ func CalcExcessBlobGas(config *params.ChainConfig, parent *types.Header, headTim
 		parentExcessBlobGas = *parent.ExcessBlobGas
 		parentBlobGasUsed = *parent.BlobGasUsed
 	}
-	excessBlobGas := parentExcessBlobGas + parentBlobGasUsed
-	targetGas := uint64(targetBlobsPerBlock(config, headTimestamp)) * params.BlobTxBlobGasPerBlob
+	var (
+		excessBlobGas = parentExcessBlobGas + parentBlobGasUsed
+		target        = targetBlobsPerBlock(config, headTimestamp)
+		targetGas     = uint64(target) * params.BlobTxBlobGasPerBlob
+	)
 	if excessBlobGas < targetGas {
 		return 0
 	}
+	if !config.IsOsaka(config.LondonBlock, headTimestamp) {
+		// Pre-Osaka, we use the formula defined by EIP-4844.
+		return excessBlobGas - targetGas
+	}
+
+	// EIP-7918 (post-Osaka) introduces a different formula for computing excess.
+	var (
+		baseCost     = big.NewInt(params.BlobBaseCost)
+		reservePrice = baseCost.Mul(baseCost, parent.BaseFee)
+		blobPrice    = calcBlobPrice(config, parent)
+	)
+	if reservePrice.Cmp(blobPrice) > 0 {
+		max := MaxBlobsPerBlock(config, headTimestamp)
+		scaledExcess := parentBlobGasUsed * uint64(max-target) / uint64(max)
+		return parentExcessBlobGas + scaledExcess
+	}
 	return excessBlobGas - targetGas
 }
 
@@ -177,3 +196,9 @@ func fakeExponential(factor, numerator, denominator *big.Int) *big.Int {
 	}
 	return output.Div(output, denominator)
 }
+
+// calcBlobPrice calculates the blob price for a block.
+func calcBlobPrice(config *params.ChainConfig, header *types.Header) *big.Int {
+	blobBaseFee := CalcBlobFee(config, header)
+	return new(big.Int).Mul(blobBaseFee, big.NewInt(params.BlobTxBlobGasPerBlob))
+}

consensus/misc/eip4844/eip4844_test.go

@@ -127,3 +127,43 @@ func TestFakeExponential(t *testing.T) {
 		}
 	}
 }
+
+func TestCalcExcessBlobGasEIP7918(t *testing.T) {
+	var (
+		cfg           = params.MergedTestChainConfig
+		targetBlobs   = targetBlobsPerBlock(cfg, *cfg.CancunTime)
+		blobGasTarget = uint64(targetBlobs) * params.BlobTxBlobGasPerBlob
+	)
+	makeHeader := func(parentExcess, parentBaseFee uint64, blobsUsed int) *types.Header {
+		blobGasUsed := uint64(blobsUsed) * params.BlobTxBlobGasPerBlob
+		return &types.Header{
+			BaseFee:       big.NewInt(int64(parentBaseFee)),
+			ExcessBlobGas: &parentExcess,
+			BlobGasUsed:   &blobGasUsed,
+		}
+	}
+
+	tests := []struct {
+		name          string
+		header        *types.Header
+		wantExcessGas uint64
+	}{
+		{
+			name:          "BelowReservePrice",
+			header:        makeHeader(0, 1_000_000_000, targetBlobs),
+			wantExcessGas: blobGasTarget * 3 / 9,
+		},
+		{
+			name:          "AboveReservePrice",
+			header:        makeHeader(0, 1, targetBlobs),
+			wantExcessGas: 0,
+		},
+	}
+	for _, tc := range tests {
+		got := CalcExcessBlobGas(cfg, tc.header, *cfg.CancunTime)
+		if got != tc.wantExcessGas {
+			t.Fatalf("%s: excess-blob-gas mismatch – have %d, want %d",
+				tc.name, got, tc.wantExcessGas)
+		}
+	}
+}

params/protocol_params.go

@@ -173,6 +173,7 @@ const (
 	BlobTxBlobGasPerBlob               = 1 << 17 // Gas consumption of a single data blob (== blob byte size)
 	BlobTxMinBlobGasprice              = 1       // Minimum gas price for data blobs
 	BlobTxPointEvaluationPrecompileGas = 50000   // Gas price for the point evaluation precompile.
+	BlobBaseCost                       = 1 << 13 // Base execution gas cost for a blob.
 
 	HistoryServeWindow = 8192 // Number of blocks to serve historical block hashes for, EIP-2935.
 )