secp256k1: Support go generate w/o removing file.

This modifies the logic that deals with generating, loading, and
decompressing the compressed byte points to allow go generate to run
correctly regardless of whether or not the generated file already
exists.

It accomplishes this by making use of a function to obtain the
compressed byte points string instead of directly referencing the
variable such that the function is nil by default and only set to the
real function via the generated file.

dcrec/secp256k1/genprecomps.go

@@ -44,7 +44,7 @@ func main() {
 	base64.StdEncoding.Encode(encoded, compressed.Bytes())
 
 	fmt.Fprintln(fi, "// Copyright (c) 2015 The btcsuite developers")
-	fmt.Fprintln(fi, "// Copyright (c) 2015-2021 The Decred developers")
+	fmt.Fprintln(fi, "// Copyright (c) 2015-2022 The Decred developers")
 	fmt.Fprintln(fi, "// Use of this source code is governed by an ISC")
 	fmt.Fprintln(fi, "// license that can be found in the LICENSE file.")
 	fmt.Fprintln(fi)
@@ -54,6 +54,13 @@ func main() {
 	fmt.Fprintln(fi, "// DO NOT EDIT")
 	fmt.Fprintln(fi)
 	fmt.Fprintf(fi, "var compressedBytePoints = %q\n", string(encoded))
+	fmt.Fprintln(fi)
+	fmt.Fprintln(fi, "// Set accessor to a real function.")
+	fmt.Fprintln(fi, "func init() {")
+	fmt.Fprintln(fi, "	compressedBytePointsFn = func() string {")
+	fmt.Fprintln(fi, "		return compressedBytePoints")
+	fmt.Fprintln(fi, "	}")
+	fmt.Fprintln(fi, "}")
 
 	a1, b1, a2, b2 := secp256k1.EndomorphismVectors()
 	fmt.Println("The following values are the computed linearly " +

dcrec/secp256k1/genstatics.go

@@ -18,10 +18,6 @@ import (
 	"math/big"
 )
 
-// compressedBytePoints are dummy points used so the code which generates the
-// real values can compile.
-var compressedBytePoints = ""
-
 // SerializedBytePoints returns a serialized byte slice which contains all of
 // the possible points per 8-bit window.  This is used to when generating
 // compressedbytepoints.go.

dcrec/secp256k1/loadprecomputed.go

@@ -20,6 +20,11 @@ import (
 // accelerating scalar base multiplication.
 type bytePointTable [32][256][2]FieldVal
 
+// compressedBytePointsFn is set to a real function by the code generation to
+// return the compressed pre-computed values for accelerating scalar base
+// multiplication.
+var compressedBytePointsFn func() string
+
 // s256BytePoints houses pre-computed values used to accelerate scalar base
 // multiplication such that they are only loaded on first use.
 var s256BytePoints = func() func() *bytePointTable {
@@ -38,10 +43,10 @@ var s256BytePoints = func() func() *bytePointTable {
 	var data *bytePointTable
 	mustLoadBytePoints := func() {
 		// There will be no byte points to load when generating them.
-		bp := compressedBytePoints
-		if len(bp) == 0 {
+		if compressedBytePointsFn == nil {
 			return
 		}
+		bp := compressedBytePointsFn()
 
 		// Decompress the pre-computed table used to accelerate scalar base
 		// multiplication.