all: make vendored copy of reexec (#28382)

a little copying is better than a little dependency

-- go proverb

We have this dependency on docker, a.k.a moby: a gigantic library, and we only need ~70 LOC,
so here I tried moving it inline instead.

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

internal/cmdtest/test_cmd.go

@@ -32,7 +32,7 @@ import (
 	"text/template"
 	"time"
 
-	"github.com/docker/docker/pkg/reexec"
+	"github.com/ethereum/go-ethereum/internal/reexec"
 )
 
 func NewTestCmd(t *testing.T, data interface{}) *TestCmd {

internal/reexec/reexec.go

@@ -0,0 +1,35 @@
+// This file originates from Docker/Moby,
+// https://github.com/moby/moby/blob/master/pkg/reexec/reexec.go
+// Licensed under Apache License 2.0: https://github.com/moby/moby/blob/master/LICENSE
+// Copyright 2013-2018 Docker, Inc.
+//
+// Package reexec facilitates the busybox style reexec of the docker binary that
+// we require because of the forking limitations of using Go.  Handlers can be
+// registered with a name and the argv 0 of the exec of the binary will be used
+// to find and execute custom init paths.
+package reexec
+
+import (
+	"fmt"
+	"os"
+)
+
+var registeredInitializers = make(map[string]func())
+
+// Register adds an initialization func under the specified name
+func Register(name string, initializer func()) {
+	if _, exists := registeredInitializers[name]; exists {
+		panic(fmt.Sprintf("reexec func already registered under name %q", name))
+	}
+	registeredInitializers[name] = initializer
+}
+
+// Init is called as the first part of the exec process and returns true if an
+// initialization function was called.
+func Init() bool {
+	if initializer, ok := registeredInitializers[os.Args[0]]; ok {
+		initializer()
+		return true
+	}
+	return false
+}

internal/reexec/self_linux.go

@@ -0,0 +1,14 @@
+// This file originates from Docker/Moby,
+// https://github.com/moby/moby/blob/master/pkg/reexec/
+// Licensed under Apache License 2.0: https://github.com/moby/moby/blob/master/LICENSE
+// Copyright 2013-2018 Docker, Inc.
+
+//go:build linux
+
+package reexec
+
+// Self returns the path to the current process's binary.
+// Returns "/proc/self/exe".
+func Self() string {
+	return "/proc/self/exe"
+}

internal/reexec/self_others.go

@@ -0,0 +1,32 @@
+// This file originates from Docker/Moby,
+// https://github.com/moby/moby/blob/master/pkg/reexec/
+// Licensed under Apache License 2.0: https://github.com/moby/moby/blob/master/LICENSE
+// Copyright 2013-2018 Docker, Inc.
+
+//go:build !linux
+
+package reexec
+
+import (
+	"os"
+	"os/exec"
+	"path/filepath"
+)
+
+// Self returns the path to the current process's binary.
+// Uses os.Args[0].
+func Self() string {
+	name := os.Args[0]
+	if filepath.Base(name) == name {
+		if lp, err := exec.LookPath(name); err == nil {
+			return lp
+		}
+	}
+	// handle conversion of relative paths to absolute
+	if absName, err := filepath.Abs(name); err == nil {
+		return absName
+	}
+	// if we couldn't get absolute name, return original
+	// (NOTE: Go only errors on Abs() if os.Getwd fails)
+	return name
+}