Add cross-compilation Makefile targets and tar-based releases.

Revamp the build system to be more inline with other Prometheus exporters.
Notably add Darwin and Windows build targets, and add support for releases
using tar files.

tools/vendor/github.com/dnephin/govet/unsafeptr.go

@@ -0,0 +1,97 @@
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Check for invalid uintptr -> unsafe.Pointer conversions.
+
+package main
+
+import (
+	"go/ast"
+	"go/token"
+	"go/types"
+)
+
+func init() {
+	register("unsafeptr",
+		"check for misuse of unsafe.Pointer",
+		checkUnsafePointer,
+		callExpr)
+}
+
+func checkUnsafePointer(f *File, node ast.Node) {
+	x := node.(*ast.CallExpr)
+	if len(x.Args) != 1 {
+		return
+	}
+	if f.hasBasicType(x.Fun, types.UnsafePointer) && f.hasBasicType(x.Args[0], types.Uintptr) && !f.isSafeUintptr(x.Args[0]) {
+		f.Badf(x.Pos(), "possible misuse of unsafe.Pointer")
+	}
+}
+
+// isSafeUintptr reports whether x - already known to be a uintptr -
+// is safe to convert to unsafe.Pointer. It is safe if x is itself derived
+// directly from an unsafe.Pointer via conversion and pointer arithmetic
+// or if x is the result of reflect.Value.Pointer or reflect.Value.UnsafeAddr
+// or obtained from the Data field of a *reflect.SliceHeader or *reflect.StringHeader.
+func (f *File) isSafeUintptr(x ast.Expr) bool {
+	switch x := x.(type) {
+	case *ast.ParenExpr:
+		return f.isSafeUintptr(x.X)
+
+	case *ast.SelectorExpr:
+		switch x.Sel.Name {
+		case "Data":
+			// reflect.SliceHeader and reflect.StringHeader are okay,
+			// but only if they are pointing at a real slice or string.
+			// It's not okay to do:
+			//	var x SliceHeader
+			//	x.Data = uintptr(unsafe.Pointer(...))
+			//	... use x ...
+			//	p := unsafe.Pointer(x.Data)
+			// because in the middle the garbage collector doesn't
+			// see x.Data as a pointer and so x.Data may be dangling
+			// by the time we get to the conversion at the end.
+			// For now approximate by saying that *Header is okay
+			// but Header is not.
+			pt, ok := f.pkg.types[x.X].Type.(*types.Pointer)
+			if ok {
+				t, ok := pt.Elem().(*types.Named)
+				if ok && t.Obj().Pkg().Path() == "reflect" {
+					switch t.Obj().Name() {
+					case "StringHeader", "SliceHeader":
+						return true
+					}
+				}
+			}
+		}
+
+	case *ast.CallExpr:
+		switch len(x.Args) {
+		case 0:
+			// maybe call to reflect.Value.Pointer or reflect.Value.UnsafeAddr.
+			sel, ok := x.Fun.(*ast.SelectorExpr)
+			if !ok {
+				break
+			}
+			switch sel.Sel.Name {
+			case "Pointer", "UnsafeAddr":
+				t, ok := f.pkg.types[sel.X].Type.(*types.Named)
+				if ok && t.Obj().Pkg().Path() == "reflect" && t.Obj().Name() == "Value" {
+					return true
+				}
+			}
+
+		case 1:
+			// maybe conversion of uintptr to unsafe.Pointer
+			return f.hasBasicType(x.Fun, types.Uintptr) && f.hasBasicType(x.Args[0], types.UnsafePointer)
+		}
+
+	case *ast.BinaryExpr:
+		switch x.Op {
+		case token.ADD, token.SUB:
+			return f.isSafeUintptr(x.X) && !f.isSafeUintptr(x.Y)
+		}
+	}
+	return false
+}