tool/gocross: a tool for building Tailscale binaries

Signed-off-by: David Anderson <danderson@tailscale.com>

tool/gocross/env.go

@@ -0,0 +1,131 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package main
+
+import (
+	"fmt"
+	"os"
+	"sort"
+	"strings"
+)
+
+// Environment starts from an initial set of environment variables, and tracks
+// mutations to the environment. It can then apply those mutations to the
+// environment, or produce debugging output that illustrates the changes it
+// would make.
+type Environment struct {
+	init  map[string]string
+	set   map[string]string
+	unset map[string]bool
+
+	setenv   func(string, string) error
+	unsetenv func(string) error
+}
+
+// NewEnvironment returns an Environment initialized from os.Environ.
+func NewEnvironment() *Environment {
+	init := map[string]string{}
+	for _, env := range os.Environ() {
+		fs := strings.SplitN(env, "=", 2)
+		if len(fs) != 2 {
+			panic("bad environ provided")
+		}
+		init[fs[0]] = fs[1]
+	}
+
+	return newEnvironmentForTest(init, os.Setenv, os.Unsetenv)
+}
+
+func newEnvironmentForTest(init map[string]string, setenv func(string, string) error, unsetenv func(string) error) *Environment {
+	return &Environment{
+		init:     init,
+		set:      map[string]string{},
+		unset:    map[string]bool{},
+		setenv:   setenv,
+		unsetenv: unsetenv,
+	}
+}
+
+// Set sets the environment variable k to v.
+func (e *Environment) Set(k, v string) {
+	e.set[k] = v
+	delete(e.unset, k)
+}
+
+// Unset removes the environment variable k.
+func (e *Environment) Unset(k string) {
+	delete(e.set, k)
+	e.unset[k] = true
+}
+
+// IsSet reports whether the environment variable k is set.
+func (e *Environment) IsSet(k string) bool {
+	if e.unset[k] {
+		return false
+	}
+	if _, ok := e.init[k]; ok {
+		return true
+	}
+	if _, ok := e.set[k]; ok {
+		return true
+	}
+	return false
+}
+
+// Get returns the value of the environment variable k, or defaultVal if it is
+// not set.
+func (e *Environment) Get(k, defaultVal string) string {
+	if e.unset[k] {
+		return defaultVal
+	}
+	if v, ok := e.set[k]; ok {
+		return v
+	}
+	if v, ok := e.init[k]; ok {
+		return v
+	}
+	return defaultVal
+}
+
+// Apply applies all pending mutations to the environment.
+func (e *Environment) Apply() error {
+	for k, v := range e.set {
+		if err := e.setenv(k, v); err != nil {
+			return fmt.Errorf("setting %q: %v", k, err)
+		}
+		e.init[k] = v
+		delete(e.set, k)
+	}
+	for k := range e.unset {
+		if err := e.unsetenv(k); err != nil {
+			return fmt.Errorf("unsetting %q: %v", k, err)
+		}
+		delete(e.init, k)
+		delete(e.unset, k)
+	}
+	return nil
+}
+
+// Diff returns a string describing the pending mutations to the environment.
+func (e *Environment) Diff() string {
+	lines := make([]string, 0, len(e.set)+len(e.unset))
+	for k, v := range e.set {
+		old, ok := e.init[k]
+		if ok {
+			lines = append(lines, fmt.Sprintf("%s=%s (was %s)", k, v, old))
+		} else {
+			lines = append(lines, fmt.Sprintf("%s=%s (was <nil>)", k, v))
+		}
+	}
+	for k := range e.unset {
+		old, ok := e.init[k]
+		if ok {
+			lines = append(lines, fmt.Sprintf("%s=<nil> (was %s)", k, old))
+		} else {
+			lines = append(lines, fmt.Sprintf("%s=<nil> (was <nil>)", k))
+		}
+	}
+	sort.Strings(lines)
+	return strings.Join(lines, "\n")
+}