internal/stdlib: refactor getGoRepo, add clone tests
Refactor getGoRepo to handle both test and cloned repos. This will
enable adding a third option, getting the Go repo from a local copy.
Also add a test for checking the contents of a cloned repo, to make
sure the clone logic is correct. Flag-protect it because it's slow
and hits an external server.
Change-Id: I9de50bf12fccbd4fa35aa0566bdee3cddb25edf8
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/347403
Trust: Jonathan Amsterdam <jba@google.com>
Run-TryBot: Jonathan Amsterdam <jba@google.com>
TryBot-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jamal Carvalho <jamal@golang.org>
diff --git a/internal/stdlib/stdlib.go b/internal/stdlib/stdlib.go
index d875773..2c57d76 100644
--- a/internal/stdlib/stdlib.go
+++ b/internal/stdlib/stdlib.go
@@ -200,10 +200,19 @@
)
// getGoRepo returns a repo object for the Go repo at version.
-func getGoRepo(v string) (_ *git.Repository, err error) {
- defer derrors.Wrap(&err, "getGoRepo(%q)", v)
+func getGoRepo(version string) (_ *git.Repository, _ plumbing.ReferenceName, err error) {
+ defer derrors.Wrap(&err, "getGoRepo(%q)", version)
+ if UseTestData {
+ return getTestGoRepo(version)
+ }
+ return cloneGoRepo(version)
+}
- var ref plumbing.ReferenceName
+// cloneGoRepo returns a repo object for the Go repo at version by cloning the
+// Go repo.
+func cloneGoRepo(v string) (_ *git.Repository, ref plumbing.ReferenceName, err error) {
+ defer derrors.Wrap(&err, "cloneGoRepo(%q)", v)
+
if v == version.Master {
ref = plumbing.HEAD
} else if SupportedBranches[v] {
@@ -211,21 +220,25 @@
} else {
tag, err := TagForVersion(v)
if err != nil {
- return nil, err
+ return nil, "", err
}
ref = plumbing.NewTagReferenceName(tag)
}
- return git.Clone(memory.NewStorage(), nil, &git.CloneOptions{
+ repo, err := git.Clone(memory.NewStorage(), nil, &git.CloneOptions{
URL: GoRepoURL,
ReferenceName: ref,
SingleBranch: true,
Depth: 1,
Tags: git.NoTags,
})
+ if err != nil {
+ return nil, "", err
+ }
+ return repo, ref, nil
}
// getTestGoRepo gets a Go repo for testing.
-func getTestGoRepo(v string) (_ *git.Repository, err error) {
+func getTestGoRepo(v string) (_ *git.Repository, _ plumbing.ReferenceName, err error) {
defer derrors.Wrap(&err, "getTestGoRepo(%q)", v)
if v == TestMasterVersion {
v = version.Master
@@ -236,15 +249,15 @@
fs := osfs.New(filepath.Join(testhelper.TestDataPath("testdata"), v))
repo, err := git.Init(memory.NewStorage(), fs)
if err != nil {
- return nil, fmt.Errorf("git.Initi: %v", err)
+ return nil, "", fmt.Errorf("git.Initi: %v", err)
}
wt, err := repo.Worktree()
if err != nil {
- return nil, fmt.Errorf("repo.Worktree: %v", err)
+ return nil, "", fmt.Errorf("repo.Worktree: %v", err)
}
// Add all files in the directory.
if _, err := wt.Add(""); err != nil {
- return nil, fmt.Errorf("wt.Add(): %v", err)
+ return nil, "", fmt.Errorf("wt.Add(): %v", err)
}
_, err = wt.Commit("", &git.CommitOptions{All: true, Author: &object.Signature{
Name: "Joe Random",
@@ -252,9 +265,9 @@
When: TestCommitTime,
}})
if err != nil {
- return nil, fmt.Errorf("wt.Commit: %v", err)
+ return nil, "", fmt.Errorf("wt.Commit: %v", err)
}
- return repo, nil
+ return repo, plumbing.HEAD, nil
}
// Versions returns all the versions of Go that are relevant to the discovery
@@ -316,28 +329,24 @@
}
func zipInternal(requestedVersion string) (_ *zip.Reader, resolvedVersion string, commitTime time.Time, prefix string, err error) {
- var repo *git.Repository
- if UseTestData {
- repo, err = getTestGoRepo(requestedVersion)
- } else {
- if requestedVersion == version.Latest {
- requestedVersion, err = semanticVersion(requestedVersion)
- if err != nil {
- return nil, "", time.Time{}, "", err
- }
+ if !UseTestData && requestedVersion == version.Latest {
+ requestedVersion, err = semanticVersion(requestedVersion)
+ if err != nil {
+ return nil, "", time.Time{}, "", err
}
- repo, err = getGoRepo(requestedVersion)
}
+ repo, refName, err := getGoRepo(requestedVersion)
if err != nil {
return nil, "", time.Time{}, "", err
}
var buf bytes.Buffer
z := zip.NewWriter(&buf)
- head, err := repo.Head()
+
+ ref, err := repo.Reference(refName, true)
if err != nil {
return nil, "", time.Time{}, "", err
}
- commit, err := repo.CommitObject(head.Hash())
+ commit, err := repo.CommitObject(ref.Hash())
if err != nil {
return nil, "", time.Time{}, "", err
}
diff --git a/internal/stdlib/stdlib_test.go b/internal/stdlib/stdlib_test.go
index 3555b2f..0e20402 100644
--- a/internal/stdlib/stdlib_test.go
+++ b/internal/stdlib/stdlib_test.go
@@ -6,6 +6,7 @@
import (
"errors"
+ "flag"
"io/fs"
"reflect"
"testing"
@@ -14,6 +15,8 @@
"golang.org/x/pkgsite/internal/version"
)
+var clone = flag.Bool("clone", false, "test actual clones of the Go repo")
+
func TestTagForVersion(t *testing.T) {
for _, test := range []struct {
name string
@@ -142,39 +145,65 @@
if !gotTime.Equal(TestCommitTime) {
t.Errorf("commit time: got %s, want %s", gotTime, TestCommitTime)
}
- wantFiles := map[string]bool{
- "LICENSE": true,
- "errors/errors.go": true,
- "errors/errors_test.go": true,
- }
- if semver.Compare(resolvedVersion, "v1.13.0") > 0 || resolvedVersion == TestMasterVersion {
- wantFiles["cmd/README.vendor"] = true
- }
+ checkContentDirFiles(t, cdir, resolvedVersion)
+ })
+ }
+}
- const readmeVendorFile = "README.vendor"
- if _, err := fs.Stat(cdir, readmeVendorFile); !errors.Is(err, fs.ErrNotExist) {
- t.Fatalf("fs.Stat returned %v; want %q to be removed", err, readmeVendorFile)
- }
- err = fs.WalkDir(cdir, ".", func(path string, d fs.DirEntry, err error) error {
- if err != nil {
- return err
- }
- if d.IsDir() {
- return nil
- }
- delete(wantFiles, path)
- return nil
- })
+func TestContentDirClone(t *testing.T) {
+ if !*clone {
+ t.Skip("-clone not supplied")
+ }
+ for _, resolvedVersion := range []string{
+ "v1.3.2",
+ "v1.14.6",
+ version.Master,
+ version.Latest,
+ } {
+ t.Run(resolvedVersion, func(t *testing.T) {
+ cdir, _, _, err := ContentDir(resolvedVersion)
if err != nil {
t.Fatal(err)
}
- if len(wantFiles) > 0 {
- t.Errorf("zip missing files: %v", reflect.ValueOf(wantFiles).MapKeys())
- }
+ checkContentDirFiles(t, cdir, resolvedVersion)
})
}
}
+func checkContentDirFiles(t *testing.T, cdir fs.FS, resolvedVersion string) {
+ wantFiles := map[string]bool{
+ "LICENSE": true,
+ "errors/errors.go": true,
+ "errors/errors_test.go": true,
+ }
+ if semver.Compare(resolvedVersion, "v1.13.0") > 0 || resolvedVersion == TestMasterVersion {
+ wantFiles["cmd/README.vendor"] = true
+ }
+ if semver.Compare(resolvedVersion, "v1.14.0") > 0 {
+ wantFiles["context/context.go"] = true
+ }
+ const readmeVendorFile = "README.vendor"
+ if _, err := fs.Stat(cdir, readmeVendorFile); !errors.Is(err, fs.ErrNotExist) {
+ t.Fatalf("fs.Stat returned %v; want %q to be removed", err, readmeVendorFile)
+ }
+ err := fs.WalkDir(cdir, ".", func(path string, d fs.DirEntry, err error) error {
+ if err != nil {
+ return err
+ }
+ if d.IsDir() {
+ return nil
+ }
+ delete(wantFiles, path)
+ return nil
+ })
+ if err != nil {
+ t.Fatal(err)
+ }
+ if len(wantFiles) > 0 {
+ t.Errorf("zip missing files: %v", reflect.ValueOf(wantFiles).MapKeys())
+ }
+}
+
func TestZipInfo(t *testing.T) {
UseTestData = true
defer func() { UseTestData = false }()
diff --git a/internal/stdlib/testdata/v1.14.6/src/context/context.go b/internal/stdlib/testdata/v1.14.6/src/context/context.go
new file mode 100644
index 0000000..b561968
--- /dev/null
+++ b/internal/stdlib/testdata/v1.14.6/src/context/context.go
@@ -0,0 +1,554 @@
+// 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.
+
+// Package context defines the Context type, which carries deadlines,
+// cancellation signals, and other request-scoped values across API boundaries
+// and between processes.
+//
+// Incoming requests to a server should create a Context, and outgoing
+// calls to servers should accept a Context. The chain of function
+// calls between them must propagate the Context, optionally replacing
+// it with a derived Context created using WithCancel, WithDeadline,
+// WithTimeout, or WithValue. When a Context is canceled, all
+// Contexts derived from it are also canceled.
+//
+// The WithCancel, WithDeadline, and WithTimeout functions take a
+// Context (the parent) and return a derived Context (the child) and a
+// CancelFunc. Calling the CancelFunc cancels the child and its
+// children, removes the parent's reference to the child, and stops
+// any associated timers. Failing to call the CancelFunc leaks the
+// child and its children until the parent is canceled or the timer
+// fires. The go vet tool checks that CancelFuncs are used on all
+// control-flow paths.
+//
+// Programs that use Contexts should follow these rules to keep interfaces
+// consistent across packages and enable static analysis tools to check context
+// propagation:
+//
+// Do not store Contexts inside a struct type; instead, pass a Context
+// explicitly to each function that needs it. The Context should be the first
+// parameter, typically named ctx:
+//
+// func DoSomething(ctx context.Context, arg Arg) error {
+// // ... use ctx ...
+// }
+//
+// Do not pass a nil Context, even if a function permits it. Pass context.TODO
+// if you are unsure about which Context to use.
+//
+// Use context Values only for request-scoped data that transits processes and
+// APIs, not for passing optional parameters to functions.
+//
+// The same Context may be passed to functions running in different goroutines;
+// Contexts are safe for simultaneous use by multiple goroutines.
+//
+// See https://blog.golang.org/context for example code for a server that uses
+// Contexts.
+package context
+
+import (
+ "errors"
+ "internal/reflectlite"
+ "sync"
+ "sync/atomic"
+ "time"
+)
+
+// A Context carries a deadline, a cancellation signal, and other values across
+// API boundaries.
+//
+// Context's methods may be called by multiple goroutines simultaneously.
+type Context interface {
+ // Deadline returns the time when work done on behalf of this context
+ // should be canceled. Deadline returns ok==false when no deadline is
+ // set. Successive calls to Deadline return the same results.
+ Deadline() (deadline time.Time, ok bool)
+
+ // Done returns a channel that's closed when work done on behalf of this
+ // context should be canceled. Done may return nil if this context can
+ // never be canceled. Successive calls to Done return the same value.
+ // The close of the Done channel may happen asynchronously,
+ // after the cancel function returns.
+ //
+ // WithCancel arranges for Done to be closed when cancel is called;
+ // WithDeadline arranges for Done to be closed when the deadline
+ // expires; WithTimeout arranges for Done to be closed when the timeout
+ // elapses.
+ //
+ // Done is provided for use in select statements:
+ //
+ // // Stream generates values with DoSomething and sends them to out
+ // // until DoSomething returns an error or ctx.Done is closed.
+ // func Stream(ctx context.Context, out chan<- Value) error {
+ // for {
+ // v, err := DoSomething(ctx)
+ // if err != nil {
+ // return err
+ // }
+ // select {
+ // case <-ctx.Done():
+ // return ctx.Err()
+ // case out <- v:
+ // }
+ // }
+ // }
+ //
+ // See https://blog.golang.org/pipelines for more examples of how to use
+ // a Done channel for cancellation.
+ Done() <-chan struct{}
+
+ // If Done is not yet closed, Err returns nil.
+ // If Done is closed, Err returns a non-nil error explaining why:
+ // Canceled if the context was canceled
+ // or DeadlineExceeded if the context's deadline passed.
+ // After Err returns a non-nil error, successive calls to Err return the same error.
+ Err() error
+
+ // Value returns the value associated with this context for key, or nil
+ // if no value is associated with key. Successive calls to Value with
+ // the same key returns the same result.
+ //
+ // Use context values only for request-scoped data that transits
+ // processes and API boundaries, not for passing optional parameters to
+ // functions.
+ //
+ // A key identifies a specific value in a Context. Functions that wish
+ // to store values in Context typically allocate a key in a global
+ // variable then use that key as the argument to context.WithValue and
+ // Context.Value. A key can be any type that supports equality;
+ // packages should define keys as an unexported type to avoid
+ // collisions.
+ //
+ // Packages that define a Context key should provide type-safe accessors
+ // for the values stored using that key:
+ //
+ // // Package user defines a User type that's stored in Contexts.
+ // package user
+ //
+ // import "context"
+ //
+ // // User is the type of value stored in the Contexts.
+ // type User struct {...}
+ //
+ // // key is an unexported type for keys defined in this package.
+ // // This prevents collisions with keys defined in other packages.
+ // type key int
+ //
+ // // userKey is the key for user.User values in Contexts. It is
+ // // unexported; clients use user.NewContext and user.FromContext
+ // // instead of using this key directly.
+ // var userKey key
+ //
+ // // NewContext returns a new Context that carries value u.
+ // func NewContext(ctx context.Context, u *User) context.Context {
+ // return context.WithValue(ctx, userKey, u)
+ // }
+ //
+ // // FromContext returns the User value stored in ctx, if any.
+ // func FromContext(ctx context.Context) (*User, bool) {
+ // u, ok := ctx.Value(userKey).(*User)
+ // return u, ok
+ // }
+ Value(key interface{}) interface{}
+}
+
+// Canceled is the error returned by Context.Err when the context is canceled.
+var Canceled = errors.New("context canceled")
+
+// DeadlineExceeded is the error returned by Context.Err when the context's
+// deadline passes.
+var DeadlineExceeded error = deadlineExceededError{}
+
+type deadlineExceededError struct{}
+
+func (deadlineExceededError) Error() string { return "context deadline exceeded" }
+func (deadlineExceededError) Timeout() bool { return true }
+func (deadlineExceededError) Temporary() bool { return true }
+
+// An emptyCtx is never canceled, has no values, and has no deadline. It is not
+// struct{}, since vars of this type must have distinct addresses.
+type emptyCtx int
+
+func (*emptyCtx) Deadline() (deadline time.Time, ok bool) {
+ return
+}
+
+func (*emptyCtx) Done() <-chan struct{} {
+ return nil
+}
+
+func (*emptyCtx) Err() error {
+ return nil
+}
+
+func (*emptyCtx) Value(key interface{}) interface{} {
+ return nil
+}
+
+func (e *emptyCtx) String() string {
+ switch e {
+ case background:
+ return "context.Background"
+ case todo:
+ return "context.TODO"
+ }
+ return "unknown empty Context"
+}
+
+var (
+ background = new(emptyCtx)
+ todo = new(emptyCtx)
+)
+
+// Background returns a non-nil, empty Context. It is never canceled, has no
+// values, and has no deadline. It is typically used by the main function,
+// initialization, and tests, and as the top-level Context for incoming
+// requests.
+func Background() Context {
+ return background
+}
+
+// TODO returns a non-nil, empty Context. Code should use context.TODO when
+// it's unclear which Context to use or it is not yet available (because the
+// surrounding function has not yet been extended to accept a Context
+// parameter).
+func TODO() Context {
+ return todo
+}
+
+// A CancelFunc tells an operation to abandon its work.
+// A CancelFunc does not wait for the work to stop.
+// A CancelFunc may be called by multiple goroutines simultaneously.
+// After the first call, subsequent calls to a CancelFunc do nothing.
+type CancelFunc func()
+
+// WithCancel returns a copy of parent with a new Done channel. The returned
+// context's Done channel is closed when the returned cancel function is called
+// or when the parent context's Done channel is closed, whichever happens first.
+//
+// Canceling this context releases resources associated with it, so code should
+// call cancel as soon as the operations running in this Context complete.
+func WithCancel(parent Context) (ctx Context, cancel CancelFunc) {
+ c := newCancelCtx(parent)
+ propagateCancel(parent, &c)
+ return &c, func() { c.cancel(true, Canceled) }
+}
+
+// newCancelCtx returns an initialized cancelCtx.
+func newCancelCtx(parent Context) cancelCtx {
+ return cancelCtx{Context: parent}
+}
+
+// goroutines counts the number of goroutines ever created; for testing.
+var goroutines int32
+
+// propagateCancel arranges for child to be canceled when parent is.
+func propagateCancel(parent Context, child canceler) {
+ done := parent.Done()
+ if done == nil {
+ return // parent is never canceled
+ }
+
+ select {
+ case <-done:
+ // parent is already canceled
+ child.cancel(false, parent.Err())
+ return
+ default:
+ }
+
+ if p, ok := parentCancelCtx(parent); ok {
+ p.mu.Lock()
+ if p.err != nil {
+ // parent has already been canceled
+ child.cancel(false, p.err)
+ } else {
+ if p.children == nil {
+ p.children = make(map[canceler]struct{})
+ }
+ p.children[child] = struct{}{}
+ }
+ p.mu.Unlock()
+ } else {
+ atomic.AddInt32(&goroutines, +1)
+ go func() {
+ select {
+ case <-parent.Done():
+ child.cancel(false, parent.Err())
+ case <-child.Done():
+ }
+ }()
+ }
+}
+
+// &cancelCtxKey is the key that a cancelCtx returns itself for.
+var cancelCtxKey int
+
+// parentCancelCtx returns the underlying *cancelCtx for parent.
+// It does this by looking up parent.Value(&cancelCtxKey) to find
+// the innermost enclosing *cancelCtx and then checking whether
+// parent.Done() matches that *cancelCtx. (If not, the *cancelCtx
+// has been wrapped in a custom implementation providing a
+// different done channel, in which case we should not bypass it.)
+func parentCancelCtx(parent Context) (*cancelCtx, bool) {
+ done := parent.Done()
+ if done == closedchan || done == nil {
+ return nil, false
+ }
+ p, ok := parent.Value(&cancelCtxKey).(*cancelCtx)
+ if !ok {
+ return nil, false
+ }
+ p.mu.Lock()
+ ok = p.done == done
+ p.mu.Unlock()
+ if !ok {
+ return nil, false
+ }
+ return p, true
+}
+
+// removeChild removes a context from its parent.
+func removeChild(parent Context, child canceler) {
+ p, ok := parentCancelCtx(parent)
+ if !ok {
+ return
+ }
+ p.mu.Lock()
+ if p.children != nil {
+ delete(p.children, child)
+ }
+ p.mu.Unlock()
+}
+
+// A canceler is a context type that can be canceled directly. The
+// implementations are *cancelCtx and *timerCtx.
+type canceler interface {
+ cancel(removeFromParent bool, err error)
+ Done() <-chan struct{}
+}
+
+// closedchan is a reusable closed channel.
+var closedchan = make(chan struct{})
+
+func init() {
+ close(closedchan)
+}
+
+// A cancelCtx can be canceled. When canceled, it also cancels any children
+// that implement canceler.
+type cancelCtx struct {
+ Context
+
+ mu sync.Mutex // protects following fields
+ done chan struct{} // created lazily, closed by first cancel call
+ children map[canceler]struct{} // set to nil by the first cancel call
+ err error // set to non-nil by the first cancel call
+}
+
+func (c *cancelCtx) Value(key interface{}) interface{} {
+ if key == &cancelCtxKey {
+ return c
+ }
+ return c.Context.Value(key)
+}
+
+func (c *cancelCtx) Done() <-chan struct{} {
+ c.mu.Lock()
+ if c.done == nil {
+ c.done = make(chan struct{})
+ }
+ d := c.done
+ c.mu.Unlock()
+ return d
+}
+
+func (c *cancelCtx) Err() error {
+ c.mu.Lock()
+ err := c.err
+ c.mu.Unlock()
+ return err
+}
+
+type stringer interface {
+ String() string
+}
+
+func contextName(c Context) string {
+ if s, ok := c.(stringer); ok {
+ return s.String()
+ }
+ return reflectlite.TypeOf(c).String()
+}
+
+func (c *cancelCtx) String() string {
+ return contextName(c.Context) + ".WithCancel"
+}
+
+// cancel closes c.done, cancels each of c's children, and, if
+// removeFromParent is true, removes c from its parent's children.
+func (c *cancelCtx) cancel(removeFromParent bool, err error) {
+ if err == nil {
+ panic("context: internal error: missing cancel error")
+ }
+ c.mu.Lock()
+ if c.err != nil {
+ c.mu.Unlock()
+ return // already canceled
+ }
+ c.err = err
+ if c.done == nil {
+ c.done = closedchan
+ } else {
+ close(c.done)
+ }
+ for child := range c.children {
+ // NOTE: acquiring the child's lock while holding parent's lock.
+ child.cancel(false, err)
+ }
+ c.children = nil
+ c.mu.Unlock()
+
+ if removeFromParent {
+ removeChild(c.Context, c)
+ }
+}
+
+// WithDeadline returns a copy of the parent context with the deadline adjusted
+// to be no later than d. If the parent's deadline is already earlier than d,
+// WithDeadline(parent, d) is semantically equivalent to parent. The returned
+// context's Done channel is closed when the deadline expires, when the returned
+// cancel function is called, or when the parent context's Done channel is
+// closed, whichever happens first.
+//
+// Canceling this context releases resources associated with it, so code should
+// call cancel as soon as the operations running in this Context complete.
+func WithDeadline(parent Context, d time.Time) (Context, CancelFunc) {
+ if cur, ok := parent.Deadline(); ok && cur.Before(d) {
+ // The current deadline is already sooner than the new one.
+ return WithCancel(parent)
+ }
+ c := &timerCtx{
+ cancelCtx: newCancelCtx(parent),
+ deadline: d,
+ }
+ propagateCancel(parent, c)
+ dur := time.Until(d)
+ if dur <= 0 {
+ c.cancel(true, DeadlineExceeded) // deadline has already passed
+ return c, func() { c.cancel(false, Canceled) }
+ }
+ c.mu.Lock()
+ defer c.mu.Unlock()
+ if c.err == nil {
+ c.timer = time.AfterFunc(dur, func() {
+ c.cancel(true, DeadlineExceeded)
+ })
+ }
+ return c, func() { c.cancel(true, Canceled) }
+}
+
+// A timerCtx carries a timer and a deadline. It embeds a cancelCtx to
+// implement Done and Err. It implements cancel by stopping its timer then
+// delegating to cancelCtx.cancel.
+type timerCtx struct {
+ cancelCtx
+ timer *time.Timer // Under cancelCtx.mu.
+
+ deadline time.Time
+}
+
+func (c *timerCtx) Deadline() (deadline time.Time, ok bool) {
+ return c.deadline, true
+}
+
+func (c *timerCtx) String() string {
+ return contextName(c.cancelCtx.Context) + ".WithDeadline(" +
+ c.deadline.String() + " [" +
+ time.Until(c.deadline).String() + "])"
+}
+
+func (c *timerCtx) cancel(removeFromParent bool, err error) {
+ c.cancelCtx.cancel(false, err)
+ if removeFromParent {
+ // Remove this timerCtx from its parent cancelCtx's children.
+ removeChild(c.cancelCtx.Context, c)
+ }
+ c.mu.Lock()
+ if c.timer != nil {
+ c.timer.Stop()
+ c.timer = nil
+ }
+ c.mu.Unlock()
+}
+
+// WithTimeout returns WithDeadline(parent, time.Now().Add(timeout)).
+//
+// Canceling this context releases resources associated with it, so code should
+// call cancel as soon as the operations running in this Context complete:
+//
+// func slowOperationWithTimeout(ctx context.Context) (Result, error) {
+// ctx, cancel := context.WithTimeout(ctx, 100*time.Millisecond)
+// defer cancel() // releases resources if slowOperation completes before timeout elapses
+// return slowOperation(ctx)
+// }
+func WithTimeout(parent Context, timeout time.Duration) (Context, CancelFunc) {
+ return WithDeadline(parent, time.Now().Add(timeout))
+}
+
+// WithValue returns a copy of parent in which the value associated with key is
+// val.
+//
+// Use context Values only for request-scoped data that transits processes and
+// APIs, not for passing optional parameters to functions.
+//
+// The provided key must be comparable and should not be of type
+// string or any other built-in type to avoid collisions between
+// packages using context. Users of WithValue should define their own
+// types for keys. To avoid allocating when assigning to an
+// interface{}, context keys often have concrete type
+// struct{}. Alternatively, exported context key variables' static
+// type should be a pointer or interface.
+func WithValue(parent Context, key, val interface{}) Context {
+ if key == nil {
+ panic("nil key")
+ }
+ if !reflectlite.TypeOf(key).Comparable() {
+ panic("key is not comparable")
+ }
+ return &valueCtx{parent, key, val}
+}
+
+// A valueCtx carries a key-value pair. It implements Value for that key and
+// delegates all other calls to the embedded Context.
+type valueCtx struct {
+ Context
+ key, val interface{}
+}
+
+// stringify tries a bit to stringify v, without using fmt, since we don't
+// want context depending on the unicode tables. This is only used by
+// *valueCtx.String().
+func stringify(v interface{}) string {
+ switch s := v.(type) {
+ case stringer:
+ return s.String()
+ case string:
+ return s
+ }
+ return "<not Stringer>"
+}
+
+func (c *valueCtx) String() string {
+ return contextName(c.Context) + ".WithValue(type " +
+ reflectlite.TypeOf(c.key).String() +
+ ", val " + stringify(c.val) + ")"
+}
+
+func (c *valueCtx) Value(key interface{}) interface{} {
+ if c.key == key {
+ return c.val
+ }
+ return c.Context.Value(key)
+}
