Commit e4dc7f1e authored by Sameer Ajmani's avatar Sameer Ajmani

context: update documentation on cancelation and go vet check.

Also replace double spaces after periods with single spaces.

Change-Id: Iedaea47595c5ce64e7e8aa3a368f36d49061c555
Reviewed-on: https://go-review.googlesource.com/24431Reviewed-by: default avatarAlan Donovan <adonovan@google.com>
Reviewed-by: default avatarIan Lance Taylor <iant@golang.org>
parent 3c6ed76d
...@@ -6,32 +6,35 @@ ...@@ -6,32 +6,35 @@
// cancelation signals, and other request-scoped values across API boundaries // cancelation signals, and other request-scoped values across API boundaries
// and between processes. // and between processes.
// //
// Incoming requests to a server should create a Context, and outgoing calls to // Incoming requests to a server should create a Context, and outgoing
// servers should accept a Context. The chain of function calls between them // calls to servers should accept a Context. The chain of function
// must propagate the Context, optionally replacing it with a derived Context // calls between them must propagate the Context, optionally replacing
// created using WithCancel, WithDeadline, WithTimeout, or WithValue. These // it with a derived Context created using WithCancel, WithDeadline,
// Context values form a tree: when a Context is canceled, all Contexts derived // WithTimeout, or WithValue. When a Context is canceled, all
// from it are also canceled. // Contexts derived from it are also canceled.
// //
// The WithCancel, WithDeadline, and WithTimeout functions return a derived // The WithCancel, WithDeadline, and WithTimeout functions take a
// Context and a CancelFunc. Calling the CancelFunc cancels the new Context and // Context (the parent) and return a derived Context (the child) and a
// any Contexts derived from it, removes the Context from the parent's tree, and // CancelFunc. Calling the CancelFunc cancels the child and its
// stops any associated timers. Failing to call the CancelFunc leaks the // children, removes the parent's reference to the child, and stops
// associated resources until the parent Context is canceled or the timer fires. // 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 // Programs that use Contexts should follow these rules to keep interfaces
// consistent across packages and enable static analysis tools to check context // consistent across packages and enable static analysis tools to check context
// propagation: // propagation:
// //
// Do not store Contexts inside a struct type; instead, pass a Context // 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 // explicitly to each function that needs it. The Context should be the first
// parameter, typically named ctx: // parameter, typically named ctx:
// //
// func DoSomething(ctx context.Context, arg Arg) error { // func DoSomething(ctx context.Context, arg Arg) error {
// // ... use ctx ... // // ... use ctx ...
// } // }
// //
// Do not pass a nil Context, even if a function permits it. Pass context.TODO // Do not pass a nil Context, even if a function permits it. Pass context.TODO
// if you are unsure about which Context to use. // if you are unsure about which Context to use.
// //
// Use context Values only for request-scoped data that transits processes and // Use context Values only for request-scoped data that transits processes and
...@@ -58,13 +61,13 @@ import ( ...@@ -58,13 +61,13 @@ import (
// Context's methods may be called by multiple goroutines simultaneously. // Context's methods may be called by multiple goroutines simultaneously.
type Context interface { type Context interface {
// Deadline returns the time when work done on behalf of this context // Deadline returns the time when work done on behalf of this context
// should be canceled. Deadline returns ok==false when no deadline is // should be canceled. Deadline returns ok==false when no deadline is
// set. Successive calls to Deadline return the same results. // set. Successive calls to Deadline return the same results.
Deadline() (deadline time.Time, ok bool) Deadline() (deadline time.Time, ok bool)
// Done returns a channel that's closed when work done on behalf of this // 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 // context should be canceled. Done may return nil if this context can
// never be canceled. Successive calls to Done return the same value. // never be canceled. Successive calls to Done return the same value.
// //
// WithCancel arranges for Done to be closed when cancel is called; // WithCancel arranges for Done to be closed when cancel is called;
// WithDeadline arranges for Done to be closed when the deadline // WithDeadline arranges for Done to be closed when the deadline
...@@ -93,24 +96,24 @@ type Context interface { ...@@ -93,24 +96,24 @@ type Context interface {
// a Done channel for cancelation. // a Done channel for cancelation.
Done() <-chan struct{} Done() <-chan struct{}
// Err returns a non-nil error value after Done is closed. Err returns // Err returns a non-nil error value after Done is closed. Err returns
// Canceled if the context was canceled or DeadlineExceeded if the // Canceled if the context was canceled or DeadlineExceeded if the
// context's deadline passed. No other values for Err are defined. // context's deadline passed. No other values for Err are defined.
// After Done is closed, successive calls to Err return the same value. // After Done is closed, successive calls to Err return the same value.
Err() error Err() error
// Value returns the value associated with this context for key, or nil // Value returns the value associated with this context for key, or nil
// if no value is associated with key. Successive calls to Value with // if no value is associated with key. Successive calls to Value with
// the same key returns the same result. // the same key returns the same result.
// //
// Use context values only for request-scoped data that transits // Use context values only for request-scoped data that transits
// processes and API boundaries, not for passing optional parameters to // processes and API boundaries, not for passing optional parameters to
// functions. // functions.
// //
// A key identifies a specific value in a Context. Functions that wish // A key identifies a specific value in a Context. Functions that wish
// to store values in Context typically allocate a key in a global // to store values in Context typically allocate a key in a global
// variable then use that key as the argument to context.WithValue and // variable then use that key as the argument to context.WithValue and
// Context.Value. A key can be any type that supports equality; // Context.Value. A key can be any type that supports equality;
// packages should define keys as an unexported type to avoid // packages should define keys as an unexported type to avoid
// collisions. // collisions.
// //
...@@ -129,7 +132,7 @@ type Context interface { ...@@ -129,7 +132,7 @@ type Context interface {
// // This prevents collisions with keys defined in other packages. // // This prevents collisions with keys defined in other packages.
// type key int // type key int
// //
// // userKey is the key for user.User values in Contexts. It is // // userKey is the key for user.User values in Contexts. It is
// // unexported; clients use user.NewContext and user.FromContext // // unexported; clients use user.NewContext and user.FromContext
// // instead of using this key directly. // // instead of using this key directly.
// var userKey key = 0 // var userKey key = 0
...@@ -160,7 +163,7 @@ func (deadlineExceededError) Error() string { return "context deadline exceeded" ...@@ -160,7 +163,7 @@ func (deadlineExceededError) Error() string { return "context deadline exceeded"
func (deadlineExceededError) Timeout() bool { return true } func (deadlineExceededError) Timeout() bool { return true }
// An emptyCtx is never canceled, has no values, and has no deadline. It is not // 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. // struct{}, since vars of this type must have distinct addresses.
type emptyCtx int type emptyCtx int
...@@ -196,17 +199,17 @@ var ( ...@@ -196,17 +199,17 @@ var (
) )
// Background returns a non-nil, empty Context. It is never canceled, has no // 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, // values, and has no deadline. It is typically used by the main function,
// initialization, and tests, and as the top-level Context for incoming // initialization, and tests, and as the top-level Context for incoming
// requests. // requests.
func Background() Context { func Background() Context {
return background return background
} }
// TODO returns a non-nil, empty Context. Code should use context.TODO when // 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 // 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 // surrounding function has not yet been extended to accept a Context
// parameter). TODO is recognized by static analysis tools that determine // parameter). TODO is recognized by static analysis tools that determine
// whether Contexts are propagated correctly in a program. // whether Contexts are propagated correctly in a program.
func TODO() Context { func TODO() Context {
return todo return todo
...@@ -266,7 +269,7 @@ func propagateCancel(parent Context, child canceler) { ...@@ -266,7 +269,7 @@ func propagateCancel(parent Context, child canceler) {
} }
// parentCancelCtx follows a chain of parent references until it finds a // parentCancelCtx follows a chain of parent references until it finds a
// *cancelCtx. This function understands how each of the concrete types in this // *cancelCtx. This function understands how each of the concrete types in this
// package represents its parent. // package represents its parent.
func parentCancelCtx(parent Context) (*cancelCtx, bool) { func parentCancelCtx(parent Context) (*cancelCtx, bool) {
for { for {
...@@ -296,14 +299,14 @@ func removeChild(parent Context, child canceler) { ...@@ -296,14 +299,14 @@ func removeChild(parent Context, child canceler) {
p.mu.Unlock() p.mu.Unlock()
} }
// A canceler is a context type that can be canceled directly. The // A canceler is a context type that can be canceled directly. The
// implementations are *cancelCtx and *timerCtx. // implementations are *cancelCtx and *timerCtx.
type canceler interface { type canceler interface {
cancel(removeFromParent bool, err error) cancel(removeFromParent bool, err error)
Done() <-chan struct{} Done() <-chan struct{}
} }
// A cancelCtx can be canceled. When canceled, it also cancels any children // A cancelCtx can be canceled. When canceled, it also cancels any children
// that implement canceler. // that implement canceler.
type cancelCtx struct { type cancelCtx struct {
Context Context
...@@ -355,8 +358,8 @@ func (c *cancelCtx) cancel(removeFromParent bool, err error) { ...@@ -355,8 +358,8 @@ func (c *cancelCtx) cancel(removeFromParent bool, err error) {
} }
// WithDeadline returns a copy of the parent context with the deadline adjusted // 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, // 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 // WithDeadline(parent, d) is semantically equivalent to parent. The returned
// context's Done channel is closed when the deadline expires, when 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 // cancel function is called, or when the parent context's Done channel is
// closed, whichever happens first. // closed, whichever happens first.
...@@ -388,8 +391,8 @@ func WithDeadline(parent Context, deadline time.Time) (Context, CancelFunc) { ...@@ -388,8 +391,8 @@ func WithDeadline(parent Context, deadline time.Time) (Context, CancelFunc) {
return c, func() { c.cancel(true, Canceled) } return c, func() { c.cancel(true, Canceled) }
} }
// A timerCtx carries a timer and a deadline. It embeds a cancelCtx to // 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 // implement Done and Err. It implements cancel by stopping its timer then
// delegating to cancelCtx.cancel. // delegating to cancelCtx.cancel.
type timerCtx struct { type timerCtx struct {
cancelCtx cancelCtx
...@@ -451,7 +454,7 @@ func WithValue(parent Context, key, val interface{}) Context { ...@@ -451,7 +454,7 @@ func WithValue(parent Context, key, val interface{}) Context {
return &valueCtx{parent, key, val} return &valueCtx{parent, key, val}
} }
// A valueCtx carries a key-value pair. It implements Value for that key and // A valueCtx carries a key-value pair. It implements Value for that key and
// delegates all other calls to the embedded Context. // delegates all other calls to the embedded Context.
type valueCtx struct { type valueCtx struct {
Context Context
......
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