context #

Context

This module 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 with_cancel, with_deadline, with_timeout, or with_value. When a Context is canceled, all Contexts derived from it are also canceled.

The with_cancel, with_deadline, and with_timeout functions take a Context (the parent) and return a derived Context (the child). Calling the cancel function cancels the child and its children, removes the parent's reference to the child, and stops any associated timers.

Programs that use Contexts should follow these rules to keep interfaces consistent across different modules.

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, just to make it more consistent.

Examples

In this section you can see some usage examples for this module

Context With Cancellation

import context

// This example demonstrates the use of a cancelable context to prevent a
// routine leak. By the end of the example function, the routine started
// by gen will return without leaking.
fn example_with_cancel() {
    // gen generates integers in a separate routine and
    // sends them to the returned channel.
    // The callers of gen need to cancel the context once
    // they are done consuming generated integers not to leak
    // the internal routine started by gen.
    gen := fn (ctx context.Context) chan int {
        dst := chan int{}
        go fn (ctx context.Context, dst chan int) {
            mut v := 0
            ch := ctx.done()
            for {
                select {
                   _ := <-ch {
                       // returning not to leak the routine
                       return
                   }
                   dst <- v {
                       v++
                   }
                }
            }
        }(ctx, dst)
        return dst
    }

    ctx := context.with_cancel(context.background())
    defer {
        context.cancel(ctx)
    }

    ch := gen(ctx)
    for i in 0 .. 5 {
        v := <-ch
        assert i == v
    }
}

Context With Deadline

import context
import time

const (
    // a reasonable duration to block in an example
    short_duration = 1 * time.millisecond
)

// This example passes a context with an arbitrary deadline to tell a blocking
// function that it should abandon its work as soon as it gets to it.
fn example_with_deadline() {
    dur := time.now().add(short_duration)
    ctx := context.with_deadline(context.background(), dur)

    defer {
        // Even though ctx will be expired, it is good practice to call its
        // cancellation function in any case. Failure to do so may keep the
        // context and its parent alive longer than necessary.
        context.cancel(ctx)
    }

    ctx_ch := ctx.done()
    select {
        _ := <-ctx_ch {}
        1 * time.second {
            panic('This should not happen')
        }
    }
}

Context With Timeout

import context
import time

const (
    // a reasonable duration to block in an example
    short_duration = 1 * time.millisecond
)

// This example passes a context with a timeout to tell a blocking function that
// it should abandon its work after the timeout elapses.
fn example_with_timeout() {
    // Pass a context with a timeout to tell a blocking function that it
    // should abandon its work after the timeout elapses.
    ctx := context.with_timeout(context.background(), short_duration)
    defer {
        context.cancel(ctx)
    }

    ctx_ch := ctx.done()
    select {
        _ := <-ctx_ch {}
        1 * time.second {
            panic('This should not happen')
        }
    }
}

Context With Value

import context

type ValueContextKey = string

// This example demonstrates how a value can be passed to the context
// and also how to retrieve it if it exists.
fn example_with_value() {
    f := fn (ctx context.Context, key ValueContextKey) string {
        if value := ctx.value(key) {
            if !isnil(value) {
                return *(&string(value))
            }
        }
        return 'key not found'
    }

    key := ValueContextKey('language')
    value := 'VAL'
    ctx := context.with_value(context.background(), key, &value)

    assert value == f(ctx, key)
    assert 'key not found' == f(ctx, ValueContextKey('color'))
}

fn background #

fn background() Context

background returns an 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.

fn cancel #

fn cancel(ctx Context)

fn todo #

fn todo() Context

todo returns an empty Context. Code should use 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).

fn with_cancel #

fn with_cancel(parent Context) Context

with_cancel 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.

fn with_deadline #

fn with_deadline(parent Context, d time.Time) Context

with_deadline 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, with_deadline(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.

fn with_timeout #

fn with_timeout(parent Context, timeout time.Duration) Context

with_timeout returns with_deadline(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

fn with_value #

fn with_value(parent Context, key string, value voidptr) Context

with_value 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 with_value should define their own types for keys

interface Canceler #

interface Canceler {
	id string
	cancel(remove_from_parent bool, err IError)
	done() chan int
}

interface Context #

interface Context {
	deadline() ?time.Time
	done() chan int
	err() IError
	value(key string) ?voidptr
	str() string
}

type EmptyContext #

type EmptyContext = int

An EmptyContext is never canceled, has no values, and has no deadline. It is not struct{}, since vars of this type must have distinct addresses.

fn (EmptyContext) deadline #

fn (ctx EmptyContext) deadline() ?time.Time

fn (EmptyContext) done #

fn (ctx EmptyContext) done() chan int

fn (EmptyContext) err #

fn (ctx EmptyContext) err() IError

fn (EmptyContext) value #

fn (ctx EmptyContext) value(key string) ?voidptr

fn (EmptyContext) str #

fn (ctx EmptyContext) str() string

struct CancelContext #

struct CancelContext {
	id string
mut:
	context  Context
	mutex    &sync.Mutex
	done     chan int
	children map[string]Canceler
	err      IError
}

A CancelContext can be canceled. When canceled, it also cancels any children that implement Canceler.

fn (CancelContext) deadline #

fn (ctx CancelContext) deadline() ?time.Time

fn (CancelContext) done #

fn (mut ctx CancelContext) done() chan int

fn (CancelContext) err #

fn (mut ctx CancelContext) err() IError

fn (CancelContext) value #

fn (ctx CancelContext) value(key string) ?voidptr

fn (CancelContext) str #

fn (ctx CancelContext) str() string

struct TimerContext #

struct TimerContext {
	id string
mut:
	cancel_ctx CancelContext
	deadline   time.Time
}

A TimerContext carries a timer and a deadline. It embeds a CancelContext to implement done and err. It implements cancel by stopping its timer then delegating to CancelContext.cancel

fn (TimerContext) deadline #

fn (ctx TimerContext) deadline() ?time.Time

fn (TimerContext) done #

fn (mut ctx TimerContext) done() chan int

fn (TimerContext) err #

fn (mut ctx TimerContext) err() IError

fn (TimerContext) value #

fn (ctx TimerContext) value(key string) ?voidptr

fn (TimerContext) cancel #

fn (mut ctx TimerContext) cancel(remove_from_parent bool, err IError)

fn (TimerContext) str #

fn (ctx TimerContext) str() string

struct ValueContext #

struct ValueContext {
	key   string
	value voidptr
mut:
	context Context
}

A ValueContext carries a key-value pair. It implements Value for that key and delegates all other calls to the embedded Context.

fn (ValueContext) deadline #

fn (ctx ValueContext) deadline() ?time.Time

fn (ValueContext) done #

fn (ctx ValueContext) done() chan int

fn (ValueContext) err #

fn (ctx ValueContext) err() IError

fn (ValueContext) value #

fn (ctx ValueContext) value(key string) ?voidptr

fn (ValueContext) str #

fn (ctx ValueContext) str() string