vlib/context: add onecontext as submodule (#12549)

Author: ulises-jeremias

cmd/tools/vtest-self.v

@@ -8,6 +8,7 @@ const github_job = os.getenv('GITHUB_JOB')
 
 const (
 	skip_test_files               = [
+		'vlib/context/onecontext/onecontext_test.v',
 		'vlib/context/deadline_test.v' /* sometimes blocks */,
 		'vlib/mysql/mysql_orm_test.v' /* mysql not installed */,
 		'vlib/pg/pg_orm_test.v' /* pg not installed */,

vlib/context/cancel.v

@@ -51,9 +51,10 @@ mut:
 pub fn with_cancel(mut parent Context) (Context, CancelFn) {
 	mut c := new_cancel_context(parent)
 	propagate_cancel(mut parent, mut c)
-	return Context(c), fn [mut c] () {
+	cancel_fn := fn [mut c] () {
 		c.cancel(true, canceled)
 	}
+	return Context(c), CancelFn(cancel_fn)
 }
 
 // new_cancel_context returns an initialized CancelContext.

vlib/context/deadline.v

@@ -44,9 +44,10 @@ pub fn with_deadline(mut parent Context, d time.Time) (Context, CancelFn) {
 	dur := d - time.now()
 	if dur.nanoseconds() <= 0 {
 		ctx.cancel(true, deadline_exceeded) // deadline has already passed
-		return Context(ctx), fn [mut ctx] () {
+		cancel_fn := fn [mut ctx] () {
 			ctx.cancel(true, canceled)
 		}
+		return Context(ctx), CancelFn(cancel_fn)
 	}
 
 	if ctx.err() is none {
@@ -55,9 +56,11 @@ pub fn with_deadline(mut parent Context, d time.Time) (Context, CancelFn) {
 			ctx.cancel(true, deadline_exceeded)
 		}(mut ctx, dur)
 	}
-	return Context(ctx), fn [mut ctx] () {
+
+	cancel_fn := fn [mut ctx] () {
 		ctx.cancel(true, canceled)
 	}
+	return Context(ctx), CancelFn(cancel_fn)
 }
 
 // with_timeout returns with_deadline(parent, time.now().add(timeout)).

vlib/context/onecontext/README.md

@@ -0,0 +1,40 @@
+# onecontext
+
+A library to merge existing V contexts.
+
+## Overview
+
+Have you ever faced the situation where you have to merge multiple existing contexts?
+If not, then you might, eventually.
+
+For example, we can face the situation where we are building an application
+using a library that gives us a global context.
+This context expires once the application is stopped.
+
+Meanwhile, we are exposing a service like this:
+
+```v ignore
+fn (f Foo) get(ctx context.Context, bar Bar) ?Baz {
+	. . .
+}
+```
+
+Here, we receive another context provided by the service.
+
+Then, in the `get` implementation, we want for example to query a database and
+we must provide a context for that.
+
+Ideally, we would like to provide a merged context that would expire either:
+
+- When the application is stopped
+- Or when the received service context expires
+
+This is exactly the purpose of this library.
+
+In our case, we can now merge the two contexts in a single one like this:
+
+```v ignore
+ctx, cancel := onecontext.merge(ctx1, ctx2)
+```
+
+This returns a merged context that we can now propagate

vlib/context/onecontext/onecontext.v

@@ -0,0 +1,146 @@
+module onecontext
+
+import context
+import sync
+import time
+
+// canceled is the error returned when the cancel function is called on a merged context
+pub const canceled = error('canceled context')
+
+struct OneContext {
+mut:
+	ctx        context.Context
+	ctxs       []context.Context
+	done       chan int
+	err        IError = none
+	err_mutex  sync.Mutex
+	cancel_fn  context.CancelFn
+	cancel_ctx context.Context
+}
+
+// merge allows to merge multiple contexts
+// it returns the merged context
+pub fn merge(ctx context.Context, ctxs ...context.Context) (context.Context, context.CancelFn) {
+	mut background := context.background()
+	cancel_ctx, cancel := context.with_cancel(mut &background)
+	mut octx := &OneContext{
+		done: chan int{cap: 3}
+		ctx: ctx
+		ctxs: ctxs
+		cancel_fn: cancel
+		cancel_ctx: cancel_ctx
+	}
+	go octx.run()
+	return context.Context(octx), context.CancelFn(cancel)
+}
+
+pub fn (octx OneContext) deadline() ?time.Time {
+	mut min := time.Time{}
+
+	if deadline := octx.ctx.deadline() {
+		min = deadline
+	}
+
+	for ctx in octx.ctxs {
+		if deadline := ctx.deadline() {
+			if min.unix_time() == 0 || deadline < min {
+				min = deadline
+			}
+		}
+	}
+
+	if min.unix_time() == 0 {
+		return none
+	}
+
+	return min
+}
+
+pub fn (octx OneContext) done() chan int {
+	return octx.done
+}
+
+pub fn (mut octx OneContext) err() IError {
+	octx.err_mutex.@lock()
+	defer {
+		octx.err_mutex.unlock()
+	}
+	return octx.err
+}
+
+pub fn (octx OneContext) value(key context.Key) ?context.Any {
+	if value := octx.ctx.value(key) {
+		return value
+	}
+
+	for ctx in octx.ctxs {
+		if value := ctx.value(key) {
+			return value
+		}
+	}
+
+	return none
+}
+
+pub fn (mut octx OneContext) run() {
+	mut wrapped_ctx := &octx.ctx
+	if octx.ctxs.len == 1 {
+		mut first_ctx := &octx.ctxs[0]
+		octx.run_two_contexts(mut wrapped_ctx, mut first_ctx)
+		return
+	}
+
+	octx.run_multiple_contexts(mut wrapped_ctx)
+	for mut ctx in octx.ctxs {
+		octx.run_multiple_contexts(mut &ctx)
+	}
+}
+
+pub fn (octx OneContext) str() string {
+	return ''
+}
+
+pub fn (mut octx OneContext) cancel(err IError) {
+	octx.cancel_fn()
+	octx.err_mutex.@lock()
+	octx.err = err
+	octx.err_mutex.unlock()
+	if !octx.done.closed {
+		octx.done <- 0
+		octx.done.close()
+	}
+}
+
+pub fn (mut octx OneContext) run_two_contexts(mut ctx1 context.Context, mut ctx2 context.Context) {
+	go fn (mut octx OneContext, mut ctx1 context.Context, mut ctx2 context.Context) {
+		octx_cancel_done := octx.cancel_ctx.done()
+		c1done := ctx1.done()
+		c2done := ctx2.done()
+		select {
+			_ := <-octx_cancel_done {
+				octx.cancel(onecontext.canceled)
+			}
+			_ := <-c1done {
+				octx.cancel(ctx1.err())
+			}
+			_ := <-c2done {
+				octx.cancel(ctx1.err())
+			}
+		}
+	}(mut &octx, mut &ctx1, mut &ctx2)
+}
+
+pub fn (mut octx OneContext) run_multiple_contexts(mut ctx context.Context) {
+	go fn (mut octx OneContext, mut ctx context.Context) {
+		octx_cancel_done := octx.cancel_ctx.done()
+		cdone := ctx.done()
+		select {
+			_ := <-octx_cancel_done {
+				octx.cancel(onecontext.canceled)
+			}
+			_ := <-cdone {
+				octx.cancel(ctx.err())
+			}
+		}
+	}(mut &octx, mut &ctx)
+}