context: add a new context module, based on Golang's context, intended to be used in webservers (#9563)

vlib/context/value.v

@@ -0,0 +1,56 @@
+module context
+
+import time
+
+// A ValueContext carries a key-value pair. It implements Value for that key and
+// delegates all other calls to the embedded Context.
+pub struct ValueContext {
+	key   string
+	value voidptr
+mut:
+	context 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
+pub fn with_value(parent Context, key string, value voidptr) &ValueContext {
+	if isnil(key) {
+		panic('nil key')
+	}
+	return &ValueContext{
+		context: parent
+		key: key
+		value: value
+	}
+}
+
+pub fn (ctx ValueContext) deadline() ?time.Time {
+	return ctx.context.deadline()
+}
+
+pub fn (ctx ValueContext) done() chan int {
+	return ctx.context.done()
+}
+
+pub fn (ctx ValueContext) err() string {
+	return ctx.context.err()
+}
+
+pub fn (ctx ValueContext) value(key string) ?voidptr {
+	if ctx.key == key {
+		return ctx.value
+	}
+	return ctx.context.value(key)
+}
+
+pub fn (ctx ValueContext) str() string {
+	return context_name(ctx.context) + '.with_value'
+}