**Description**:
Enforce order of start and shutdown of extensions according to their
internally declared dependencies
**Link to tracking Issue**:
Resolves#8732
**Motivation**:
This is an alternative approach to #8733 which uses declaration order in
the config to start extensions. That approach (a) enforces order when
it's not always necessary to enforce, and (b) exposes unnecessary
complexity to the user by making them responsible for the order.
This PR instead derives the desired order of extensions based on the
dependencies they declare by implementing a `DependentExtension`
interface. That means that extensions that must depend on others can
expose this interface and be guaranteed to start after their
dependencies, while other extensions can be started in arbitrary order
(same as happens today because of iterating over a map).
The extensions that have dependencies have two options to expose them:
1. if the dependency is always static (e.g. `jaeger_query` extension
depending on `jaeger_storage` as in the OP), the extension can express
this statically as well, by returning a predefined ID of the dependent
extension
2. in cases where dependencies are dynamic, the extension can read the
names of the dependencies from its configuration.
The 2nd scenario is illustrated by the following configuration. Here
each complex extension knows that it needs dependencies that implement
`storage` and `encoding` interfaces (both existing APIs in collector &
contrib), but does not know statically which instances of those, the
actual names are supplied by the user in the configuration.
```yaml
extensions:
complex_extension_1:
storage: filestorage
encoding: otlpencoding
complex_extension_2:
storage: dbstorage
encoding: jsonencoding
filestorage:
...
dbstorage:
...
otlpencoding:
jsonencoding:
```
**Changes**:
* Introduce `DependentExtension` optional interface
* Change `Extensions` constructor to derive the required order using a
directed graph (similar to pipelines)
* Inherited from #8733 - use new ordered list of IDs to
start/stop/notify extensions in the desired order (previously a map was
used to iterate over, which resulted in random order).
* Tests
**Testing**:
Unit tests
---------
Signed-off-by: Yuri Shkuro <github@ysh.us>
Co-authored-by: Antoine Toulme <antoine@lunar-ocean.com>
Yuri Shkuro