Implements Fetch in MuxCache (#1326)

* Implements `Fetch` in `MuxCache`

`MuxCache` doesn't implement the `Fetch` method and it returns an error.
However `MuxCache` doesn't have any logic instead it delegates to the appropriate
cache based on the request. This means that we can just use the same logic as `CreateWatch`
to delegate to the appropriate cache's `Fetch` method.

`Linear` cache doesnt implement `Fetch` either so this part of the call will fail but the rest
of the caches will work as expected.

Additionally it was a bit confusing to debug the issue since the error message resembles quite a
bit a `gRPC Unimplemented` error. So I updated the error message in `Linear` cache to be more specific.

Signed-off-by: sotiris <[email protected]>

* make error lowercase

Signed-off-by: sotiris <[email protected]>

---------

Signed-off-by: sotiris <[email protected]>

Author: davinci26

pkg/cache/v3/linear.go

@@ -570,7 +570,7 @@ func (cache *LinearCache) getVersion() string {
 }
 
 func (cache *LinearCache) Fetch(context.Context, *Request) (Response, error) {
-	return nil, errors.New("not implemented")
+	return nil, errors.New("fetch is not implemented by LinearCache")
 }
 
 // NumResources returns the number of resources currently in the cache.

pkg/cache/v3/mux.go

@@ -16,7 +16,6 @@ package cache
 
 import (
 	"context"
-	"errors"
 	"fmt"
 )
 
@@ -54,6 +53,13 @@ func (mux *MuxCache) CreateDeltaWatch(request *DeltaRequest, sub Subscription, v
 	return cache.CreateDeltaWatch(request, sub, value)
 }
 
-func (mux *MuxCache) Fetch(context.Context, *Request) (Response, error) {
-	return nil, errors.New("not implemented")
+func (mux *MuxCache) Fetch(ctx context.Context, request *Request) (Response, error) {
+	key := mux.Classify(request)
+	cache, exists := mux.Caches[key]
+	if !exists {
+		return nil, fmt.Errorf("no cache defined for key %s", key)
+	}
+	// Not all caches may implement Fetch; we delegate to the selected cache and if
+	// it does not implement Fetch, it will return an error.
+	return cache.Fetch(ctx, request)
 }