config: dynamic attestation configuration through S3 backed API (#1808)

internal/api/fetcher/fetcher.go

@@ -0,0 +1,105 @@
+/*
+Copyright (c) Edgeless Systems GmbH
+
+SPDX-License-Identifier: AGPL-3.0-only
+*/
+
+/*
+Package fetcher implements a client for the versions API.
+
+The fetcher is used to get information from the versions API without having to
+authenticate with AWS, where the API is currently hosted. This package should be
+used in user-facing application code most of the time, like the CLI.
+*/
+package fetcher
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"net/http"
+)
+
+// fetcher fetches versions API resources without authentication.
+type fetcher struct {
+	httpc HTTPClient
+}
+
+// NewHTTPClient returns a new http client.
+func NewHTTPClient() HTTPClient {
+	return &http.Client{Transport: &http.Transport{DisableKeepAlives: true}} // DisableKeepAlives fixes concurrency issue see https://stackoverflow.com/a/75816347
+}
+
+func newFetcherWith(client HTTPClient) *fetcher {
+	return &fetcher{
+		httpc: client,
+	}
+}
+
+func newFetcher() *fetcher {
+	return newFetcherWith(NewHTTPClient()) // DisableKeepAlives fixes concurrency issue see https://stackoverflow.com/a/75816347
+}
+
+type apiObject interface {
+	ValidateRequest() error
+	Validate() error
+	URL() (string, error)
+}
+
+func fetch[T apiObject](ctx context.Context, c HTTPClient, obj T) (T, error) {
+	if err := obj.ValidateRequest(); err != nil {
+		return *new(T), fmt.Errorf("validating request for %T: %w", obj, err)
+	}
+
+	url, err := obj.URL()
+	if err != nil {
+		return *new(T), fmt.Errorf("getting URL for %T: %w", obj, err)
+	}
+
+	req, err := http.NewRequestWithContext(ctx, http.MethodGet, url, http.NoBody)
+	if err != nil {
+		return *new(T), fmt.Errorf("creating request for %T: %w", obj, err)
+	}
+
+	resp, err := c.Do(req)
+	if err != nil {
+		return *new(T), fmt.Errorf("sending request for %T: %w", obj, err)
+	}
+	defer resp.Body.Close()
+	switch resp.StatusCode {
+	case http.StatusOK:
+	case http.StatusNotFound:
+		return *new(T), &NotFoundError{fmt.Errorf("requesting resource at %s returned status code 404", url)}
+	default:
+		return *new(T), fmt.Errorf("unexpected status code %d while requesting resource", resp.StatusCode)
+	}
+
+	var newObj T
+	if err := json.NewDecoder(resp.Body).Decode(&newObj); err != nil {
+		return *new(T), fmt.Errorf("decoding %T: %w", obj, err)
+	}
+
+	if newObj.Validate() != nil {
+		return *new(T), fmt.Errorf("received invalid %T: %w", newObj, newObj.Validate())
+	}
+
+	return newObj, nil
+}
+
+// NotFoundError is an error that is returned when a resource is not found.
+type NotFoundError struct {
+	err error
+}
+
+func (e *NotFoundError) Error() string {
+	return fmt.Sprintf("the requested resource was not found: %s", e.err.Error())
+}
+
+func (e *NotFoundError) Unwrap() error {
+	return e.err
+}
+
+// HTTPClient is an interface for http clients.
+type HTTPClient interface {
+	Do(req *http.Request) (*http.Response, error)
+}