Discovery: Conditionally embed latest discovery files for local cache

Conditionally embedding based on
https://github.com/golang/go/issues/44484#issuecomment-948137497

To use it, wget the discovery files into the internal/discovery
folder (same JSON name) after verifying the signature. Then build with -tags=release

3 files changed, 78 insertions, 3 deletions

diff --git a/internal/discovery/discovery.go b/internal/discovery/discovery.go
index e561951..1c1531e 100644
--- a/internal/discovery/discovery.go
+++ b/internal/discovery/discovery.go
@@ -12,6 +12,9 @@ import (
 	"github.com/go-errors/errors"
 )
 
+// HasCache denotes whether or not we have an embedded cache available
+var HasCache bool
+
 // Discovery is the main structure used for this package.
 type Discovery struct {
 	// The httpClient for sending HTTP requests
@@ -73,7 +76,7 @@ func (discovery *Discovery) file(jsonFile string, previousVersion uint64, struct
 	// Parse JSON to extract version and list
 	if err = json.Unmarshal(body, structure); err != nil {
 		return errors.WrapPrefix(err,
-			fmt.Sprintf("failed getting file: %s from the Discovery server", jsonFile), 0)
+			fmt.Sprintf("failed parsing discovery file: %s from the server", jsonFile), 0)
 	}
 
 	return nil
@@ -175,6 +178,38 @@ func (discovery *Discovery) DetermineServersUpdate() bool {
 	return !time.Now().Before(upd)
 }
 
+func (discovery *Discovery) previousOrganizations() (*types.DiscoveryOrganizations, error) {
+	// If the version field is not zero then we have a cached struct
+	// We also immediately return this copy if we have no embedded JSON
+	if discovery.organizations.Version != 0 || !HasCache {
+		return &discovery.organizations, nil
+	}
+
+	// We do not have a cached struct, this we need to get it using the embedded JSON
+	var eo types.DiscoveryOrganizations
+	if err := json.Unmarshal(eOrganizations, &eo); err != nil {
+		return nil, errors.WrapPrefix(err, "failed parsing discovery organizations from the embedded cache", 0)
+	}
+	discovery.organizations = eo
+	return &eo, nil
+}
+
+func (discovery *Discovery) previousServers() (*types.DiscoveryServers, error) {
+	// If the version field is not zero then we have a cached struct
+	// We also immediately return this copy if we have no embedded JSON
+	if discovery.servers.Version != 0 || !HasCache {
+		return &discovery.servers, nil
+	}
+
+	// We do not have a cached struct, this we need to get it using the embedded JSON
+	var es types.DiscoveryServers
+	if err := json.Unmarshal(eServers, &es); err != nil {
+		return nil, errors.WrapPrefix(err, "failed parsing discovery servers from the embedded cache", 0)
+	}
+	discovery.servers = es
+	return &es, nil
+}
+
 // Organizations returns the discovery organizations
 // If there was an error, a cached copy is returned if available.
 func (discovery *Discovery) Organizations() (*types.DiscoveryOrganizations, error) {
@@ -185,7 +220,9 @@ func (discovery *Discovery) Organizations() (*types.DiscoveryOrganizations, erro
 	err := discovery.file(file, discovery.organizations.Version, &discovery.organizations)
 	if err != nil {
 		// Return previous with an error
-		return &discovery.organizations, err
+		// TODO: Log here if we fail to get previous
+		orgs, _ := discovery.previousOrganizations()
+		return orgs, err
 	}
 	discovery.organizations.Timestamp = time.Now()
 	return &discovery.organizations, nil
@@ -201,7 +238,9 @@ func (discovery *Discovery) Servers() (*types.DiscoveryServers, error) {
 	err := discovery.file(file, discovery.servers.Version, &discovery.servers)
 	if err != nil {
 		// Return previous with an error
-		return &discovery.servers, err
+		// TODO: Log here if we fail to get previous
+		srvs, _ := discovery.previousServers()
+		return srvs, err
 	}
 	// Update servers timestamp
 	discovery.servers.Timestamp = time.Now()
diff --git a/internal/discovery/discovery_embed.go b/internal/discovery/discovery_embed.go
new file mode 100644
index 0000000..efd85dd
--- /dev/null
+++ b/internal/discovery/discovery_embed.go
@@ -0,0 +1,20 @@
+//go:build release
+// +build release
+
+package discovery
+
+import _ "embed"
+
+// eServers are the embedded server list
+//
+//go:embed server_list.json
+var eServers []byte
+
+// eOrganizations are the embedded organizations
+//
+//go:embed organization_list.json
+var eOrganizations []byte
+
+func init() {
+	HasCache = true
+}
diff --git a/internal/discovery/discovery_noembed.go b/internal/discovery/discovery_noembed.go
new file mode 100644
index 0000000..5c97331
--- /dev/null
+++ b/internal/discovery/discovery_noembed.go
@@ -0,0 +1,16 @@
+//go:build !release
+// +build !release
+
+package discovery
+
+// eServers are the embedded server list
+// In this case this is empty because we do not embed during development
+var eServers []byte
+
+// eOrganizations are the embedded organizations
+// In this case this is empty because we do not embed during development
+var eOrganizations []byte
+
+func init() {
+	HasCache = false
+}