internal/exec/engine: write empty cache config when not provided

When the user-provided config is empty write an empty cache config.
Additionally restrict fetching configs to only `fetch` stages.

internal/exec/engine.go

@@ -24,6 +24,7 @@ import (
 	"net/url"
 	"os"
 	"strconv"
+	"strings"
 	"time"
 
 	"github.com/coreos/go-systemd/v22/journal"
@@ -108,15 +109,19 @@ func (e Engine) Run(stageName string) error {
 		e.Fetcher.Offline = true
 	}
 
-	cfg, err := e.acquireConfig()
+	cfg, err := e.acquireConfig(stageName)
 	if err == resource.ErrNeedNet && stageName == "fetch-offline" {
 		err = SignalNeedNet()
 		if err != nil {
 			e.Logger.Crit("failed to signal neednet: %v", err)
 		}
 		return err
 	} else if err == errors.ErrEmpty {
-		e.Logger.Info("%v: ignoring user-provided config", err)
+		e.Logger.Info("%v: ignoring user-provided config & writing empty cache config", err)
+		if err = renameio.WriteFile(e.ConfigCache, []byte("{}"), 0640); err != nil {
+			e.Logger.Crit("failed to empty write cached config: %v", err)
+			return err
+		}
 	} else if err != nil {
 		e.Logger.Crit("failed to acquire config: %v", err)
 		return err
@@ -162,11 +167,24 @@ func logStructuredJournalEntry(config string, src string, isReferenced bool) err
 	return nil
 }
 
-// acquireConfig returns the configuration, first checking a local cache
-// before attempting to fetch it from the provider.
-func (e *Engine) acquireConfig() (cfg types.Config, err error) {
+// acquireConfig returns the configuration from a local cache. If one
+// does not exist then it will attempt to fetch the configuration from
+// the provider when in fetch stages.
+func (e *Engine) acquireConfig(stageName string) (cfg types.Config, err error) {
+	cfg, err = e.acquireCachedConfig()
+	if err != nil {
+		if os.IsNotExist(err) && strings.HasPrefix(stageName, "fetch") {
+			e.Logger.Info("fetching provider config")
+			cfg, err = e.acquireProviderConfig()
+			return
+		}
+	}
+	return
+}
 
-	// First try read the config @ e.ConfigCache.
+// acquireCachedConfig returns the configuration from a local cache if
+// available
+func (e *Engine) acquireCachedConfig() (cfg types.Config, err error) {
 	b, err := ioutil.ReadFile(e.ConfigCache)
 	if err == nil {
 		if err = json.Unmarshal(b, &cfg); err != nil {
@@ -179,9 +197,13 @@ func (e *Engine) acquireConfig() (cfg types.Config, err error) {
 			e.Logger.Crit("failed to update timeouts and CAs for fetcher: %v", err)
 			return
 		}
-		return
 	}
+	return
+}
 
+// acquireProviderConfig attempts to fetch the configuration from the
+// provider.
+func (e *Engine) acquireProviderConfig() (cfg types.Config, err error) {
 	// Create a new http client and fetcher with the timeouts set via the flags,
 	// since we don't have a config with timeout values we can use
 	timeout := int(e.FetchTimeout.Seconds())
@@ -222,7 +244,7 @@ func (e *Engine) acquireConfig() (cfg types.Config, err error) {
 	}
 
 	// Populate the config cache.
-	b, err = json.Marshal(cfg)
+	b, err := json.Marshal(cfg)
 	if err != nil {
 		e.Logger.Crit("failed to marshal cached config: %v", err)
 		return