[fix] [exporterhelper] Fix nil ptr dereference on storage stop (#8718)

When persistent queue fails to start, for example due to bad
permissions, then persistent storage does not get allocated. Thus,
ensure stop storage method is only called if actually initialized.

**Description:** 
Fixing a bug that resulted in panic when de-referencing nil pointer. The
issue is that in the persistent queue fail to start due to say bad
folder permissions, then the `Start` method does not actually set the
associated persistent storage, and thus when shutting down it tries to
stop the storage and panics.

.chloggen/fix-nil-storage-panic-on-bad-start.yaml

@@ -0,0 +1,25 @@
+# Use this changelog template to create an entry for release notes.
+
+# One of 'breaking', 'deprecation', 'new_component', 'enhancement', 'bug_fix'
+change_type: bug_fix
+
+# The name of the component, or a single word describing the area of concern, (e.g. otlpreceiver)
+component: exporterhelper
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Fix nil pointer dereference when stopping persistent queue after a start encountered an error
+
+# One or more tracking issues or pull requests related to the change
+issues: [8718]
+
+# (Optional) One or more lines of additional information to render under the primary note.
+# These lines will be padded with 2 spaces and then inserted directly into the document.
+# Use pipe (|) for multiline entries.
+subtext:
+
+# Optional: The change log or logs in which this entry should be included.
+# e.g. '[user]' or '[user, api]'
+# Include 'user' if the change is relevant to end users.
+# Include 'api' if there is a change to a library API.
+# Default: '[user]'
+change_logs: []

exporter/exporterhelper/internal/persistent_queue.go

@@ -16,7 +16,9 @@ import (
 var (
 	// Monkey patching for unit test
 	stopStorage = func(queue *persistentQueue) {
-		queue.storage.stop()
+		if queue.storage != nil {
+			queue.storage.stop()
+		}
 	}
 	errNoStorageClient    = errors.New("no storage client extension found")
 	errWrongExtensionType = errors.New("requested extension is not a storage extension")

exporter/exporterhelper/internal/persistent_queue_test.go

@@ -292,3 +292,12 @@ func TestInvalidStorageExtensionType(t *testing.T) {
 	assert.ErrorIs(t, err, errWrongExtensionType)
 	assert.Nil(t, client)
 }
+
+func TestPersistentQueue_StopAfterBadStart(t *testing.T) {
+	pq := NewPersistentQueue(1, 1, component.ID{}, newFakeTracesRequestMarshalerFunc(),
+		newFakeTracesRequestUnmarshalerFunc())
+	// verify that stopping a un-start/started w/error queue does not panic
+	assert.NotPanics(t, func() {
+		pq.Stop()
+	})
+}