[receiver/journald] Fix bug where failed startup could bury error message due to panic during shutdown (open-telemetry#31550)

Fixes open-telemetry#31476

Author: djaglowski

.chloggen/journald-protect-shutdown.yaml

@@ -0,0 +1,27 @@
+# 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. filelogreceiver)
+component: journaldreceiver
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Fix bug where failed startup could bury error message due to panic during shutdown
+
+# Mandatory: One or more tracking issues related to the change. You can use the PR number here if no issue exists.
+issues: [31476]
+
+# (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:
+
+# If your change doesn't affect end users or the exported elements of any package,
+# you should instead start your pull request title with [chore] or use the "Skip Changelog" label.
+# 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: []

pkg/stanza/operator/input/journald/journald.go

@@ -348,7 +348,9 @@ func (operator *Input) parseJournalEntry(line []byte) (*entry.Entry, string, err
 
 // Stop will stop generating logs.
 func (operator *Input) Stop() error {
-	operator.cancel()
+	if operator.cancel != nil {
+		operator.cancel()
+	}
 	operator.wg.Wait()
 	return nil
 }