[8.19] (backport #8215) fix(diagnostics): handle log rotation races

changelog/fragments/1747938268-fix-diag-race-condition.yaml

@@ -0,0 +1,4 @@
+kind: bug-fix
+summary: Address a race condition that can occur in Agent diagnostics if log rotation runs while logs are being zipped.
+component: elastic-agent
+pull_request: https://github.com/elastic/elastic-agent/pull/8215

internal/pkg/diagnostics/diagnostics.go

@@ -314,7 +314,7 @@ func writeErrorResult(zw *zip.Writer, path string, errBody string) error {
 if err != nil {
 return fmt.Errorf("error writing header for error.txt file for component: %w", err)
 }
-_, err = w.Write([]byte(fmt.Sprintf("%s\n", errBody)))
+_, err = fmt.Fprintf(w, "%s\n", errBody)
 if err != nil {
 return fmt.Errorf("error writing error.txt file for component: %w", err)
 }
@@ -507,7 +507,12 @@ func zipLogsWithPath(pathsHome, commitName string, collectServices, excludeEvent
 return nil
 }
 
-return saveLogs(name, path, zw)
+// Add the file to the zip.
+// Ignore files that don't exist to account for races with log rotation.
+if err := saveLogs(name, path, zw); err != nil && !errors.Is(err, fs.ErrNotExist) {
+return err
+}
+return nil
 })
 }