fix(diagnostics): handle log rotation races (#8215)

Handle a race condition that can occur in Agent diagnostics if log rotation happens while logs are being zipped. There is a time window between when `filepath.WalkDir` reads the directory contents and when log files are opened for reading. During this time window, log rotation can happen, resulting in files that no longer exist and cannot be added to the diagnostic archive. To handle this, `fs.ErrNotExist` errors are ignored when attempting to add log files. Also, address and unrelated linter warning by using fmt.Fprintf. (cherry picked from commit 751acc1)

Author: andrewkroh

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
 })
 }