Check if Filebeat log harvester tries to open named pipe (elastic#20450)

## What does this PR do?

This PR adds a check before opening a file for harvester Filebeat. If the file is a named pipe, an error is returned and the file is not opened.

## Why is it important?

Previously if someone wanted to open a named pipe without a writer, Filebeat hangs.

## Related issues

Closes elastic#18682

CHANGELOG.next.asciidoc

@@ -507,6 +507,7 @@ https://github.com/elastic/beats/compare/v7.0.0-alpha2...master[Check the HEAD d
 - Add support for additional fields and FirewallMatchEvent type events in CrowdStrike module {pull}20138[20138]
 - Add event.ingested for Suricata module {pull}20220[20220]
 - Add event.ingested to all Filebeat modules. {pull}20386[20386]
+- Return error when log harvester tries to open a named pipe. {issue}18682[18682] {pull}20450[20450]
 
 *Heartbeat*
 

filebeat/input/log/harvester.go

@@ -505,6 +505,14 @@ func (h *Harvester) shouldExportLine(line string) bool {
 // is returned and the harvester is closed. The file will be picked up again the next time
 // the file system is scanned
 func (h *Harvester) openFile() error {
+	fi, err := os.Stat(h.state.Source)
+	if err != nil {
+		return fmt.Errorf("failed to stat source file %s: %v", h.state.Source, err)
+	}
+	if fi.Mode()&os.ModeNamedPipe != 0 {
+		return fmt.Errorf("failed to open file %s, named pipes are not supported", h.state.Source)
+	}
+
 	f, err := file_helper.ReadOpen(h.state.Source)
 	if err != nil {
 		return fmt.Errorf("Failed opening %s: %s", h.state.Source, err)