feat: extend information printed in the iPXE script, add retries

With this change, iPXE now prints information about available network
interfaces and configured IP after DHCP.

It should also retry DHCP but it's hard to test in my environment.

Also different interface should be tried if loading `/ipxe` script
fails.

Example:

```
net0: 4e:65:61:9e:84:a4 using undionly on 0000:00:02.0 (closed)
  [Link:up, TX:0 TXE:1 RX:0 RXE:0]
  [TXE: 1 x "Network unreachable (http://ipxe.org/28086011)"]
Configuring (net0 4e:65:61:9e:84:a4)...... ok
net0: 172.25.0.3/255.255.255.0 gw 172.25.0.1
http://172.24.0.2:8081/ipxe... ok
/env/agent-amd64/vmlinuz... ok
```

Signed-off-by: Andrey Smirnov <andrey.smirnov@talos-systems.com>

app/sidero-controller-manager/internal/ipxe/ipxe_server.go

@@ -40,11 +40,57 @@ const bootFile = `#!ipxe
 chain ipxe?uuid=${uuid}&mac=${mac:hexhyp}&domain=${domain}&hostname=${hostname}&serial=${serial}&arch=${buildarch}
 `
 
-// bootTemplate is embedded into iPXE binary when that binary is sent to the node.
-//
-// bootTemplate should be kept in sync with the bootFile above.
-var bootTemplate = template.Must(template.New("iPXE embedded").Parse(`dhcp
-chain http://{{ .Endpoint }}:{{ .Port }}/ipxe?uuid=${uuid}&mac=${mac:hexhyp}&domain=${domain}&hostname=${hostname}&serial=${serial}&arch=${buildarch}
+// BootTemplate is embedded into iPXE binary when that binary is sent to the node.
+var BootTemplate = template.Must(template.New("iPXE embedded").Parse(`
+prompt --key 0x02 --timeout 2000 Press Ctrl-B for the iPXE command line... && shell ||
+
+# print interfaces
+ifstat
+
+# retry 10 times overall
+set attempts:int32 10
+set x:int32 0
+
+:retry_loop
+
+	set idx:int32 0
+
+	:loop
+		# try DHCP on each available interface
+		isset ${net${idx}/mac} || goto exhausted
+
+		ifclose
+		dhcp net${idx} && goto boot
+
+	:next_iface
+		inc idx && goto loop
+
+	:boot
+		# attempt boot, if fails try next iface
+		route
+
+		chain http://{{ .Endpoint }}:{{ .Port }}/ipxe?uuid=${uuid}&mac=${net${idx}/mac:hexhyp}&domain=${domain}&hostname=${hostname}&serial=${serial}&arch=${buildarch} || goto next_iface
+
+:exhausted
+	echo
+	echo Failed to iPXE boot successfully via all interfaces
+
+	iseq ${x} ${attempts} && goto fail ||
+
+	echo Retrying...
+	echo
+
+	inc x
+	goto retry_loop
+
+:fail
+	echo
+	echo Failed to get a valid response after ${attempts} attempts
+	echo
+
+	echo Rebooting in 5 seconds...
+	sleep 5
+	reboot
 `))
 
 // ipxeTemplate is returned as response to `chain` request from the bootFile/bootTemplate to boot actual OS (or Sidero agent).
@@ -194,7 +240,7 @@ func RegisterIPXE(mux *http.ServeMux, endpoint string, port int, args string, bo
 
 	var embeddedScriptBuf bytes.Buffer
 
-	if err := bootTemplate.Execute(&embeddedScriptBuf, map[string]string{
+	if err := BootTemplate.Execute(&embeddedScriptBuf, map[string]string{
 		"Endpoint": apiEndpoint,
 		"Port":     strconv.Itoa(iPXEPort),
 	}); err != nil {

app/sidero-controller-manager/internal/ipxe/ipxe_test.go

@@ -4,11 +4,26 @@
 
 package ipxe_test
 
-import "testing"
+import (
+	"bytes"
+	"testing"
 
-func TestEmpty(t *testing.T) {
-	// added for accurate coverage estimation
-	//
-	// please remove it once any unit-test is added
-	// for this package
+	"github.com/stretchr/testify/assert"
+
+	"github.com/talos-systems/sidero/app/sidero-controller-manager/internal/ipxe"
+)
+
+func TestEmbeddedLength(t *testing.T) {
+	var buf bytes.Buffer
+
+	assert.NoError(t, ipxe.BootTemplate.Execute(&buf, struct {
+		Endpoint string
+		Port     string
+	}{ // use bigger values here to get maximum length of the script
+		Endpoint: "[2001:470:6d:30e:e5b8:903e:3701:7332]",
+		Port:     "12345",
+	}))
+
+	// iPXE script should fit length of the reserved space in the iPXE binary
+	assert.Less(t, buf.Len(), 1000)
 }