e2e: Add Client Intro test framework and initial test. (#26639)

The new client intro test mimics the Consul and Vault compat tests
and uses local agents to perform the required setup. This method
allows us the flexibility moving forward to test when enforcement
mode is in strict.

The test suite will now be triggered from the test-e2e CI run
and can also be called by a make target.

e2e/execagent/execagent.go

@@ -12,6 +12,7 @@ import (
 	"path/filepath"
 	"text/template"
 
+	"github.com/hashicorp/go-hclog"
 	"github.com/hashicorp/nomad/api"
 )
 
@@ -37,7 +38,8 @@ var (
 
 	agentTemplate = template.Must(template.New("agent").Parse(`
 enable_debug = true
-log_level = "{{ or .LogLevel "DEBUG" }}"
+name         = "{{ or .AgentName "nomad-e2e-test-agent" }}"
+log_level    = "{{ or .LogLevel "DEBUG" }}"
 
 ports {
   http = {{.HTTP}}
@@ -58,6 +60,11 @@ client {
   options = {
     "driver.raw_exec.enable" = "1"
   }
+  {{- $retry_join_length := len .RetryJoinAddrs }}{{ if not (eq $retry_join_length 0) }}
+  server_join {
+    retry_join = [{{ range $index, $element := .RetryJoinAddrs }}{{if $index}}, {{end}}"{{$element}}"{{ end }}]
+  }
+  {{ end }}
 }
 {{ end }}
 `))
@@ -69,7 +76,22 @@ type AgentTemplateVars struct {
 	Serf         int
 	EnableClient bool
 	EnableServer bool
-	LogLevel     string
+
+	// AgentName is the name to apply to the Nomad agent. This is optional, but
+	// allows for multiple agents to be run on the same host. If not set, it
+	// will default to "nomad-e2e-test-agent".
+	AgentName string
+
+	LogLevel string
+
+	// NodePool is the Nomad node pool to assign the agent to when running with
+	// client mode enabled. This will default to the "default" node pool if not
+	// set.
+	NodePool string
+
+	// RetryJoinAddrs is a list of addresses to use for the retry_join config
+	// block.
+	RetryJoinAddrs []string
 }
 
 func newAgentTemplateVars() (*AgentTemplateVars, error) {
@@ -87,14 +109,32 @@ func newAgentTemplateVars() (*AgentTemplateVars, error) {
 	}
 
 	vars := AgentTemplateVars{
-		HTTP: httpPort,
-		RPC:  rpcPort,
-		Serf: serfPort,
+		HTTP:     httpPort,
+		RPC:      rpcPort,
+		Serf:     serfPort,
+		LogLevel: hclog.Warn.String(),
+		NodePool: "default",
 	}
 
 	return &vars, nil
 }
 
+// SetMode is a helper function to allow setting the agent mode (client, server,
+// or both).
+func (a *AgentTemplateVars) SetMode(mode AgentMode) {
+	switch mode {
+	case ModeClient:
+		a.EnableClient = true
+		a.EnableServer = false
+	case ModeServer:
+		a.EnableClient = false
+		a.EnableServer = true
+	case ModeBoth:
+		a.EnableClient = true
+		a.EnableServer = true
+	}
+}
+
 func writeConfig(path string, vars *AgentTemplateVars) error {
 	f, err := os.OpenFile(path, os.O_RDWR|os.O_CREATE, 0644)
 	if err != nil {
@@ -222,6 +262,65 @@ func NewClientServerPair(bin string, serverOut, clientOut io.Writer) (
 	return
 }
 
+// TemplateVariableCallbackFunc is a callback function that allow callers to
+// modify the template variables before the config file is written out.
+type TemplateVariableCallbackFunc func(c *AgentTemplateVars)
+
+func NewSingleModeAgent(
+	bin, baseDir string,
+	mode AgentMode,
+	writer io.Writer,
+	varCallbackFn TemplateVariableCallbackFunc,
+) (*NomadAgent, error) {
+
+	templateVars, err := newAgentTemplateVars()
+	if err != nil {
+		return nil, err
+	}
+
+	// Allow the caller to modify the template variables before we write out the
+	// config file.
+	if varCallbackFn != nil {
+		varCallbackFn(templateVars)
+	}
+
+	// Set the mode (client, server, or both)
+	templateVars.SetMode(mode)
+
+	baseDataDir := BaseDir
+
+	if baseDir != "" {
+		baseDataDir = baseDir
+	}
+
+	if err := os.MkdirAll(baseDataDir, 0755); err != nil {
+		return nil, err
+	}
+
+	agentDir, err := os.MkdirTemp(baseDataDir, "agent")
+	if err != nil {
+		return nil, err
+	}
+
+	agentConfig := filepath.Join(agentDir, "agent.hcl")
+	if err := writeConfig(agentConfig, templateVars); err != nil {
+		return nil, err
+	}
+
+	nomadAgent := &NomadAgent{
+		BinPath:  bin,
+		DataDir:  agentDir,
+		ConfFile: agentConfig,
+		Vars:     templateVars,
+		Cmd:      exec.Command(bin, "agent", "-config", agentConfig, "-data-dir", agentDir),
+	}
+
+	nomadAgent.Cmd.Stdout = writer
+	nomadAgent.Cmd.Stderr = writer
+
+	return nomadAgent, nil
+}
+
 // Start the agent command.
 func (n *NomadAgent) Start() error {
 	return n.Cmd.Start()