feat: Add support for systemd notify (#1954)

- Integrated systemd notification support using github.com/coreos/go-systemd/v22/daemon.
- Updated manager/runner.go to signal readiness to systemd.
- Modified .golangci.yml to include github.com/coreos/go-systemd in linters-settings.
- Updated dependencies in go.mod and go.sum for github.com/coreos/go-systemd/v22 v22.5.0.

.golangci.yml

@@ -86,6 +86,7 @@ linters-settings:
           - github.com/pkg/errors
           - github.com/stretchr/testify/assert
           - github.com/stretchr/testify/require
+          - github.com/coreos/go-systemd
 
 run:
   timeout: 10m

cli.go

@@ -4,6 +4,7 @@
 package main
 
 import (
+	"context"
 	"flag"
 	"fmt"
 	"io"
@@ -13,6 +14,7 @@ import (
 	"sync"
 	"time"
 
+	"github.com/coreos/go-systemd/v22/daemon"
 	"github.com/hashicorp/consul-template/config"
 	"github.com/hashicorp/consul-template/logging"
 	"github.com/hashicorp/consul-template/manager"
@@ -105,11 +107,38 @@ func (cli *CLI) Run(args []string) int {
 		return ExitCodeOK
 	}
 
+	// Create a context with cancellation
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	// Create a channel for signaling readiness to the systemd init system.
+	readyCh := make(chan struct{})
+
+	// Start a goroutine that listens for signals on the readyCh channel.
+	// When a signal (an empty struct) is received, it notifies systemd that
+	// the application is ready by calling daemon.SdNotify with SdNotifyReady.
+	// If an error occurs during the notification, it logs a warning message.
+	go func() {
+		for {
+			select {
+			case <-readyCh:
+				_, err := daemon.SdNotify(false, daemon.SdNotifyReady)
+				if err != nil {
+					log.Printf("[WARN] failed to signal readiness to systemd: %v", err)
+				}
+			case <-ctx.Done():
+				return
+			}
+		}
+	}()
+
 	// Initial runner
 	runner, err := manager.NewRunner(config, dry)
 	if err != nil {
 		return logError(err, ExitCodeRunnerError)
 	}
+
+	runner.SetReadyChannel(readyCh)
 	go runner.Start()
 
 	// Listen for monitored signals
@@ -137,6 +166,7 @@ func (cli *CLI) Run(args []string) int {
 		case <-service_os.Shutdown_Channel():
 			fmt.Fprintf(cli.errStream, "Cleaning up...\n")
 			runner.StopImmediately()
+			cancel() // Cancel the context to stop the readyCh goroutine
 			return ExitCodeInterrupt
 		case s := <-cli.signalCh:
 			log.Printf("[DEBUG] (cli) receiving signal %q", s)
@@ -163,6 +193,7 @@ func (cli *CLI) Run(args []string) int {
 				if err != nil {
 					return logError(err, ExitCodeRunnerError)
 				}
+				runner.SetReadyChannel(readyCh)
 				go runner.Start()
 			case *config.KillSignal:
 				fmt.Fprintf(cli.errStream, "Cleaning up...\n")

go.mod

@@ -43,6 +43,7 @@ require (
 	github.com/Masterminds/semver/v3 v3.2.0 // indirect
 	github.com/armon/go-metrics v0.4.1 // indirect
 	github.com/cenkalti/backoff/v3 v3.2.2 // indirect
+	github.com/coreos/go-systemd/v22 v22.5.0
 	github.com/fatih/color v1.17.0 // indirect
 	github.com/go-jose/go-jose/v3 v3.0.3 // indirect
 	github.com/google/uuid v1.3.0 // indirect

go.sum

@@ -29,6 +29,8 @@ github.com/cenkalti/backoff/v3 v3.2.2/go.mod h1:cIeZDE3IrqwwJl6VUwCN6trj1oXrTS4r
 github.com/cespare/xxhash/v2 v2.1.1/go.mod h1:VGX0DQ3Q6kWi7AoAeZDth3/j3BFtOZR5XLFGgcrjCOs=
 github.com/circonus-labs/circonus-gometrics v2.3.1+incompatible/go.mod h1:nmEj6Dob7S7YxXgwXpfOuvO54S+tGdZdw9fuRZt25Ag=
 github.com/circonus-labs/circonusllhist v0.1.3/go.mod h1:kMXHVDlOchFAehlya5ePtbp5jckzBHf4XRpQvBOLI+I=
+github.com/coreos/go-systemd/v22 v22.5.0 h1:RrqgGjYQKalulkV8NGVIfkXQf6YYmOyiJKk8iXXhfZs=
+github.com/coreos/go-systemd/v22 v22.5.0/go.mod h1:Y58oyj3AT4RCenI/lSvhwexgC+NSVTIJ3seZv2GcEnc=
 github.com/davecgh/go-spew v1.1.0/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
 github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
 github.com/davecgh/go-spew v1.1.2-0.20180830191138-d8f796af33cc h1:U9qPSI2PIWSS1VwoXQT9A3Wy9MM3WgvqSxFWenqJduM=
@@ -52,6 +54,7 @@ github.com/go-logfmt/logfmt v0.4.0/go.mod h1:3RMwSq7FuexP4Kalkev3ejPJsZTpXXBr9+V
 github.com/go-stack/stack v1.8.0/go.mod h1:v0f6uXyyMGvRgIKkXu+yp6POWl0qKG85gN/melR3HDY=
 github.com/go-test/deep v1.0.2 h1:onZX1rnHT3Wv6cqNgYyFOOlgVKJrksuCMCRvJStbMYw=
 github.com/go-test/deep v1.0.2/go.mod h1:wGDj63lr65AM2AQyKZd/NYHGb0R+1RLqB8NKt3aSFNA=
+github.com/godbus/dbus/v5 v5.0.4/go.mod h1:xhWf0FNVPg57R7Z0UbKHbJfkEywrmjJnf7w5xrFpKfA=
 github.com/gogo/protobuf v1.1.1/go.mod h1:r8qH/GZQm5c6nD/R0oafs1akxWv10x8SbQlK7atdtwQ=
 github.com/golang/protobuf v1.2.0/go.mod h1:6lQm79b+lXiMfvg/cZm0SGofjICqVBUtrP5yJMmIC1U=
 github.com/golang/protobuf v1.3.1/go.mod h1:6lQm79b+lXiMfvg/cZm0SGofjICqVBUtrP5yJMmIC1U=

manager/runner.go

@@ -132,6 +132,12 @@ type Runner struct {
 	// Runner config. This prevents risk of data races when reading config for
 	// other elements started by the Runner, like template functions.
 	finalConfigCopy config.Config
+
+	// readyCh is a channel used to signal readiness to the systemd init system.
+	// When a struct{} is sent on this channel, it triggers a notification to systemd
+	// that the application is ready. This helps in managing application state and
+	// integration with systemd's readiness protocol.
+	readyCh chan struct{}
 }
 
 // RenderEvent captures the time and events that occurred for a template
@@ -309,6 +315,14 @@ func (r *Runner) Start() {
 
 		if r.allTemplatesRendered() {
 			log.Printf("[DEBUG] (runner) all templates rendered")
+
+			// If the readyCh channel is not nil, send an empty struct to signal readiness.
+			// This will notify the systemd init system that the application is ready to
+			// handle requests. This mechanism integrates with systemd's readiness protocol.
+			if r.readyCh != nil {
+				r.readyCh <- struct{}{}
+			}
+
 			// Enable quiescence for all templates if we have specified wait
 			// intervals.
 		NEXT_Q:
@@ -722,6 +736,13 @@ func (r *Runner) Run() error {
 	return nil
 }
 
+// SetReadyChannel sets the readyCh channel which is used to signal readiness to the systemd init system.
+// The channel should be a struct{} channel, and when an empty struct is sent on this channel,
+// it will trigger a notification to systemd that the application is ready.
+func (r *Runner) SetReadyChannel(ch chan struct{}) {
+	r.readyCh = ch
+}
+
 type templateRunCtx struct {
 	// commands is the set of commands that will be executed after all templates
 	// have run. When adding to the commands, care should be taken not to