TUN-8630: Check checksum of downloaded binary to compare to current for auto-updating

In the rare case that the updater downloads the same binary (validated via checksum)
we want to make sure that the updater does not attempt to upgrade and restart the cloudflared
process. The binaries are equivalent and this would provide no value.

However, we are covering this case because there was an errant deployment of cloudflared
that reported itself as an older version and was then stuck in an infinite loop
attempting to upgrade to the latest version which didn't exist. By making sure that
the binary is different ensures that the upgrade will be attempted and cloudflared
will be restarted to run the new version.

This change only affects cloudflared tunnels running with default settings or
`--no-autoupdate=false` which allows cloudflared to auto-update itself in-place. Most
distributions that handle package management at the operating system level are
not affected by this change.

cmd/cloudflared/cliutil/build_info.go

@@ -1,7 +1,10 @@
 package cliutil
 
 import (
+	"crypto/sha256"
 	"fmt"
+	"io"
+	"os"
 	"runtime"
 
 	"github.com/rs/zerolog"
@@ -13,6 +16,7 @@ type BuildInfo struct {
 	GoArch             string `json:"go_arch"`
 	BuildType          string `json:"build_type"`
 	CloudflaredVersion string `json:"cloudflared_version"`
+	Checksum           string `json:"checksum"`
 }
 
 func GetBuildInfo(buildType, version string) *BuildInfo {
@@ -22,11 +26,12 @@ func GetBuildInfo(buildType, version string) *BuildInfo {
 		GoArch:             runtime.GOARCH,
 		BuildType:          buildType,
 		CloudflaredVersion: version,
+		Checksum:           currentBinaryChecksum(),
 	}
 }
 
 func (bi *BuildInfo) Log(log *zerolog.Logger) {
-	log.Info().Msgf("Version %s", bi.CloudflaredVersion)
+	log.Info().Msgf("Version %s (Checksum %s)", bi.CloudflaredVersion, bi.Checksum)
 	if bi.BuildType != "" {
 		log.Info().Msgf("Built%s", bi.GetBuildTypeMsg())
 	}
@@ -51,3 +56,28 @@ func (bi *BuildInfo) GetBuildTypeMsg() string {
 func (bi *BuildInfo) UserAgent() string {
 	return fmt.Sprintf("cloudflared/%s", bi.CloudflaredVersion)
 }
+
+// FileChecksum opens a file and returns the SHA256 checksum.
+func FileChecksum(filePath string) (string, error) {
+	f, err := os.Open(filePath)
+	if err != nil {
+		return "", err
+	}
+	defer f.Close()
+
+	h := sha256.New()
+	if _, err := io.Copy(h, f); err != nil {
+		return "", err
+	}
+
+	return fmt.Sprintf("%x", h.Sum(nil)), nil
+}
+
+func currentBinaryChecksum() string {
+	currentPath, err := os.Executable()
+	if err != nil {
+		return ""
+	}
+	sum, _ := FileChecksum(currentPath)
+	return sum
+}