Suggest brew upgrade gh when new version detected

When the update notifier is enabled and a new version was detected, show
a Homebrew upgrade notice if:
- the release was at least 24 hours ago; and
- the current `gh` binary is under the Homebrew prefix.

cmd/gh/main.go

@@ -8,7 +8,9 @@ import (
 	"os"
 	"os/exec"
 	"path"
+	"path/filepath"
 	"strings"
+	"time"
 
 	surveyCore "github.com/AlecAivazis/survey/v2/core"
 	"github.com/cli/cli/api"
@@ -161,13 +163,16 @@ func main() {
 
 	newRelease := <-updateMessageChan
 	if newRelease != nil {
-		msg := fmt.Sprintf("%s %s → %s\n%s",
+		ghExe, _ := os.Executable()
+		fmt.Fprintf(stderr, "\n\n%s %s → %s\n",
 			ansi.Color("A new release of gh is available:", "yellow"),
 			ansi.Color(buildVersion, "cyan"),
-			ansi.Color(newRelease.Version, "cyan"),
+			ansi.Color(newRelease.Version, "cyan"))
+		if suggestBrewUpgrade(newRelease, ghExe) {
+			fmt.Fprintf(stderr, "To upgrade, run: %s\n", "brew update && brew upgrade gh")
+		}
+		fmt.Fprintf(stderr, "%s\n\n",
 			ansi.Color(newRelease.URL, "yellow"))
-
-		fmt.Fprintf(stderr, "\n\n%s\n\n", msg)
 	}
 }
 
@@ -259,3 +264,24 @@ func apiVerboseLog() api.ClientOption {
 	colorize := utils.IsTerminal(os.Stderr)
 	return api.VerboseLog(colorable.NewColorable(os.Stderr), logTraffic, colorize)
 }
+
+// Suggest to `brew upgrade gh` only if gh was found under homebrew prefix and when the release was
+// published over 24h ago, allowing homebrew-core ample time to merge the formula bump.
+func suggestBrewUpgrade(rel *update.ReleaseInfo, ghBinary string) bool {
+	if rel.PublishedAt.IsZero() || time.Since(rel.PublishedAt) < time.Duration(time.Hour*24) {
+		return false
+	}
+
+	brewExe, err := safeexec.LookPath("brew")
+	if err != nil {
+		return false
+	}
+
+	brewPrefixBytes, err := exec.Command(brewExe, "--prefix").Output()
+	if err != nil {
+		return false
+	}
+
+	brewBinPrefix := filepath.Join(strings.TrimSpace(string(brewPrefixBytes)), "bin") + string(filepath.Separator)
+	return strings.HasPrefix(ghBinary, brewBinPrefix)
+}

internal/update/update.go

@@ -18,8 +18,9 @@ var gitDescribeSuffixRE = regexp.MustCompile(`\d+-\d+-g[a-f0-9]{8}$`)
 
 // ReleaseInfo stores information about a release
 type ReleaseInfo struct {
-	Version string `json:"tag_name"`
-	URL     string `json:"html_url"`
+	Version     string    `json:"tag_name"`
+	URL         string    `json:"html_url"`
+	PublishedAt time.Time `json:"published_at"`
 }
 
 type StateEntry struct {