add version info automatically if you build with go 1.18+

.github/workflows/ci.yml

@@ -9,6 +9,7 @@ jobs:
       fail-fast: false
       matrix:
         go-version:
+          - 1.18.x
           - 1.17.x
           - 1.16.x
           - 1.15.x

mage/main.go

@@ -66,13 +66,6 @@ const (
 
 var debug = log.New(ioutil.Discard, "DEBUG: ", log.Ltime|log.Lmicroseconds)
 
-// set by ldflags when you "mage build"
-var (
-	commitHash = "<not set>"
-	timestamp  = "<not set>"
-	gitTag     = "<not set>"
-)
-
 //go:generate stringer -type=Command
 
 // Command tracks invocations of mage that run without targets or other flags.
@@ -146,8 +139,8 @@ func ParseAndRun(stdout, stderr io.Writer, stdin io.Reader, args []string) int {
 
 	switch cmd {
 	case Version:
-		out.Println("Mage Build Tool", gitTag)
-		out.Println("Build Date:", timestamp)
+		out.Println("Mage Build Tool", getTag())
+		out.Println("Commit Date:", timestamp)
 		out.Println("Commit:", commitHash)
 		out.Println("built with:", runtime.Version())
 		return 0

mage/version.go

@@ -0,0 +1,87 @@
+//go:build go1.18
+// +build go1.18
+
+package mage
+
+import (
+	"encoding/json"
+	"net/http"
+	dbg "runtime/debug"
+	"sort"
+	"strings"
+	"time"
+)
+
+// set by ldflags when you "mage build"
+var (
+	commitHash = "<not set>"
+	timestamp  = "<not set>"
+	gitTag     = "<not set>"
+)
+
+func init() {
+	info, ok := dbg.ReadBuildInfo()
+	if !ok {
+		return
+	}
+	for _, kv := range info.Settings {
+		switch kv.Key {
+		case "vcs.revision":
+			commitHash = kv.Value
+		case "vcs.time":
+			timestamp = kv.Value
+		}
+	}
+}
+
+// uses the commit hash to get the git tag (if one exists) from github
+func getTag() string {
+	debug.Println("requesting tag info from github via https://api.github.com/repos/magefile/mage/git/refs/tags")
+
+	http.DefaultClient.Timeout = 300 * time.Millisecond
+	resp, err := http.DefaultClient.Get("https://api.github.com/repos/magefile/mage/git/refs/tags")
+	if err != nil {
+		debug.Println("unable to request tag info from github:", err)
+		return ""
+	}
+	defer resp.Body.Close()
+	if resp.StatusCode != http.StatusOK {
+		debug.Println("bad response requesting mage tag info from github:", resp.StatusCode)
+		return ""
+	}
+
+	var tags []tag
+	err = json.NewDecoder(resp.Body).Decode(&tags)
+	if err != nil {
+		debug.Println("error unmarshalling mage tag info from github:", err)
+		return ""
+	}
+
+	var found []string
+	for _, t := range tags {
+		if t.Object.Sha == commitHash && strings.HasPrefix(t.Ref, "refs/tags/") {
+			found = append(found, t.Ref[len("refs/tags/"):])
+		}
+	}
+	if len(found) == 0 {
+		debug.Println("no git tag found for commit hash:", commitHash)
+		return "<no tag for this commit>"
+	}
+	if len(found) == 1 {
+		return found[0]
+	}
+	// more than one tag for this commit, report the highest tag number.
+	sort.Strings(found)
+	return found[len(found)-1]
+}
+
+type tag struct {
+	Ref    string `json:"ref"`
+	NodeID string `json:"node_id"`
+	URL    string `json:"url"`
+	Object struct {
+		Sha  string `json:"sha"`
+		Type string `json:"type"`
+		URL  string `json:"url"`
+	} `json:"object"`
+}

mage/version_old.go

@@ -0,0 +1,15 @@
+//go:build !go1.18
+// +build !go1.18
+
+package mage
+
+// set by ldflags when you "mage build"
+var (
+	commitHash = "<not set>"
+	timestamp  = "<not set>"
+	gitTag     = ""
+)
+
+func getTag() string {
+	return gitTag
+}