version: use Go 1.18's git stamping as default implementation

cmd/tailscale/depaware.txt

@@ -197,7 +197,7 @@ tailscale.com/cmd/tailscale dependencies: (generated by github.com/tailscale/dep
         reflect                                                      from crypto/x509+
         regexp                                                       from github.com/tailscale/goupnp/httpu+
         regexp/syntax                                                from regexp
-        runtime/debug                                                from golang.org/x/sync/singleflight
+        runtime/debug                                                from golang.org/x/sync/singleflight+
         sort                                                         from compress/flate+
         strconv                                                      from compress/flate+
         strings                                                      from bufio+

version/print.go

@@ -15,7 +15,11 @@ func String() string {
 	ret.WriteString(Short)
 	ret.WriteByte('\n')
 	if GitCommit != "" {
-		fmt.Fprintf(&ret, "  tailscale commit: %s\n", GitCommit)
+		var dirty string
+		if GitDirty {
+			dirty = "-dirty"
+		}
+		fmt.Fprintf(&ret, "  tailscale commit: %s%s\n", GitCommit, dirty)
 	}
 	if ExtraGitCommit != "" {
 		fmt.Fprintf(&ret, "  other commit: %s\n", ExtraGitCommit)

version/version.go

@@ -6,28 +6,64 @@
 package version
 
 import (
+	"runtime/debug"
 	"strings"
 
 	tailscaleroot "tailscale.com"
 )
 
 // Long is a full version number for this build, of the form
-// "x.y.z-commithash", or "date.yyyymmdd" if no actual version was
-// provided.
-var Long = "date.20220107"
+// "x.y.z-commithash" for builds stamped in the usual way (see
+// build_dist.sh in the root) or, for binaries built by hand with the
+// go tool, it's of the form "1.23.0-dev20220316-t29837428937{,-dirty}"
+// where "1.23.0" comes from ../VERSION.txt and the part after dev
+// is YYYYMMDD of the commit time, and the part after -t is the commit
+// hash. The dirty suffix is whether there are uncommitted changes.
+var Long = ""
 
 // Short is a short version number for this build, of the form
-// "x.y.z", or "date.yyyymmdd" if no actual version was provided.
+// "x.y.z" for builds stamped in the usual way (see
+// build_dist.sh in the root) or, for binaries built by hand with the                                                                            // go tool, it's like Long's dev form, but ending at the date part,
+// of the form "1.23.0-dev20220316".
 var Short = ""
 
 func init() {
-	// If it hasn't been link-stamped with -X (via build_dist.sh or similar),
-	// then use the VERSION.txt file in the root and the date in the Long
-	// variable above which we occasionally bump by hand.
-	if Short == "" {
-		Long = strings.TrimSpace(tailscaleroot.Version) + "-" + Long
+	if Long != "" && Short != "" {
+		// Built in the recommended way, using build_dist.sh.
+		return
+	}
+
+	bi, ok := debug.ReadBuildInfo()
+	if !ok {
+		Long = strings.TrimSpace(tailscaleroot.Version) + "-ERR-BuildInfo"
 		Short = Long
+		return
+	}
+	var dirty string // "-dirty" suffix if dirty
+	var commitHashAbbrev, commitDate string
+	for _, s := range bi.Settings {
+		switch s.Key {
+		case "vcs.revision":
+			GitCommit = s.Value
+			if len(s.Value) >= 9 {
+				commitHashAbbrev = s.Value[:9]
+			}
+		case "vcs.time":
+			if len(s.Value) >= len("yyyy-mm-dd") {
+				commitDate = s.Value[:len("yyyy-mm-dd")]
+				commitDate = strings.ReplaceAll(commitDate, "-", "")
+			}
+		case "vcs.modified":
+			if s.Value == "true" {
+				dirty = "-dirty"
+				GitDirty = true
+			}
+		}
 	}
+
+	// Backup path, using Go 1.18's built-in git stamping.
+	Short = strings.TrimSpace(tailscaleroot.Version) + "-dev" + commitDate
+	Long = Short + "-t" + commitHashAbbrev + dirty
 }
 
 // GitCommit, if non-empty, is the git commit of the
@@ -36,6 +72,11 @@ func init() {
 // --exclude "*" --dirty --abbrev=200`.
 var GitCommit = ""
 
+// GitDirty is whether Go stamped the binary has having dirty version
+// control changes in the working directory (debug.ReadBuildInfo
+// setting "vcs.modified" was true).
+var GitDirty bool
+
 // ExtraGitCommit, if non-empty, is the git commit of a "supplemental"
 // repository at which Tailscale was built. Its format is the same as
 // gitCommit.