Report the Go build info version when linker flags set none (closes #422)

A binary from go install, the README's first command, reported itself as version dev, a development build, although Go had recorded the tag it was built from. When the linker flags leave the version, commit or commit date unset, they are now filled in from the Go build info, so such a binary reports its tag and counts as a release. Builds through make, script/docker and goreleaser are unchanged.

Model: opus-5-5

internal/globals/globals.go

@@ -1,19 +1,24 @@
 // Package globals holds application-wide metadata (name, version,
-// commit) that is populated at build time via linker flags.
+// commit) that is populated at build time via linker flags, or from the
+// build info Go records in the binary when the linker flags set none.
 package globals
 
 import (
 	"regexp"
+	"runtime/debug"
 	"strings"
 	"time"
+
+	"golang.org/x/mod/module"
 )
 
 // Appname is the application name, populated from main().
 var Appname = "vaultik" //nolint:gochecknoglobals // set via -ldflags at build time
 
 // DevVersion is the version a binary reports when it was built without
-// git metadata: script/version emits it outside a git checkout, and an
-// unstamped `go build` keeps it. goreleaser's snapshot template stamps
+// git metadata: script/version emits it outside a git checkout, and a
+// binary keeps it when neither its linker flags nor its Go build info
+// name a version. goreleaser's snapshot template stamps
 // it followed by "-" and the commit it was built from. It is
 // deliberately not a number.
 const DevVersion = "dev"
@@ -32,6 +37,54 @@ var Commit = Unknown //nolint:gochecknoglobals // set via -ldflags at build time
 // CommitDate is the ISO-8601 date of the commit, populated from main().
 var CommitDate = Unknown //nolint:gochecknoglobals // set via -ldflags at build time
 
+// develVersion is the main module version Go records when it cannot
+// tell which version it built.
+const develVersion = "(devel)"
+
+// FillUnsetFromBuildInfo applies FillUnset to Version, Commit and
+// CommitDate, with the build info of the running binary.
+func FillUnsetFromBuildInfo() {
+	info, ok := debug.ReadBuildInfo()
+	if !ok {
+		return
+	}
+
+	Version, Commit, CommitDate = FillUnset(
+		info, Version, Commit, CommitDate)
+}
+
+// FillUnset returns version, commit and commitDate. Each one that
+// still holds its default ("dev" or empty for the version, "unknown"
+// for the others) is replaced by the value Go recorded in info. A
+// `go install` of a tag sets no linker flags, but Go records the tag
+// as the main module's version. Go records the commit and its time
+// only when it builds from a git checkout. The time is cut to a date,
+// as the Makefile stamps it.
+func FillUnset(
+	info *debug.BuildInfo, version, commit, commitDate string,
+) (string, string, string) {
+	if (version == "" || version == DevVersion) &&
+		info.Main.Version != "" && info.Main.Version != develVersion {
+		version = info.Main.Version
+	}
+
+	for _, s := range info.Settings {
+		switch s.Key {
+		case "vcs.revision":
+			if commit == Unknown {
+				commit = s.Value
+			}
+		case "vcs.time":
+			t, err := time.Parse(time.RFC3339, s.Value)
+			if commitDate == Unknown && err == nil {
+				commitDate = t.UTC().Format(time.DateOnly)
+			}
+		}
+	}
+
+	return version, commit, commitDate
+}
+
 // Author identifies the upstream author of vaultik.
 const Author = "Jeffrey Paul <sneak@sneak.berlin>"
 
@@ -68,8 +121,10 @@ func New() (*Globals, error) {
 // a release. "dev" and goreleaser's snapshot "dev-<sha>" count, and so
 // does what `git describe --tags --always --dirty` gives a make or
 // docker build of an untagged commit: the bare short commit, or
-// tag-N-gHASH on a commit after a tag. Any version ending in "-dirty"
-// counts, a modified checkout of a tag ("v1.0.0-dirty") included.
+// tag-N-gHASH on a commit after a tag. A Go pseudo-version counts: Go
+// records one for a build of an untagged commit without linker flags.
+// Any version ending in "-dirty" or "+dirty" counts, a modified
+// checkout of a tag ("v1.0.0-dirty", "v1.0.0+dirty") included.
 // A plain tag such as "v1.0.0" or "1.0.0" is a release.
 //
 // The empty string counts too. Nothing that knows its version reports
@@ -82,7 +137,8 @@ func New() (*Globals, error) {
 func IsDevVersion(v string) bool {
 	if v == "" || v == Unknown || v == DevVersion ||
 		strings.HasPrefix(v, DevVersion+"-") ||
-		strings.HasSuffix(v, "-dirty") {
+		strings.HasSuffix(v, "-dirty") || strings.HasSuffix(v, "+dirty") ||
+		module.IsPseudoVersion(v) {
 		return true
 	}