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_test.go

@@ -1,6 +1,7 @@
 package globals_test
 
 import (
+	"runtime/debug"
 	"testing"
 
 	"sneak.berlin/go/vaultik/internal/globals"
@@ -59,12 +60,20 @@ func TestIsDevVersion(t *testing.T) {
 		{"1.0.0-rc.1-12-g877eb2f", true},
 		// A tagged commit with uncommitted changes is not that tag.
 		{"v1.0.0-dirty", true},
+		// What Go records for a build without linker flags: a
+		// pseudo-version for an untagged commit, "+dirty" for a
+		// modified checkout.
+		{"v1.0.0-rc3.0.20261111001226-abcdefabcdef", true},
+		{"v0.0.0-20261111001226-abcdefabcdef", true},
+		{"v1.0.1-0.20261111001226-abcdefabcdef", true},
+		{"v1.0.0+dirty", true},
 		// What a tagged build produces (goreleaser's .Version strips
 		// the leading "v"; script/version keeps it).
 		{"1.0.0", false},
 		{"0.1.0", false},
 		{"1.0.0-rc.1", false},
 		{"v1.0.0", false},
+		{"v1.0.0-rc3", false},
 		// A release must not be mistaken for a dev build just because
 		// the string happens to contain "dev".
 		{"1.0.0-dev", false},
@@ -86,10 +95,9 @@ func TestIsDevVersion(t *testing.T) {
 	}
 }
 
-// TestDefaultVersionIsDev pins the linker-flag contract: an unstamped
-// binary (no -ldflags at all, which is what `go build ./...` and `go
-// install` produce) must report itself as a development build rather
-// than as some default release number.
+// TestDefaultVersionIsDev pins the default: a binary whose linker flags
+// and build info name no version must report itself as a development
+// build rather than as some default release number.
 func TestDefaultVersionIsDev(t *testing.T) {
 	t.Parallel()
 
@@ -98,3 +106,104 @@ func TestDefaultVersionIsDev(t *testing.T) {
 			globals.DevVersion)
 	}
 }
+
+// buildInfo returns build info with the given main module version and,
+// when revision is not empty, the commit settings Go records for a
+// build from a git checkout.
+func buildInfo(version, revision string) *debug.BuildInfo {
+	info := &debug.BuildInfo{
+		Main: debug.Module{Path: "sneak.berlin/go/vaultik", Version: version},
+	}
+
+	if revision != "" {
+		info.Settings = []debug.BuildSetting{
+			{Key: "vcs.revision", Value: revision},
+			{Key: "vcs.time", Value: "2026-11-10T23:12:26Z"},
+		}
+	}
+
+	return info
+}
+
+// TestFillUnset covers a binary from `go install` of a tag, which
+// carries no linker flags: it must report the tag. Values the linker
+// flags set are kept.
+func TestFillUnset(t *testing.T) {
+	t.Parallel()
+
+	const (
+		tag = "v1.0.0-rc3"
+		rev = "0123456789abcdef0123456789abcdef01234567"
+	)
+
+	cases := []struct {
+		name                    string
+		info                    *debug.BuildInfo
+		version, commit, date   string
+		wantVersion, wantCommit string
+		wantDate                string
+	}{
+		{
+			name:        "go install of a tag",
+			info:        buildInfo(tag, ""),
+			version:     globals.DevVersion,
+			commit:      globals.Unknown,
+			date:        globals.Unknown,
+			wantVersion: tag,
+			wantCommit:  globals.Unknown,
+			wantDate:    globals.Unknown,
+		},
+		{
+			name:        "empty version",
+			info:        buildInfo(tag, ""),
+			version:     "",
+			commit:      globals.Unknown,
+			date:        globals.Unknown,
+			wantVersion: tag,
+			wantCommit:  globals.Unknown,
+			wantDate:    globals.Unknown,
+		},
+		{
+			name:        "go build in a git checkout",
+			info:        buildInfo(tag, rev),
+			version:     globals.DevVersion,
+			commit:      globals.Unknown,
+			date:        globals.Unknown,
+			wantVersion: tag,
+			wantCommit:  rev,
+			wantDate:    "2026-11-10",
+		},
+		{
+			name:        "Go could not tell the version",
+			info:        buildInfo("(devel)", ""),
+			version:     globals.DevVersion,
+			commit:      globals.Unknown,
+			date:        globals.Unknown,
+			wantVersion: globals.DevVersion,
+			wantCommit:  globals.Unknown,
+			wantDate:    globals.Unknown,
+		},
+		{
+			name:        "linker flags set",
+			info:        buildInfo(tag, rev),
+			version:     "v1.0.0-rc2-4-gabcdef1",
+			commit:      "abcdef1",
+			date:        "2026-11-01",
+			wantVersion: "v1.0.0-rc2-4-gabcdef1",
+			wantCommit:  "abcdef1",
+			wantDate:    "2026-11-01",
+		},
+	}
+
+	for _, tc := range cases {
+		version, commit, date := globals.FillUnset(
+			tc.info, tc.version, tc.commit, tc.date)
+
+		if version != tc.wantVersion || commit != tc.wantCommit ||
+			date != tc.wantDate {
+			t.Errorf("%s: got %q, %q, %q; want %q, %q, %q", tc.name,
+				version, commit, date,
+				tc.wantVersion, tc.wantCommit, tc.wantDate)
+		}
+	}
+}