cmd: use git-archive for version information

Signed-off-by: Hank Donnay <hdonnay@redhat.com>

.gitattributes

@@ -0,0 +1,2 @@
+cmd/build.go export-subst
+*.go diff=golang

cmd/build.go

@@ -5,40 +5,63 @@ import (
 	"bytes"
 	"context"
 	"os/exec"
+	"runtime/debug"
 	"time"
 )
 
-// This is a helper for development. In production, we shouldn't assume that the
-// process is running in a git repository or that git is installed. Our build
-// system does this for release builds.
+// Injected via git-export(1). See that man page and gitattributes(5).
+const (
+	// Needs a length check because GitHub zipballs/tarballs don't do the
+	// describe and just strip the pattern.
+	describe = `$Format:%(describe:match=v4.*)$`
+	revision = `$Format:%h (%cI)$`
+)
 
 func init() {
-	defer func() {
-		if Version == "" {
-			Version = `???`
-		}
-	}()
-	ctx, done := context.WithTimeout(context.Background(), 5*time.Second)
-	defer done()
-	if Version != "" {
+	switch {
+	case Version != "":
 		// Had our version injected at build: do nothing.
-		return
-	}
-	if _, err := exec.LookPath("git"); err != nil {
-		// Couldn't find a git binary: do nothing.
-		return
-	}
-	if err := exec.CommandContext(ctx, "git", "rev-parse", "--show-toplevel").Run(); err != nil {
-		// Couldn't find a git repository: do nothing.
-		return
+	case len(describe) > 0 && describe[0] != '$':
+		Version = describe
+	case revision[0] == '$':
+		// This is a helper for development. In production, we shouldn't assume
+		// that the process is running in a git repository or that git is
+		// installed. This is quite possibly wrong if run from the wrong working
+		// directory.
+		Version = `(random source build)`
+		ctx, done := context.WithTimeout(context.Background(), 5*time.Second)
+		defer done()
+		if _, err := exec.LookPath("git"); err != nil {
+			// Couldn't find a git binary: do nothing.
+			break
+		}
+		if err := exec.CommandContext(ctx, "git", "rev-parse", "--show-toplevel").Run(); err != nil {
+			// Couldn't find a git repository: do nothing.
+			break
+		}
+		out, err := exec.CommandContext(ctx, "git", "describe").Output()
+		if err != nil {
+			// Couldn't describe the current commit: do nothing.
+			break
+		}
+		Version = string(bytes.TrimSpace(out))
+	default:
+		Version = revision
 	}
-	out, err := exec.CommandContext(ctx, "git", "describe").Output()
-	if err != nil {
-		// Couldn't describe the current commit: do nothing.
-		return
+
+	// If we can read out the current binary's debug info, append the claircore
+	// version if there was a replacement.
+	if info, ok := debug.ReadBuildInfo(); ok {
+		for _, m := range info.Deps {
+			if m.Path != "github.com/quay/claircore" {
+				continue
+			}
+			if m.Replace != nil && m.Replace.Version != m.Version {
+				Version += " (claircore " + m.Replace.Version + ")"
+			}
+		}
 	}
-	Version = string(bytes.TrimSpace(out))
 }
 
-// Version is a version string, injected at build time for release builds.
+// Version is a version string, injected at release time for release builds.
 var Version string