version: Add versioning for github tarballs

When building from github tarballs, getting the version from `git
describe` won't work, or worse, will be irrelevant [1]
However, we can make use of $Format$ and .gitattributes as described in
[2] to automatically substitute the correct version in a go variable
when `git archive` is used.
In particular, the correct version number will automatically be
substituted when GitHub creates release tarballs.

[1] rpms/gvisor-tap-vsock.spec uses `%autosetup -Sgit -n %{name}-%{version}` which unpacks
the release tarball in a newly created git repository.

[2] https://icinga.com/blog/2022/05/25/embedding-git-commit-information-in-go-binaries/

Signed-off-by: Christophe Fergeau <[email protected]>

.gitattributes

@@ -0,0 +1 @@
+/pkg/types/version.go export-subst

pkg/types/version.go

@@ -4,11 +4,15 @@ import (
 	"flag"
 	"fmt"
 	"runtime/debug"
+	"strings"
 )
 
 var (
 	// set using the '-X github.com/containers/gvisor-tap-vsock/pkg/types.gitVersion' linker flag
 	gitVersion = ""
+	// set through .gitattributes when `git archive` is used
+	// see https://icinga.com/blog/2022/05/25/embedding-git-commit-information-in-go-binaries/
+	gitArchiveVersion = "$Format:%(describe)$"
 )
 
 type version struct {
@@ -36,6 +40,9 @@ func (ver *version) ShowVersion() bool {
 
 func moduleVersion() string {
 	switch {
+	// This will be substituted when building from a GitHub tarball
+	case !strings.HasPrefix(gitArchiveVersion, "$Format:"):
+		return gitArchiveVersion
 	// This will be set when building from git using make
 	case gitVersion != "":
 		return gitVersion

rpm/gvisor-tap-vsock.spec

@@ -92,8 +92,8 @@ CGO_CFLAGS=$(echo $CGO_CFLAGS | sed 's/-specs=\/usr\/lib\/rpm\/redhat\/redhat-an
 export CGO_CFLAGS+=" -m64 -mtune=generic -fcf-protection=full"
 %endif
 
-# reset LDFLAGS for plugins and gvisor binaries, but ensure gvproxy --version and such is set
-LDFLAGS="-X github.com/containers/gvisor-tap-vsock/pkg/types.gitVersion=%{version}"
+# reset LDFLAGS for plugins and gvisor binaries
+LDFLAGS=''
 
 # build gvisor-tap-vsock binaries
 %gobuild -o bin/gvproxy ./cmd/gvproxy