Rely on Go-maintained commit value instead of handling that ourselves

Should not change behavior

Signed-off-by: Miloslav Trmač <mitr@redhat.com>

Makefile

@@ -81,10 +81,8 @@ endif
 CONTAINER_GOSRC = /src/github.com/containers/skopeo
 CONTAINER_RUN ?= $(CONTAINER_CMD) --security-opt label=disable -v $(CURDIR):$(CONTAINER_GOSRC) -w $(CONTAINER_GOSRC) $(SKOPEO_CIDEV_CONTAINER_FQIN)
 
-GIT_COMMIT := $(shell GIT_CEILING_DIRECTORIES=$$(cd ..; pwd) git rev-parse HEAD 2> /dev/null || true)
-
 EXTRA_LDFLAGS ?=
-SKOPEO_LDFLAGS := -ldflags '-X main.gitCommit=${GIT_COMMIT} $(EXTRA_LDFLAGS)'
+SKOPEO_LDFLAGS := -ldflags '$(EXTRA_LDFLAGS)'
 
 MANPAGES_MD = $(wildcard docs/*.md)
 MANPAGES ?= $(MANPAGES_MD:%.md=%)

cmd/skopeo/main.go

@@ -3,6 +3,7 @@ package main
 import (
 	"context"
 	"fmt"
+	"runtime/debug"
 	"strings"
 	"time"
 
@@ -15,10 +16,6 @@ import (
 	"go.podman.io/storage/pkg/reexec"
 )
 
-// gitCommit will be the hash that the binary was built from
-// and will be populated by the Makefile
-var gitCommit = ""
-
 var defaultUserAgent = "skopeo/" + version.Version
 
 type globalOptions struct {
@@ -70,8 +67,10 @@ func createApp() (*cobra.Command, *globalOptions) {
 		// (skopeo --tls-verify inspect) (causes a warning) and (skopeo inspect --tls-verify) (no warning).
 		TraverseChildren: true,
 	}
-	if gitCommit != "" {
-		rootCommand.Version = fmt.Sprintf("%s commit: %s", version.Version, gitCommit)
+	// We don’t use debug.ReadBuildInfo to automate version.Version, because that would not work well for builds from
+	// a released tarball (e.g. RPM builds).
+	if commit := gitCommit(); commit != "" {
+		rootCommand.Version = fmt.Sprintf("%s commit: %s", version.Version, commit)
 	} else {
 		rootCommand.Version = version.Version
 	}
@@ -112,6 +111,20 @@ func createApp() (*cobra.Command, *globalOptions) {
 	return rootCommand, &opts
 }
 
+// gitCommit returns the git commit for this codebase, if we are built from a git repo; "" otherwise.
+func gitCommit() string {
+	bi, ok := debug.ReadBuildInfo()
+	if !ok {
+		logrus.Fatal("runtime.ReadBuildInfo failed")
+	}
+	for _, e := range bi.Settings {
+		if e.Key == "vcs.revision" {
+			return e.Value
+		}
+	}
+	return ""
+}
+
 // before is run by the cli package for any command, before running the command-specific handler.
 func (opts *globalOptions) before(cmd *cobra.Command, args []string) error {
 	if opts.debug {