allow buildah cross compile for a darwin target

the goal here is to allow ourselves to cross compile buildah for a darwin
target.  we are doing this to eventually protect from regressions that could
creep into buildah so we don't dig ourselves a deeper hole.

the simplified and non-variable approach to the make darwin was done with
intent to keep this simple until we can exploit things a little more.

once this PR merges, i will create a CI test that will test for regressions
on a make darwin.  we should also be doing a gofmt with a darwin target so the
!linux|darwin tagged files are also checked for completeness.  initially the
test can be optional for passing with the long-term idea that it be made
a firm requirement at the buildah maintainers behest.

Signed-off-by: baude <bbaude@redhat.com>

Closes: #840
Approved by: rhatdan

Makefile

@@ -20,6 +20,9 @@ all: buildah imgtype docs
 buildah: *.go imagebuildah/*.go bind/*.go cmd/buildah/*.go docker/*.go pkg/cli/*.go pkg/parse/*.go unshare/*.c unshare/*.go util/*.go
 	$(GO) build $(LDFLAGS) -o buildah $(BUILDFLAGS) ./cmd/buildah
 
+darwin:
+	GOOS=darwin $(GO) build $(LDFLAGS) -o buildah.darwin -tags "containers_image_openpgp" ./cmd/buildah
+
 imgtype: *.go docker/*.go util/*.go tests/imgtype/imgtype.go
 	$(GO) build $(LDFLAGS) -o imgtype $(BUILDFLAGS) ./tests/imgtype/imgtype.go
 

cmd/buildah/unshare_unsupported.go

@@ -2,6 +2,10 @@
 
 package main
 
+import (
+	"github.com/urfave/cli"
+)
+
 var (
 	unshareCommand = cli.Command{
 		Name:           "unshare",

run.go

@@ -1145,9 +1145,9 @@ func runUsingRuntimeMain() {
 		os.Exit(1)
 	}
 	// Set ourselves up to read the container's exit status.  We're doing this in a child process
-	// so that we won't mess with the setting in a caller of the library.
-	if err := unix.Prctl(unix.PR_SET_CHILD_SUBREAPER, uintptr(1), 0, 0, 0); err != nil {
-		fmt.Fprintf(os.Stderr, "prctl(PR_SET_CHILD_SUBREAPER, 1): %v\n", err)
+	// so that we won't mess with the setting in a caller of the library. This stubs to OS specific
+	// calls
+	if err := setChildProcess(); err != nil {
 		os.Exit(1)
 	}
 	// Run the container, start to finish.

run_linux.go

@@ -0,0 +1,17 @@
+// +build linux
+
+package buildah
+
+import (
+	"fmt"
+	"golang.org/x/sys/unix"
+	"os"
+)
+
+func setChildProcess() error {
+	if err := unix.Prctl(unix.PR_SET_CHILD_SUBREAPER, uintptr(1), 0, 0, 0); err != nil {
+		fmt.Fprintf(os.Stderr, "prctl(PR_SET_CHILD_SUBREAPER, 1): %v\n", err)
+		return err
+	}
+	return nil
+}

run_unsupport.go

@@ -0,0 +1,11 @@
+// +build !linux
+
+package buildah
+
+import (
+	"github.com/pkg/errors"
+)
+
+func setChildProcess() error {
+	return errors.New("function not supported on non-linux systems")
+}

vendor.conf

@@ -42,7 +42,7 @@ github.com/ostreedev/ostree-go aeb02c6b6aa2889db3ef62f7855650755befd460
 github.com/pborman/uuid master
 github.com/pkg/errors master
 github.com/pquerna/ffjson d49c2bc1aa135aad0c6f4fc2056623ec78f5d5ac
-github.com/projectatomic/libpod 781eec27b52c842fc83c8b1c97fbf825065f3b0c
+github.com/projectatomic/libpod master
 github.com/sirupsen/logrus master
 github.com/syndtr/gocapability master
 github.com/tchap/go-patricia master