generate: Respect --os when generating default templates

Also add --host-specific checks to SetPlatformOS and SetPlatformArch.
The SetPlatformArch check is currently just a warning (details in
eda1ac7, validate: With --host-specific, compare config platform
vs. runtime, 2016-08-16) but I've added an error to its API so callers
will be ready when we do actually raise errors (e.g. for '--arch arm'
on an amd64 box).

Also add IsSet checks to --os and --arch so we don't clobber their
template values.  Before this commit on my amd64 Linux box:

  $ echo '{"platform": {"os": "windows", "arch": "386"}}' >template.json
  $ ocitools generate --template template.json | jq .platform
  {
    "os": "linux",
    "arch": "amd64"
  }

After this commit:

  $ ocitools generate --template template.json | jq .platform
  {
    "os": "windows",
    "arch": "386"
  }

Signed-off-by: W. Trevor King <wking@tremily.us>

Author: wking

cmd/oci-runtime-tool/generate.go

@@ -3,7 +3,6 @@ package main
 import (
 	"fmt"
 	"os"
-	"runtime"
 	"strconv"
 	"strings"
 
@@ -15,7 +14,7 @@ import (
 
 var generateFlags = []cli.Flag{
 	cli.StringFlag{Name: "apparmor", Usage: "specifies the the apparmor profile for the container"},
-	cli.StringFlag{Name: "arch", Value: runtime.GOARCH, Usage: "architecture the container is created for"},
+	cli.StringFlag{Name: "arch", Usage: "architecture the container is created for"},
 	cli.StringSliceFlag{Name: "args", Usage: "command to run in the container"},
 	cli.StringSliceFlag{Name: "bind", Usage: "bind mount directories src:dest[:options...]"},
 	cli.StringSliceFlag{Name: "cap-add", Usage: "add Linux capabilities"},
@@ -52,7 +51,7 @@ var generateFlags = []cli.Flag{
 	cli.StringFlag{Name: "network", Usage: "network namespace"},
 	cli.BoolFlag{Name: "no-new-privileges", Usage: "set no new privileges bit for the container process"},
 	cli.IntFlag{Name: "oom-score-adj", Usage: "oom_score_adj for the container"},
-	cli.StringFlag{Name: "os", Value: runtime.GOOS, Usage: "operating system the container is created for"},
+	cli.StringFlag{Name: "os", Usage: "operating system the container is created for"},
 	cli.StringFlag{Name: "output", Usage: "output file (defaults to stdout)"},
 	cli.StringFlag{Name: "pid", Usage: "pid namespace"},
 	cli.StringSliceFlag{Name: "poststart", Usage: "set command to run in poststart hooks"},
@@ -92,7 +91,12 @@ var generateCommand = cli.Command{
 	Before: before,
 	Action: func(context *cli.Context) error {
 		// Start from the default template.
-		specgen, err := generate.New(nil)
+		var osPointer *string
+		if context.IsSet("os") {
+			goos := context.String("os")
+			osPointer = &goos
+		}
+		specgen, err := generate.New(osPointer)
 		if err != nil {
 			return err
 		}
@@ -143,8 +147,19 @@ func setupSpec(g *generate.Generator, context *cli.Context) error {
 		g.SetHostname(context.String("hostname"))
 	}
 
-	g.SetPlatformOS(context.String("os"))
-	g.SetPlatformArch(context.String("arch"))
+	if context.IsSet("os") {
+		err := g.SetPlatformOS(context.String("os"))
+		if err != nil {
+			return err
+		}
+	}
+
+	if context.IsSet("arch") {
+		err := g.SetPlatformArch(context.String("arch"))
+		if err != nil {
+			return err
+		}
+	}
 
 	if context.IsSet("label") {
 		annotations := context.StringSlice("label")

generate/generate.go

@@ -11,6 +11,7 @@ import (
 
 	rspec "github.com/opencontainers/runtime-spec/specs-go"
 	"github.com/opencontainers/runtime-tools/generate/seccomp"
+	"github.com/Sirupsen/logrus"
 	"github.com/syndtr/gocapability/capability"
 )
 
@@ -284,15 +285,23 @@ func (g *Generator) RemoveAnnotation(key string) {
 }
 
 // SetPlatformOS sets g.spec.Process.OS.
-func (g *Generator) SetPlatformOS(os string) {
+func (g *Generator) SetPlatformOS(os string) error {
+	if g.HostSpecific && os != runtime.GOOS {
+		return fmt.Errorf("cannot set platform.os to %s on a %s host", os, runtime.GOOS)
+	}
 	g.initSpec()
 	g.spec.Platform.OS = os
+	return nil
 }
 
 // SetPlatformArch sets g.spec.Platform.Arch.
-func (g *Generator) SetPlatformArch(arch string) {
+func (g *Generator) SetPlatformArch(arch string) error {
+	if g.HostSpecific && arch != runtime.GOARCH {
+		logrus.Warnf("setting platform.arch to %s on a %s host", arch, runtime.GOARCH)
+	}
 	g.initSpec()
 	g.spec.Platform.Arch = arch
+	return nil
 }
 
 // SetProcessUID sets g.spec.Process.User.UID.

man/oci-runtime-tool-generate.1.md

@@ -173,7 +173,13 @@ read the configuration from `config.json`.
   Specifies oom_score_adj for the container.
 
 **--os**=OS
-  Operating system used within the container
+  Operating system used within the container.  When no template is
+  set, the default is to use the `GOOS` ocitools was built with and to
+  default other settings based on this setting.  For example, if
+  `GOOS` is `linux`, `ocitools generate` will generate a default Linux
+  config, `ocitools generate --os=windows` will generate a default
+  Windows config, and `ocitools --host-specific generate --os=windows`
+  will exit with an error.
 
 **--output**=PATH
   Instead of writing the configuration JSON to stdout, write it to a