context: detect and report conflicting options

`WithChroot` and `WithSnapshot` conflicts to each other,
and the users can get surprising results if they try to use them
at the same time.

Now: chroot is expected to be much more popular than snapshots,
being the latter useful mostly for tests or offline debug/troubleshoot,
but still the code should detect and handle these conflicts.

This is made a bit awkward because the current `context` pkg API.
Once we are free to break compatibility, we can make the flow more
linear.

Signed-off-by: Francesco Romani <fromani@redhat.com>

pkg/context/context.go

@@ -7,6 +7,8 @@
 package context
 
 import (
+	"fmt"
+
 	"github.com/jaypipes/ghw/pkg/option"
 	"github.com/jaypipes/ghw/pkg/snapshot"
 )
@@ -22,6 +24,7 @@ type Context struct {
 	snapshotUnpackedPath string
 	alert                option.Alerter
 	refCount             int
+	err                  error
 }
 
 // New returns a Context struct pointer that has had various options set on it
@@ -49,6 +52,13 @@ func New(opts ...*option.Option) *Context {
 		ctx.EnableTools = *merged.EnableTools
 	}
 
+	// New is not allowed to return error - it would break the established API.
+	// so the only way out is to actually do the checks here and record the error,
+	// and return it later, at the earliest possible occasion, in Setup()
+	if ctx.SnapshotPath != "" && ctx.Chroot != option.DefaultChroot {
+		// The env/client code supplied a value, but we are will overwrite it when unpacking shapshots!
+		ctx.err = fmt.Errorf("Conflicting options: chroot %q and snapshot path %q", ctx.Chroot, ctx.SnapshotPath)
+	}
 	return ctx
 }
 
@@ -85,6 +95,9 @@ func (ctx *Context) Do(fn func() error) error {
 // You should call `Setup` just once. It is safe to call `Setup` if you don't make
 // use of optional extra features - `Setup` will do nothing.
 func (ctx *Context) Setup() error {
+	if ctx.err != nil {
+		return ctx.err
+	}
 	if ctx.refCount > 1 {
 		// someone else came before and fixed things already!
 		ctx.refCount++

pkg/option/option.go

@@ -14,7 +14,10 @@ import (
 )
 
 const (
-	defaultChroot           = "/"
+	DefaultChroot = "/"
+)
+
+const (
 	envKeyChroot            = "GHW_CHROOT"
 	envKeyDisableWarnings   = "GHW_DISABLE_WARNINGS"
 	envKeyDisableTools      = "GHW_DISABLE_TOOLS"
@@ -57,7 +60,7 @@ func EnvOrDefaultChroot() string {
 	if val, exists := os.LookupEnv(envKeyChroot); exists {
 		return val
 	}
-	return defaultChroot
+	return DefaultChroot
 }
 
 // EnvOrDefaultSnapshotPath returns the value of the GHW_SNAPSHOT_PATH environs variable