zfs: introduce pkg zfs/zfscmd for command logging, status, prometheus metrics

refs #196

zfs/zfscmd/zfscmd_context.go

@@ -0,0 +1,39 @@
+package zfscmd
+
+import (
+	"context"
+
+	"github.com/zrepl/zrepl/logger"
+)
+
+type contextKey int
+
+const (
+	contextKeyLogger contextKey = iota
+	contextKeyJobID
+)
+
+type Logger = logger.Logger
+
+func WithJobID(ctx context.Context, jobID string) context.Context {
+	return context.WithValue(ctx, contextKeyJobID, jobID)
+}
+
+func getJobIDOrDefault(ctx context.Context, def string) string {
+	ret, ok := ctx.Value(contextKeyJobID).(string)
+	if !ok {
+		return def
+	}
+	return ret
+}
+
+func WithLogger(ctx context.Context, log Logger) context.Context {
+	return context.WithValue(ctx, contextKeyLogger, log)
+}
+
+func getLogger(ctx context.Context) Logger {
+	if l, ok := ctx.Value(contextKeyLogger).(Logger); ok {
+		return l
+	}
+	return logger.NewNullLogger()
+}