feat(SelfUpdate): added check for version being out of date

while we're here, might as well use an ipns dnslink entry to check
if the qri verion if out of date by comparing a hard-coded version
of the 2-n back version of qri against the hardcoded hash of the
1-n back version stored within the app itself.

api/server.go

@@ -94,6 +94,14 @@ func (s *Server) Serve() (err error) {
 	go s.ServeRPC()
 	go s.ServeWebapp()
 
+	if node, err := s.qriNode.IPFSNode(); err == nil {
+		go func() {
+			if err := core.CheckVersion(context.Background(), node.Namesys); err == core.ErrUpdateRequired {
+				s.log.Info("This version of qri is out of date, please refer to https://github.com/qri-io/qri/releases/latest for more info")
+			}
+		}()
+	}
+
 	// http.ListenAndServe will not return unless there's an error
 	return StartServer(s.cfg, server)
 }
@@ -181,6 +189,7 @@ func (s *Server) HandleIPNSPath(w http.ResponseWriter, r *http.Request) {
 		apiutil.WriteErrResponse(w, http.StatusBadRequest, fmt.Errorf("no IPFS node present: %s", err.Error()))
 		return
 	}
+
 	p, err := node.Namesys.Resolve(r.Context(), r.URL.Path[len("/ipns/"):])
 	if err != nil {
 		apiutil.WriteErrResponse(w, http.StatusBadRequest, fmt.Errorf("error resolving IPNS Name: %s", err.Error()))

core/self_update.go

@@ -0,0 +1,41 @@
+package core
+
+import (
+	"context"
+	"fmt"
+
+	namesys "gx/ipfs/QmViBzgruNUoLNBnXcx8YWbDNwV8MNGEGKkLo6JGetygdw/go-ipfs/namesys"
+)
+
+/*
+* self-updating checks IPNS entry that represents the desired hash of the previous version of
+* this program, which is the result of adding the complied binary of this program to IPFS.
+* If the returned value of a lookup differs, we have a version mismatch, and need to perform
+* an update.
+*
+* In the future we'll do this automatically, but for now we can at least warn users that they need
+* to update their version
+ */
+
+// previousVersion is a hard-coded reference the gx "lastpubver" file of the previous release
+const previousVersion = "/ipfs/QmcXZCLAgUdvXpt1fszjNGVGn6WnhsrJahkQXY3JJqxUWJ"
+
+// prevIPNSName is the dnslink address to check for version agreement
+const prevIPNSName = "/ipns/cli.previous.qri.io"
+
+// ErrUpdateRequired means this version of qri is out of date
+var ErrUpdateRequired = fmt.Errorf("update required")
+
+// CheckVersion uses a name resolver to lookup prevIPNSName, checking if the hard-coded previousVersion
+// and the returned lookup match. If they don't, CheckVersion returns ErrUpdateRequired
+func CheckVersion(ctx context.Context, res namesys.Resolver) error {
+	p, err := res.Resolve(ctx, prevIPNSName)
+	if err != nil {
+		return fmt.Errorf("error resolving name: %s", err.Error())
+	}
+
+	if p.String() != previousVersion {
+		return ErrUpdateRequired
+	}
+	return nil
+}