Mount a share at the access the server gave this machine, and not at all when it does not export it here

modules/mounts/README.md

@@ -86,7 +86,9 @@ password: the options, fstab lines and mountinfo the tools read are redacted.
   what `watch` found in the module's state `shares`, keyed by this machine, with `wants`: each share this
   machine asks of which server and whether read-write or read-only (`[{"server", "share", "access"}]`).
   The server exports a share only to the machines that want it and that it grants, and decides the access
-  itself. Every five minutes the bundle also asks the controller's `data` verb where the mesh keeps data on
+  itself: `apply` mounts a share at the access the server's answer gives this machine (`to[]`), read-only
+  when either the share or this machine's grant is, and not at all when the server does not export it
+  here (said as failed, naming the grant that is missing). Every five minutes the bundle also asks the controller's `data` verb where the mesh keeps data on
   this machine, and leaves the paths for `apply`; a failed ask keeps the last paths. No module reads
   another's files (ADR 0260).
 

modules/mounts/cmd/mounts/apply.go

@@ -252,7 +252,13 @@ func (a *Applier) Look(ctx context.Context) Status {
 			notSet[w.Name] = ShareStatus{Name: w.Name, Kind: "nfs", Mountpoint: w.Mountpoint, State: StateFailed,
 				Why: fmt.Sprintf("the sharing machine gave the path %q, which the module does not mount", e.Path)}
 		default:
-			pairs = append(pairs, NFSPair(w, cfg.At, e))
+			mine, exported := e.ForNode(cfg.Node)
+			if !exported {
+				notSet[w.Name] = ShareStatus{Name: w.Name, Kind: "nfs", Mountpoint: w.Mountpoint, State: StateFailed,
+					Why: "the sharing machine " + st.Server + " does not export " + w.Name + " to this machine: its grants do not name " + cfg.Node}
+				continue
+			}
+			pairs = append(pairs, NFSPair(w, cfg.At, mine))
 		}
 	}
 

modules/mounts/cmd/mounts/review_test.go

@@ -306,3 +306,24 @@ func TestTheBundleKeepsTheLastDataWhenAnAskFails(t *testing.T) {
 		t.Errorf("a failed ask keeps the last paths and says why: %+v %v %q", d, err, said)
 	}
 }
+
+// The server decides each node's access (finding 5): a share it exports read-write may reach this node
+// read-only, and a share it exports to other nodes only does not reach this one at all. The mount takes
+// the access the server gave this node, never the share's own.
+func TestTheServersAccessForThisNodeIsTheMountsAccess(t *testing.T) {
+	library := ServerShare{Name: "library", Path: "/srv/library", Access: "rw", Live: true,
+		To: []ShareClient{{Node: "ws", Address: "10.0.0.3", Access: "rw"}, {Node: "laptop", Address: "10.0.0.4", Access: "ro"}}}
+	got, ok := library.ForNode("laptop")
+	if !ok || got.Access != "ro" {
+		t.Errorf("the laptop, granted read-only, gets %q (exported to it: %v)", got.Access, ok)
+	}
+	if p := NFSPair(Wanted{Name: "library", Mountpoint: "/srv/library"}, "192.0.2.10", got); !strings.Contains(p.Mount, "Options=ro,") {
+		t.Errorf("a read-only grant mounted read-write:\n%s", p.Mount)
+	}
+	if got, ok := library.ForNode("ws"); !ok || got.Access != "rw" {
+		t.Errorf("the workstation, granted read-write, gets %q (%v)", got.Access, ok)
+	}
+	if _, ok := library.ForNode("control"); ok {
+		t.Errorf("a node the server does not export to is treated as exported to")
+	}
+}

modules/mounts/cmd/mounts/units.go

@@ -71,6 +71,29 @@ type ServerShare struct {
 	GID     int    `json:"gid"`
 	Clients string `json:"clients,omitempty"`
 	Live    bool   `json:"live"`
+	// To is each node the server exports the share to, at the access it decided for that node.
+	To []ShareClient `json:"to,omitempty"`
+}
+
+// ShareClient is one node a share is exported to, as the server decided it.
+type ShareClient struct {
+	Node    string `json:"node"`
+	Address string `json:"address"`
+	Access  string `json:"access"`
+}
+
+// ForNode is the share as the server exports it to one node: its access is the lower of the share's own
+// and the node's, and false when the server does not export it to that node at all.
+func (e ServerShare) ForNode(node string) (ServerShare, bool) {
+	for _, c := range e.To {
+		if c.Node == node {
+			if c.Access == "ro" || e.Access == "ro" {
+				e.Access = "ro"
+			}
+			return e, true
+		}
+	}
+	return e, false
 }
 
 func header(name, kind string) string {

modules/mounts/cmd/mounts/write_test.go

@@ -128,7 +128,7 @@ func (f *fakeSystem) everything(except ...string) string {
 
 const config = "shares=library=/srv/library downloads=/srv/downloads:ro\n" +
 	"sources=games=smb://192.0.2.76/Share@/mnt/games usb=disk:1234-ABCD@/mnt/usb\n" +
-	"server=homeserver\nat=192.0.2.10\naccount=operator\n" +
+	"server=homeserver\nat=192.0.2.10\nnode=workstation\naccount=operator\n" +
 	"secret=/var/lib/mesh/mounts/smb-credentials.secret\nstate=/var/lib/mesh/mounts\n"
 
 func exportsAnswer(t *testing.T, shares ...ServerShare) string {
@@ -140,8 +140,8 @@ func exportsAnswer(t *testing.T, shares ...ServerShare) string {
 }
 
 var (
-	library   = ServerShare{Name: "library", Path: "/tank/media", Access: "rw", UID: 1001, GID: 2000, Live: true}
-	downloads = ServerShare{Name: "downloads", Path: "/tank/downloads", Access: "rw", UID: 1001, GID: 2000, Live: true}
+	library   = ServerShare{Name: "library", Path: "/tank/media", Access: "rw", UID: 1001, GID: 2000, Live: true, To: []ShareClient{{Node: "workstation", Address: "10.0.0.3", Access: "rw"}}}
+	downloads = ServerShare{Name: "downloads", Path: "/tank/downloads", Access: "rw", UID: 1001, GID: 2000, Live: true, To: []ShareClient{{Node: "workstation", Address: "10.0.0.3", Access: "rw"}}}
 )
 
 func TestTheSettingsAreReadAndWhatCannotBeIsSaidPlainly(t *testing.T) {