A follower joining a watched state hears its current values, and a templates archive is fetched once

A read could start a state's watch before its follower joined, and a change landing in that gap woke nothing (review
of ADR 0580 §2): a joining follower now runs over the copy's current values before any later change. The watched
reader keeps each archive once per URL, and both caches are bounded.

Claude-Session: https://claude.ai/code/session_01NcbgfJ9Gum47zkFKWRaovL

go/compose/frombus/frombustest/frombustest_test.go

@@ -157,6 +157,16 @@ func TestTheProvisionedMeshHandsAPlanWithItsProvision(t *testing.T) {
 		byKey[frombus.PlacementRead+" watched"] != 1 {
 		t.Errorf("the watched reader read %v, want a build by key each time, a manifest once, the placement watched once", byKey)
 	}
+	fetched := 0
+	keep := fromruntime.KeptFetch(func(context.Context, string) ([]byte, error) { fetched++; return []byte("tar"), nil })
+	for range 2 {
+		if body, err := keep(context.Background(), "http://store/restic/files/blobs/sha256:a"); err != nil || string(body) != "tar" {
+			t.Fatalf("the kept fetch answered %q, %v", body, err)
+		}
+	}
+	if fetched != 1 {
+		t.Errorf("a templates archive was fetched %d times, want once", fetched)
+	}
 }
 
 func planned[P any](plans map[string]P, module string) bool { _, ok := plans[module]; return ok }

go/compose/fromruntime/fromruntime.go

@@ -8,6 +8,7 @@ import (
 	"fmt"
 	"io"
 	"net/http"
+	"sync"
 
 	stdio "git.novox.be/novox/mesh-sdk/go"
 	"git.novox.be/novox/mesh-sdk/go/compose/frombus"
@@ -47,9 +48,39 @@ func (w Watched) Get(state, key string) ([]byte, bool, error) {
 	return w.WatchedStates.Get(state, key)
 }
 
-// WatchedReader is frombus's Reader over w, fetching archives over HTTP.
+// WatchedReader is frombus's Reader over w, fetching each templates archive over HTTP once (KeptFetch): a composition
+// made again on every change reads it again otherwise.
 func WatchedReader(w *stdio.WatchedStates) frombus.Reader {
-	return frombus.StatesReader(Watched{w}, HTTPFetch)
+	return frombus.StatesReader(Watched{w}, KeptFetch(HTTPFetch))
+}
+
+// archivesAtMost bounds the archives KeptFetch keeps; past it they are emptied, which costs only a fetch again.
+const archivesAtMost = 64
+
+// KeptFetch is fetch kept once per URL: an archive's URL names its blob by digest, so its body never changes under it
+// (frombus checks the digest on every read). A failed fetch is not kept.
+func KeptFetch(fetch frombus.Fetch) frombus.Fetch {
+	var mu sync.Mutex
+	kept := map[string][]byte{}
+	return func(ctx context.Context, url string) ([]byte, error) {
+		mu.Lock()
+		body, ok := kept[url]
+		mu.Unlock()
+		if ok {
+			return body, nil
+		}
+		body, err := fetch(ctx, url)
+		if err != nil {
+			return nil, err
+		}
+		mu.Lock()
+		if len(kept) >= archivesAtMost {
+			clear(kept)
+		}
+		kept[url] = body
+		mu.Unlock()
+		return body, nil
+	}
 }
 
 // HTTPFetch reads an archive over HTTP, at most 256 MiB.