Read the bus through a module's declared states in frombus, with the runtime's adapter in fromruntime, so mesh-provisioner and disk-load compose from one reader instead of two copies (issue 1955)

go/compose/fromruntime/fromruntime.go

@@ -0,0 +1,49 @@
+// Package fromruntime is compose/frombus's StatesReader over a bundle's runtime (the lead, 2026-10-29): the states a
+// module's manifest names in `reads` (frombus.StatesReads), read through stdio.State, and a templates archive fetched
+// over HTTP. Apart from frombus so that frombus stays pure for the mesh-compose command.
+package fromruntime
+
+import (
+	"context"
+	"fmt"
+	"io"
+	"net/http"
+
+	stdio "git.novox.be/novox/mesh-sdk/go"
+	"git.novox.be/novox/mesh-sdk/go/compose/frombus"
+)
+
+// States are a bundle's read states through its runtime.
+type States struct{}
+
+// Get is a key's value, false when it has none.
+func (States) Get(state, key string) ([]byte, bool, error) {
+	e, err := stdio.State(state).Get(key)
+	if err != nil || e == nil {
+		return nil, false, err
+	}
+	return e.Value, true, nil
+}
+
+// Keys are a state's keys.
+func (States) Keys(state string) ([]string, error) { return stdio.State(state).Keys() }
+
+// Reader is frombus's Reader over this bundle's read states, fetching archives over HTTP.
+func Reader() frombus.Reader { return frombus.StatesReader(States{}, HTTPFetch) }
+
+// HTTPFetch reads an archive over HTTP, at most 256 MiB.
+func HTTPFetch(ctx context.Context, url string) ([]byte, error) {
+	req, err := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)
+	if err != nil {
+		return nil, err
+	}
+	resp, err := http.DefaultClient.Do(req)
+	if err != nil {
+		return nil, err
+	}
+	defer resp.Body.Close()
+	if resp.StatusCode != http.StatusOK {
+		return nil, fmt.Errorf("%s answered %s", url, resp.Status)
+	}
+	return io.ReadAll(io.LimitReader(resp.Body, 256<<20))
+}