review: rebuild a file the mesh once wrote whole, keep links, give back a missing line end, and release a hold only after the write (hq issue 128)

internal/apply/apply.go

@@ -52,6 +52,8 @@ type Outcome struct {
 	wrote string
 	// into is what a file written into held before the mesh's keys (novox/hq ADR 0102).
 	into *store.Into
+	// kept is where this apply kept the original of a file it wrote over (novox/hq ADR 0100).
+	kept string
 	// reads is, for a container, the digest of each file it was created reading, by path — so
 	// the next apply can say which one changed (novox/hq 04-ISSUES/103).
 	reads map[string]string
@@ -441,6 +443,16 @@ func ApplyKeeping(
 			continue
 		}
 
+		// Where the original of what this file replaced was kept, carried for as long as the
+		// resource is recorded: kept by this apply, by a hold its module's cutover ends, or before.
+		held, wasHeld := known.HeldAt(resource.Identity())
+		kept := outcome.kept
+		if kept == "" && wasHeld {
+			kept = held.Kept
+		}
+		if kept == "" {
+			kept = was.Kept
+		}
 		// Only now. The record follows the fact, never leads it.
 		known.Record(store.Applied{
 			Origin: origin,
@@ -448,13 +460,19 @@ func ApplyKeeping(
 			Target: outcome.Target, AppliedAt: time.Now().UTC(),
 			Wrote: outcome.wrote,
 			Into:  outcome.into,
+			Kept:  kept,
 			Reads: outcome.reads,
 			Holds: holds(resource),
 		})
-		// Its module has been taken, and what was held for it is now the mesh's.
-		if held, wasHeld := known.HeldAt(resource.Identity()); wasHeld {
+		// Its module has been taken, and what was held for it is now the mesh's. A file written
+		// into replaced nothing that was found, so its outcome says what the write did, not that
+		// a cutover happened; its hold from when it was declared whole goes all the same — here,
+		// after the write worked, so a failed one keeps the hold and where its original is.
+		if wasHeld {
 			known.Release(held.ID)
-			outcome.Detail = takenDetail(held)
+			if f, isFile := resource.(*declaration.File); !isFile || f.Into == "" {
+				outcome.Detail = takenDetail(held)
+			}
 		}
 		if svc, ok := resource.(*declaration.Service); ok && svc.TakesOver != nil && report.Tunnel != nil {
 			// The found interface is down and the mesh's is up in its place: the tunnel changed
@@ -840,6 +858,7 @@ func applyFile(r *declaration.File, previous store.Applied, unseal Unseal, keepF
 	default:
 		out.Action = "unchanged"
 	}
+	out.kept = kept
 	if kept != "" {
 		if out.Detail != "" {
 			out.Detail += "; "