--- a/thumbcache.swift	2026-09-23 13:32:39
+++ b/thumbcache.swift	2026-09-23 13:32:39
@@ -1,23 +1,24 @@
-// EXERCISE 01 — BROKEN ON PURPOSE. Do not copy this shape into real code.
+// EXERCISE 01 — REPAIRED.
 //
 // A thumbnail pipeline. Each Thumbnailer decodes one image and reports back
 // through a completion handler it stores ON ITSELF so the work can be retried.
 //
-// Symptom as reported by QA: "memory climbs while browsing folders and never
-// comes back down, even after we navigate away and the cache is emptied."
+// The repair: the stored closure holds self WEAKLY, so the reference graph is
+// no longer a cycle and ARC releases every Thumbnailer when the cache drops
+// its last strong reference.
 //
 // The fixture browses ROUNDS folders in a row, because ONE round does not
 // distinguish the defect from healthy behaviour: free() returns memory to the
 // allocator, not to the OS, so a single round's footprint looks the same
-// either way. Growth ACROSS rounds is the discriminating measurement.
+// either way. Growth ACROSS rounds is the discriminating measurement, and it
+// is what this repair flattens.
 //
 // Build and run:
-//   swiftc -swift-version 6 -O thumbcache.swift -o /tmp/thumb_broken
-//   /tmp/thumb_broken
+//   swiftc -swift-version 6 -O thumbcache.swift -o /tmp/thumb_fixed
+//   /tmp/thumb_fixed
 //
-// This file compiles with ZERO warnings under Swift 6 strict concurrency,
-// which is the point: the compiler has nothing to say about object lifetime.
-// Every measurement is printed as key=value so a script can assert on it.
+// The observable behaviour — record count and checksum — is unchanged. Only
+// the lifetime is. Every measurement is printed as key=value.
 
 import Darwin
 
@@ -54,8 +55,8 @@
     let counters: Counters
     var pixels: [UInt8]
 
-    // Stored on self. The closure assigned to it captures self. That edge,
-    // plus this stored property, closes a cycle ARC cannot break.
+    // Stored on self, but the closure assigned to it captures self weakly,
+    // so this property no longer closes a cycle.
     var onComplete: (() -> Void)?
 
     init(index: Int, counters: Counters) {
@@ -71,8 +72,11 @@
     }
 
     func begin() {
-        // The retry handler needs the decoded pixels, so it reaches for self.
-        onComplete = {
+        // The retry handler needs the decoded pixels, so it reaches for self
+        // — but weakly. If the Thumbnailer is gone there is nothing left to
+        // retry, which is exactly the right semantics here.
+        onComplete = { [weak self] in
+            guard let self else { return }
             self.counters.checksum &+= UInt64(self.pixels[0]) &+ UInt64(self.index)
             self.counters.records += 1
         }