Fix stack overflow when destroying long Journal

Summary:
While destroying JournalDelta entries, unzip the chain as we go and
destroy each entry one by one. This prevents the stack from
overflowing on long JournalDelta chains and should fix some flaky
tests.

Reviewed By: wez

Differential Revision: D9355365

fbshipit-source-id: 31af124d318ca5d7a84314b707e1b3c71b2ccaa9

eden/fs/journal/JournalDelta.cpp

@@ -50,6 +50,17 @@ JournalDelta::JournalDelta(
     : changedFilesInOverlay{{oldName.copy(), PathChangeInfo{true, false}},
                             {newName.copy(), PathChangeInfo{true, true}}} {}
 
+JournalDelta::~JournalDelta() {
+  // O(1) stack space destruction of the delta chain.
+  JournalDeltaPtr p{std::move(previous)};
+  while (p && p.unique()) {
+    // We know we have the only reference to p, so cast away constness because
+    // we need to unset p->previous.
+    JournalDelta* q = const_cast<JournalDelta*>(p.get());
+    p = std::move(q->previous);
+  }
+}
+
 std::unique_ptr<JournalDelta> JournalDelta::merge(
     SequenceNumber limitSequence,
     bool pruneAfterLimit) const {

eden/fs/journal/JournalDelta.h

@@ -72,6 +72,8 @@ class JournalDelta {
    */
   JournalDelta(RelativePathPiece oldName, RelativePathPiece newName, Replaced);
 
+  ~JournalDelta();
+
   /** the prior delta and its chain */
   JournalDeltaPtr previous;
   /** The current sequence range.

eden/fs/journal/test/JournalTest.cpp

@@ -155,3 +155,19 @@ TEST(Journal, mergeRemoveCreateUpdate) {
       true,
       merged->changedFilesInOverlay[RelativePath{"test.txt"}].existedAfter);
 }
+
+TEST(Journal, destruction_does_not_overflow_stack_on_long_chain) {
+  Journal journal;
+  size_t N =
+#ifdef NDEBUG
+      200000 // Passes in under 200ms.
+#else
+      40000 // Passes in under 400ms.
+#endif
+      ;
+  for (size_t i = 0; i < N; ++i) {
+    auto delta = std::make_unique<JournalDelta>(
+        "foo/bar"_relpath, JournalDelta::CHANGED);
+    journal.addDelta(std::move(delta));
+  }
+}