Fix bug where gc was scanning uninit memory

VersionedMap.cpp

@@ -15,6 +15,8 @@
 #include <sys/types.h>
 #include <unistd.h>
 
+#include <memcheck.h>
+
 static_assert(std::is_standard_layout_v<weaselab::VersionedMap::MutationType>);
 static_assert(std::is_standard_layout_v<weaselab::VersionedMap::Key>);
 static_assert(std::is_standard_layout_v<weaselab::VersionedMap::Mutation>);
@@ -245,6 +247,7 @@ struct MemManager {
     if (next == firstUnaddressable) {
       mprotectSafe(base + firstUnaddressable, kUpsizeBytes,
                    PROT_READ | PROT_WRITE);
+      VALGRIND_MAKE_MEM_UNDEFINED(base + firstUnaddressable, kUpsizeBytes);
       firstUnaddressable += kUpsizeNodes;
 #if SHOW_MEMORY
       mmapBytes = getBytes();
@@ -311,49 +314,14 @@ struct MemManager {
       }
     }
 
-    // Reclaim memory on the right side
     uint32_t max = reachable.max();
     if (max == 0) {
       max = kMinAddressable - 1;
     }
+
     assert(max < next);
-    uint32_t newFirstUnaddressable = (max / kNodesPerPage + 1) * kNodesPerPage;
-    if (newFirstUnaddressable < firstUnaddressable) {
-      for (int i = newFirstUnaddressable; i < firstUnaddressable; ++i) {
-        if (base[i].entry != nullptr) {
-#if DEBUG_VERBOSE
-          if (debugVerboseEnabled) {
-            printf("Collecting %u while shrinking right\n", i);
-          }
-#endif
-          base[i].entry->delref();
-        }
-      }
-      mprotectSafe(base + newFirstUnaddressable,
-                   (firstUnaddressable - newFirstUnaddressable) * sizeof(Node),
-                   PROT_NONE);
-      firstUnaddressable = newFirstUnaddressable;
-#if SHOW_MEMORY
-      mmapBytes = getBytes();
-#endif
-    }
-    next = max + 1;
 
-    for (int i = next; i < firstUnaddressable; ++i) {
+    // Rebuild free list to prefer leftward nodes
-      if (base[i].entry != nullptr) {
-#if DEBUG_VERBOSE
-        if (debugVerboseEnabled) {
-          printf(
-              "Collecting %u while collecting right page-unaligned surplus\n",
-              i);
-        }
-#endif
-        base[i].entry->delref();
-        base[i].entry = nullptr;
-      }
-    }
-
-    // Rebuild free list and delref entries
     freeList = 0;
     reachable.iterateAbsentApproxBackwards(
         [&](uint32_t i) {
@@ -369,7 +337,33 @@ struct MemManager {
           base[i].nextFree = freeList;
           freeList = i;
         },
-        kMinAddressable, next);
+        kMinAddressable, max + 1);
+
+    // Entries to the right of max don't need to be in the freelist. They're
+    // allocated by pointer bumping.
+    for (int i = max + 1; i < next; ++i) {
+      if (base[i].entry != nullptr) {
+#if DEBUG_VERBOSE
+        if (debugVerboseEnabled) {
+          printf("Collecting %u while shrinking right\n", i);
+        }
+#endif
+        base[i].entry->delref();
+        base[i].entry = nullptr;
+      }
+    }
+
+    uint32_t newFirstUnaddressable = (max / kNodesPerPage + 1) * kNodesPerPage;
+    if (newFirstUnaddressable < firstUnaddressable) {
+      mprotectSafe(base + newFirstUnaddressable,
+                   (firstUnaddressable - newFirstUnaddressable) * sizeof(Node),
+                   PROT_NONE);
+      firstUnaddressable = newFirstUnaddressable;
+#if SHOW_MEMORY
+      mmapBytes = getBytes();
+#endif
+    }
+    next = max + 1;
   }
 
   int64_t getBytes() const {