Fix visitContiguousRanges() for empty and single chunk containers

Summary:
lastOccupiedChunks has UB with an empty container, because
the end() iterator itemPtr_ is null.

And with a single chunk, we must enter the iteration loop.

Reviewed By: nbronson

Differential Revision: D8897447

fbshipit-source-id: 7e7fd06ec7a588236eebf28f43af9ef4ab2d39cd

folly/container/detail/F14Table.h

@@ -1557,6 +1557,7 @@ class F14Table : public Policy {
   }
 
   ChunkPtr lastOccupiedChunk() const {
+    FOLLY_SAFE_DCHECK(size() > 0, "");
     if (Policy::kEnableItemIteration) {
       return begin().chunk();
     } else {
@@ -2139,9 +2140,12 @@ class F14Table : public Policy {
   // visitor should take an Item const&
   template <typename V>
   void visitItems(V&& visitor) const {
+    if (empty()) {
+      return;
+    }
     std::size_t maxChunkIndex = lastOccupiedChunk() - chunks_;
     auto chunk = &chunks_[0];
-    for (std::size_t i = 0; i < maxChunkIndex; ++i, ++chunk) {
+    for (std::size_t i = 0; i <= maxChunkIndex; ++i, ++chunk) {
       auto iter = chunk->occupiedIter();
       if (Policy::prefetchBeforeCopy()) {
         for (auto piter = iter; piter.hasNext();) {
@@ -2157,9 +2161,12 @@ class F14Table : public Policy {
   // visitor should take two Item const*
   template <typename V>
   void visitContiguousItemRanges(V&& visitor) const {
+    if (empty()) {
+      return;
+    }
     std::size_t maxChunkIndex = lastOccupiedChunk() - chunks_;
     auto chunk = &chunks_[0];
-    for (std::size_t i = 0; i < maxChunkIndex; ++i, ++chunk) {
+    for (std::size_t i = 0; i <= maxChunkIndex; ++i, ++chunk) {
       for (auto iter = chunk->occupiedRangeIter(); iter.hasNext();) {
         auto be = iter.next();
         FOLLY_SAFE_DCHECK(

folly/container/test/F14MapTest.cpp

@@ -107,10 +107,10 @@ TEST(F14Map, getAllocatedMemorySize) {
 }
 
 template <typename M>
-void runVisitContiguousRangesTest() {
+void runVisitContiguousRangesTest(int n) {
   M map;
 
-  for (int i = 0; i < 1000; ++i) {
+  for (int i = 0; i < n; ++i) {
     map[i] = i;
     map.erase(i / 2);
   }
@@ -128,6 +128,18 @@ void runVisitContiguousRangesTest() {
       iter->second = true;
     }
   });
+
+  // ensure no entries were skipped
+  for (auto& e : visited) {
+    EXPECT_TRUE(e.second);
+  }
+}
+
+template <typename M>
+void runVisitContiguousRangesTest() {
+  runVisitContiguousRangesTest<M>(0); // empty
+  runVisitContiguousRangesTest<M>(5); // single chunk
+  runVisitContiguousRangesTest<M>(1000); // many chunks
 }
 
 TEST(F14ValueMap, visitContiguousRanges) {

folly/container/test/F14SetTest.cpp

@@ -130,10 +130,10 @@ TEST(F14Set, getAllocatedMemorySize) {
 }
 
 template <typename S>
-void runVisitContiguousRangesTest() {
+void runVisitContiguousRangesTest(int n) {
   S set;
 
-  for (int i = 0; i < 1000; ++i) {
+  for (int i = 0; i < n; ++i) {
     set.insert(i);
     set.erase(i / 2);
   }
@@ -151,6 +151,18 @@ void runVisitContiguousRangesTest() {
       iter->second = true;
     }
   });
+
+  // ensure no entries were skipped
+  for (auto& e : visited) {
+    EXPECT_TRUE(e.second);
+  }
+}
+
+template <typename S>
+void runVisitContiguousRangesTest() {
+  runVisitContiguousRangesTest<S>(0); // empty
+  runVisitContiguousRangesTest<S>(5); // single chunk
+  runVisitContiguousRangesTest<S>(1000); // many chunks
 }
 
 TEST(F14ValueSet, visitContiguousRanges) {