Squashed 'src/leveldb/' changes from aca1ffc..ae6c262

ae6c262 Merge branch 'leveldb' into ripple-fork
28fa222 Looks like a bit more delay is needed to smooth the latency.
a18f3e6 Tidy up JobQueue, add ripple_core module
ab82e57 Release leveldb 1.12
02c6259 Release leveldb 1.11
5bbb544 Rate limit compactions with a 25ms pause after each complete file.
8c29c47 LevelDB issue 178 fix: cannot resize a level 0 compaction set
18b245c Added GNU/kFreeBSD kernel name (TARGET_OS)
8be9d12 CondVar::SignalAll was broken, leading to deadlocks on Windows builds. http://code.google.com/p/leveldb/issues/detail?id=149
c9fc070 Upgrade LevelDB to 1.10.0, mostly for better write stall logging.
8215b15 Tweak to variable name to support unity build

git-subtree-dir: src/leveldb
git-subtree-split: ae6c2620b2ef3d5c69e63dc0eda865d6a39fa061

AUTHORS

@@ -6,3 +6,6 @@ Google Inc.
 # Initial version authors:
 Jeffrey Dean <jeff@google.com>
 Sanjay Ghemawat <sanjay@google.com>
+
+# Partial list of contributors:
+Kevin Regan <kevin.d.regan@gmail.com>

Makefile

@@ -42,6 +42,7 @@ TESTS = \
 	env_test \
 	filename_test \
 	filter_block_test \
+	issue178_test \
 	log_test \
 	memenv_test \
 	skiplist_test \
@@ -69,7 +70,7 @@ SHARED = $(SHARED1)
 else
 # Update db.h if you change these.
 SHARED_MAJOR = 1
-SHARED_MINOR = 9
+SHARED_MINOR = 12
 SHARED1 = libleveldb.$(PLATFORM_SHARED_EXT)
 SHARED2 = $(SHARED1).$(SHARED_MAJOR)
 SHARED3 = $(SHARED1).$(SHARED_MAJOR).$(SHARED_MINOR)
@@ -146,6 +147,9 @@ filename_test: db/filename_test.o $(LIBOBJECTS) $(TESTHARNESS)
 filter_block_test: table/filter_block_test.o $(LIBOBJECTS) $(TESTHARNESS)
 	$(CXX) $(LDFLAGS) table/filter_block_test.o $(LIBOBJECTS) $(TESTHARNESS) -o $@ $(LIBS)
 
+issue178_test: issues/issue178_test.o $(LIBOBJECTS) $(TESTHARNESS)
+	$(CXX) $(LDFLAGS) issues/issue178_test.o $(LIBOBJECTS) $(TESTHARNESS) -o $@ $(LIBS)
+
 log_test: db/log_test.o $(LIBOBJECTS) $(TESTHARNESS)
 	$(CXX) $(LDFLAGS) db/log_test.o $(LIBOBJECTS) $(TESTHARNESS) -o $@ $(LIBS)
 

build_detect_platform

@@ -94,6 +94,12 @@ case "$TARGET_OS" in
         PLATFORM_LIBS="-lpthread"
         PORT_FILE=port/port_posix.cc
         ;;
+    GNU/kFreeBSD)
+        PLATFORM=OS_KFREEBSD
+        COMMON_FLAGS="$MEMCMP_FLAG -D_REENTRANT -DOS_KFREEBSD"
+        PLATFORM_LIBS="-lpthread"
+        PORT_FILE=port/port_posix.cc
+        ;;
     NetBSD)
         PLATFORM=OS_NETBSD
         COMMON_FLAGS="$MEMCMP_FLAG -D_REENTRANT -DOS_NETBSD"

db/db_impl.cc

@@ -35,6 +35,8 @@
 
 namespace leveldb {
 
+const int kNumNonTableCacheFiles = 10;
+
 // Information kept for every waiting writer
 struct DBImpl::Writer {
   Status status;
@@ -92,9 +94,9 @@ Options SanitizeOptions(const std::string& dbname,
   Options result = src;
   result.comparator = icmp;
   result.filter_policy = (src.filter_policy != NULL) ? ipolicy : NULL;
-  ClipToRange(&result.max_open_files,            20,     50000);
+  ClipToRange(&result.max_open_files,    64 + kNumNonTableCacheFiles, 50000);
-  ClipToRange(&result.write_buffer_size,         64<<10, 1<<30);
+  ClipToRange(&result.write_buffer_size, 64<<10,                      1<<30);
-  ClipToRange(&result.block_size,                1<<10,  4<<20);
+  ClipToRange(&result.block_size,        1<<10,                       4<<20);
   if (result.info_log == NULL) {
     // Open a log file in the same directory as the db
     src.env->CreateDir(dbname);  // In case it does not exist
@@ -130,12 +132,13 @@ DBImpl::DBImpl(const Options& options, const std::string& dbname)
       log_(NULL),
       tmp_batch_(new WriteBatch),
       bg_compaction_scheduled_(false),
-      manual_compaction_(NULL) {
+      manual_compaction_(NULL),
+      consecutive_compaction_errors_(0) {
   mem_->Ref();
   has_imm_.Release_Store(NULL);
 
   // Reserve ten files or so for other uses and give the rest to TableCache.
-  const int table_cache_size = options.max_open_files - 10;
+  const int table_cache_size = options.max_open_files - kNumNonTableCacheFiles;
   table_cache_ = new TableCache(dbname_, &options_, table_cache_size);
 
   versions_ = new VersionSet(dbname_, &options_, table_cache_,
@@ -310,16 +313,24 @@ Status DBImpl::Recover(VersionEdit* edit) {
     if (!s.ok()) {
       return s;
     }
+    std::set<uint64_t> expected;
+    versions_->AddLiveFiles(&expected);
     uint64_t number;
     FileType type;
     std::vector<uint64_t> logs;
     for (size_t i = 0; i < filenames.size(); i++) {
-      if (ParseFileName(filenames[i], &number, &type)
+      if (ParseFileName(filenames[i], &number, &type)) {
-          && type == kLogFile
+        expected.erase(number);
-          && ((number >= min_log) || (number == prev_log))) {
+        if (type == kLogFile && ((number >= min_log) || (number == prev_log)))
         logs.push_back(number);
       }
     }
+    if (!expected.empty()) {
+      char buf[50];
+      snprintf(buf, sizeof(buf), "%d missing files; e.g.",
+               static_cast<int>(expected.size()));
+      return Status::Corruption(buf, TableFileName(dbname_, *(expected.begin())));
+    }
 
     // Recover in the order in which the logs were generated
     std::sort(logs.begin(), logs.end());
@@ -611,6 +622,7 @@ void DBImpl::BackgroundCall() {
     Status s = BackgroundCompaction();
     if (s.ok()) {
       // Success
+      consecutive_compaction_errors_ = 0;
     } else if (shutting_down_.Acquire_Load()) {
       // Error most likely due to shutdown; do not wait
     } else {
@@ -622,7 +634,12 @@ void DBImpl::BackgroundCall() {
       Log(options_.info_log, "Waiting after background compaction error: %s",
           s.ToString().c_str());
       mutex_.Unlock();
-      env_->SleepForMicroseconds(1000000);
+      ++consecutive_compaction_errors_;
+      int seconds_to_sleep = 1;
+      for (int i = 0; i < 3 && i < consecutive_compaction_errors_ - 1; ++i) {
+        seconds_to_sleep *= 2;
+      }
+      env_->SleepForMicroseconds(seconds_to_sleep * 1000000);
       mutex_.Lock();
     }
   }
@@ -805,6 +822,9 @@ Status DBImpl::FinishCompactionOutputFile(CompactionState* compact,
           (unsigned long long) output_number,
           (unsigned long long) current_entries,
           (unsigned long long) current_bytes);
+
+      // rate-limit compaction file creation with a 100ms pause
+      env_->SleepForMicroseconds(100000);
     }
   }
   return s;
@@ -1268,10 +1288,11 @@ Status DBImpl::MakeRoomForWrite(bool force) {
     } else if (imm_ != NULL) {
       // We have filled up the current memtable, but the previous
       // one is still being compacted, so we wait.
+      Log(options_.info_log, "Current memtable full; waiting...\n");
       bg_cv_.Wait();
     } else if (versions_->NumLevelFiles(0) >= config::kL0_StopWritesTrigger) {
       // There are too many level-0 files.
-      Log(options_.info_log, "waiting...\n");
+      Log(options_.info_log, "Too many L0 files; waiting...\n");
       bg_cv_.Wait();
     } else {
       // Attempt to switch to a new memtable and trigger compaction of old

db/db_impl.h

@@ -163,6 +163,7 @@ class DBImpl : public DB {
 
   // Have we encountered a background error in paranoid mode?
   Status bg_error_;
+  int consecutive_compaction_errors_;
 
   // Per level compaction stats.  stats_[level] stores the stats for
   // compactions that produced data for the specified "level".

db/db_test.cc

@@ -33,8 +33,11 @@ class AtomicCounter {
  public:
   AtomicCounter() : count_(0) { }
   void Increment() {
+    IncrementBy(1);
+  }
+  void IncrementBy(int count) {
     MutexLock l(&mu_);
-    count_++;
+    count_ += count;
   }
   int Read() {
     MutexLock l(&mu_);
@@ -45,6 +48,10 @@ class AtomicCounter {
     count_ = 0;
   }
 };
+
+void DelayMilliseconds(int millis) {
+  Env::Default()->SleepForMicroseconds(millis * 1000);
+}
 }
 
 // Special Env used to delay background operations
@@ -69,6 +76,7 @@ class SpecialEnv : public EnvWrapper {
   AtomicCounter random_read_counter_;
 
   AtomicCounter sleep_counter_;
+  AtomicCounter sleep_time_counter_;
 
   explicit SpecialEnv(Env* base) : EnvWrapper(base) {
     delay_sstable_sync_.Release_Store(NULL);
@@ -103,7 +111,7 @@ class SpecialEnv : public EnvWrapper {
       Status Flush() { return base_->Flush(); }
       Status Sync() {
         while (env_->delay_sstable_sync_.Acquire_Load() != NULL) {
-          env_->SleepForMicroseconds(100000);
+          DelayMilliseconds(100);
         }
         return base_->Sync();
       }
@@ -174,8 +182,9 @@ class SpecialEnv : public EnvWrapper {
 
   virtual void SleepForMicroseconds(int micros) {
     sleep_counter_.Increment();
-    target()->SleepForMicroseconds(micros);
+    sleep_time_counter_.IncrementBy(micros);
   }
+
 };
 
 class DBTest {
@@ -461,6 +470,20 @@ class DBTest {
     }
     return result;
   }
+
+  bool DeleteAnSSTFile() {
+    std::vector<std::string> filenames;
+    ASSERT_OK(env_->GetChildren(dbname_, &filenames));
+    uint64_t number;
+    FileType type;
+    for (size_t i = 0; i < filenames.size(); i++) {
+      if (ParseFileName(filenames[i], &number, &type) && type == kTableFile) {
+        ASSERT_OK(env_->DeleteFile(TableFileName(dbname_, number)));
+        return true;
+      }
+    }
+    return false;
+  }
 };
 
 TEST(DBTest, Empty) {
@@ -611,7 +634,7 @@ TEST(DBTest, GetEncountersEmptyLevel) {
     }
 
     // Step 4: Wait for compaction to finish
-    env_->SleepForMicroseconds(1000000);
+    DelayMilliseconds(1000);
 
     ASSERT_EQ(NumTableFilesAtLevel(0), 0);
   } while (ChangeOptions());
@@ -1295,7 +1318,7 @@ TEST(DBTest, L0_CompactionBug_Issue44_a) {
   Reopen();
   Reopen();
   ASSERT_EQ("(a->v)", Contents());
-  env_->SleepForMicroseconds(1000000);  // Wait for compaction to finish
+  DelayMilliseconds(1000);  // Wait for compaction to finish
   ASSERT_EQ("(a->v)", Contents());
 }
 
@@ -1311,7 +1334,7 @@ TEST(DBTest, L0_CompactionBug_Issue44_b) {
   Put("","");
   Reopen();
   Put("","");
-  env_->SleepForMicroseconds(1000000);  // Wait for compaction to finish
+  DelayMilliseconds(1000);  // Wait for compaction to finish
   Reopen();
   Put("d","dv");
   Reopen();
@@ -1321,7 +1344,7 @@ TEST(DBTest, L0_CompactionBug_Issue44_b) {
   Delete("b");
   Reopen();
   ASSERT_EQ("(->)(c->cv)", Contents());
-  env_->SleepForMicroseconds(1000000);  // Wait for compaction to finish
+  DelayMilliseconds(1000);  // Wait for compaction to finish
   ASSERT_EQ("(->)(c->cv)", Contents());
 }
 
@@ -1506,6 +1529,30 @@ TEST(DBTest, NoSpace) {
   ASSERT_GE(env_->sleep_counter_.Read(), 5);
 }
 
+TEST(DBTest, ExponentialBackoff) {
+  Options options = CurrentOptions();
+  options.env = env_;
+  Reopen(&options);
+
+  ASSERT_OK(Put("foo", "v1"));
+  ASSERT_EQ("v1", Get("foo"));
+  Compact("a", "z");
+  env_->non_writable_.Release_Store(env_);  // Force errors for new files
+  env_->sleep_counter_.Reset();
+  env_->sleep_time_counter_.Reset();
+  for (int i = 0; i < 5; i++) {
+    dbfull()->TEST_CompactRange(2, NULL, NULL);
+  }
+  env_->non_writable_.Release_Store(NULL);
+
+  // Wait for compaction to finish
+  DelayMilliseconds(1000);
+
+  ASSERT_GE(env_->sleep_counter_.Read(), 5);
+  ASSERT_LT(env_->sleep_counter_.Read(), 10);
+  ASSERT_GE(env_->sleep_time_counter_.Read(), 10e6);
+}
+
 TEST(DBTest, NonWritableFileSystem) {
   Options options = CurrentOptions();
   options.write_buffer_size = 1000;
@@ -1519,7 +1566,7 @@ TEST(DBTest, NonWritableFileSystem) {
     fprintf(stderr, "iter %d; errors %d\n", i, errors);
     if (!Put("foo", big).ok()) {
       errors++;
-      env_->SleepForMicroseconds(100000);
+      DelayMilliseconds(100);
     }
   }
   ASSERT_GT(errors, 0);
@@ -1567,6 +1614,24 @@ TEST(DBTest, ManifestWriteError) {
   }
 }
 
+TEST(DBTest, MissingSSTFile) {
+  ASSERT_OK(Put("foo", "bar"));
+  ASSERT_EQ("bar", Get("foo"));
+
+  // Dump the memtable to disk.
+  dbfull()->TEST_CompactMemTable();
+  ASSERT_EQ("bar", Get("foo"));
+
+  Close();
+  ASSERT_TRUE(DeleteAnSSTFile());
+  Options options = CurrentOptions();
+  options.paranoid_checks = true;
+  Status s = TryReopen(&options);
+  ASSERT_TRUE(!s.ok());
+  ASSERT_TRUE(s.ToString().find("issing") != std::string::npos)
+      << s.ToString();
+}
+
 TEST(DBTest, FilesDeletedAfterCompaction) {
   ASSERT_OK(Put("foo", "v2"));
   Compact("a", "z");
@@ -1711,13 +1776,13 @@ TEST(DBTest, MultiThreaded) {
     }
 
     // Let them run for a while
-    env_->SleepForMicroseconds(kTestSeconds * 1000000);
+    DelayMilliseconds(kTestSeconds * 1000);
 
     // Stop the threads and wait for them to finish
     mt.stop.Release_Store(&mt);
     for (int id = 0; id < kNumThreads; id++) {
       while (mt.thread_done[id].Acquire_Load() == NULL) {
-        env_->SleepForMicroseconds(100000);
+        DelayMilliseconds(100);
       }
     }
   } while (ChangeOptions());

db/dbformat.cc

@@ -26,7 +26,7 @@ std::string ParsedInternalKey::DebugString() const {
            (unsigned long long) sequence,
            int(type));
   std::string result = "'";
-  result += user_key.ToString();
+  result += EscapeString(user_key.ToString());
   result += buf;
   return result;
 }

db/filename_test.cc

@@ -70,7 +70,7 @@ TEST(FileNameTest, Parse) {
   for (int i = 0; i < sizeof(errors) / sizeof(errors[0]); i++) {
     std::string f = errors[i];
     ASSERT_TRUE(!ParseFileName(f, &number, &type)) << f;
-  };
+  }
 }
 
 TEST(FileNameTest, Construction) {

db/version_set.cc

@@ -1331,14 +1331,19 @@ Compaction* VersionSet::CompactRange(
   }
 
   // Avoid compacting too much in one shot in case the range is large.
-  const uint64_t limit = MaxFileSizeForLevel(level);
+  // But we cannot do this for level-0 since level-0 files can overlap
-  uint64_t total = 0;
+  // and we must not pick one file and drop another older file if the
-  for (size_t i = 0; i < inputs.size(); i++) {
+  // two files overlap.
-    uint64_t s = inputs[i]->file_size;
+  if (level > 0) {
-    total += s;
+    const uint64_t limit = MaxFileSizeForLevel(level);
-    if (total >= limit) {
+    uint64_t total = 0;
-      inputs.resize(i + 1);
+    for (size_t i = 0; i < inputs.size(); i++) {
-      break;
+      uint64_t s = inputs[i]->file_size;
+      total += s;
+      if (total >= limit) {
+        inputs.resize(i + 1);
+        break;
+      }
     }
   }
 

include/leveldb/db.h

@@ -14,7 +14,7 @@ namespace leveldb {
 
 // Update Makefile if you change these
 static const int kMajorVersion = 1;
-static const int kMinorVersion = 9;
+static const int kMinorVersion = 12;
 
 struct Options;
 struct ReadOptions;

port/port_win.cc

@@ -109,12 +109,10 @@ void CondVar::Signal() {
 
 void CondVar::SignalAll() {
   wait_mtx_.Lock();
-  for(long i = 0; i < waiting_; ++i) {
+  ::ReleaseSemaphore(sem1_, waiting_, NULL);
-    ::ReleaseSemaphore(sem1_, 1, NULL);
+  while(waiting_ > 0) {
-    while(waiting_ > 0) {
+    --waiting_;
-      --waiting_;
+    ::WaitForSingleObject(sem2_, INFINITE);
-      ::WaitForSingleObject(sem2_, INFINITE);
-    }
   }
   wait_mtx_.Unlock();
 }

table/block.cc

@@ -16,7 +16,7 @@
 namespace leveldb {
 
 inline uint32_t Block::NumRestarts() const {
-  assert(size_ >= 2*sizeof(uint32_t));
+  assert(size_ >= sizeof(uint32_t));
   return DecodeFixed32(data_ + size_ - sizeof(uint32_t));
 }
 
@@ -27,11 +27,12 @@ Block::Block(const BlockContents& contents)
   if (size_ < sizeof(uint32_t)) {
     size_ = 0;  // Error marker
   } else {
-    restart_offset_ = size_ - (1 + NumRestarts()) * sizeof(uint32_t);
+    size_t max_restarts_allowed = (size_-sizeof(uint32_t)) / sizeof(uint32_t);
-    if (restart_offset_ > size_ - sizeof(uint32_t)) {
+    if (NumRestarts() > max_restarts_allowed) {
-      // The size is too small for NumRestarts() and therefore
+      // The size is too small for NumRestarts()
-      // restart_offset_ wrapped around.
       size_ = 0;
+    } else {
+      restart_offset_ = size_ - (1 + NumRestarts()) * sizeof(uint32_t);
     }
   }
 }
@@ -253,7 +254,7 @@ class Block::Iter : public Iterator {
 };
 
 Iterator* Block::NewIterator(const Comparator* cmp) {
-  if (size_ < 2*sizeof(uint32_t)) {
+  if (size_ < sizeof(uint32_t)) {
     return NewErrorIterator(Status::Corruption("bad block contents"));
   }
   const uint32_t num_restarts = NumRestarts();

table/table.cc

@@ -228,7 +228,6 @@ Status Table::InternalGet(const ReadOptions& options, const Slice& k,
         !filter->KeyMayMatch(handle.offset(), k)) {
       // Not found
     } else {
-      Slice handle = iiter->value();
       Iterator* block_iter = BlockReader(this, options, iiter->value());
       block_iter->Seek(k);
       if (block_iter->Valid()) {

table/table_test.cc

@@ -644,6 +644,36 @@ class Harness {
   Constructor* constructor_;
 };
 
+// Test empty table/block.
+TEST(Harness, Empty) {
+  for (int i = 0; i < kNumTestArgs; i++) {
+    Init(kTestArgList[i]);
+    Random rnd(test::RandomSeed() + 1);
+    Test(&rnd);
+  }
+}
+
+// Special test for a block with no restart entries.  The C++ leveldb
+// code never generates such blocks, but the Java version of leveldb
+// seems to.
+TEST(Harness, ZeroRestartPointsInBlock) {
+  char data[sizeof(uint32_t)];
+  memset(data, 0, sizeof(data));
+  BlockContents contents;
+  contents.data = Slice(data, sizeof(data));
+  contents.cachable = false;
+  contents.heap_allocated = false;
+  Block block(contents);
+  Iterator* iter = block.NewIterator(BytewiseComparator());
+  iter->SeekToFirst();
+  ASSERT_TRUE(!iter->Valid());
+  iter->SeekToLast();
+  ASSERT_TRUE(!iter->Valid());
+  iter->Seek("foo");
+  ASSERT_TRUE(!iter->Valid());
+  delete iter;
+}
+
 // Test the empty key
 TEST(Harness, SimpleEmptyKey) {
   for (int i = 0; i < kNumTestArgs; i++) {

util/cache.cc

@@ -116,7 +116,6 @@ class HandleTable {
       LRUHandle* h = list_[i];
       while (h != NULL) {
         LRUHandle* next = h->next_hash;
-        Slice key = h->key();
         uint32_t hash = h->hash;
         LRUHandle** ptr = &new_list[hash & (new_length - 1)];
         h->next_hash = *ptr;
@@ -160,7 +159,6 @@ class LRUCache {
   // mutex_ protects the following state.
   port::Mutex mutex_;
   size_t usage_;
-  uint64_t last_id_;
 
   // Dummy head of LRU list.
   // lru.prev is newest entry, lru.next is oldest entry.
@@ -170,8 +168,7 @@ class LRUCache {
 };
 
 LRUCache::LRUCache()
-    : usage_(0),
+    : usage_(0) {
-      last_id_(0) {
   // Make empty circular linked list
   lru_.next = &lru_;
   lru_.prev = &lru_;

util/coding_test.cc

@@ -109,7 +109,7 @@ TEST(Coding, Varint64) {
     values.push_back(power);
     values.push_back(power-1);
     values.push_back(power+1);
-  };
+  }
 
   std::string s;
   for (int i = 0; i < values.size(); i++) {

util/comparator.cc

@@ -66,7 +66,7 @@ class BytewiseComparatorImpl : public Comparator {
 };
 }  // namespace
 
-static port::OnceType once = LEVELDB_ONCE_INIT;
+static port::OnceType once_comparator = LEVELDB_ONCE_INIT;
 static const Comparator* bytewise;
 
 static void InitModule() {
@@ -74,7 +74,7 @@ static void InitModule() {
 }
 
 const Comparator* BytewiseComparator() {
-  port::InitOnce(&once, InitModule);
+  port::InitOnce(&once_comparator, InitModule);
   return bytewise;
 }
 

util/env_posix.cc

@@ -386,7 +386,7 @@ class PosixEnv : public Env {
   PosixEnv();
   virtual ~PosixEnv() {
     fprintf(stderr, "Destroying Env::Default()\n");
-    exit(1);
+    abort();
   }
 
   virtual Status NewSequentialFile(const std::string& fname,
@@ -467,7 +467,7 @@ class PosixEnv : public Env {
       result = IOError(fname, errno);
     }
     return result;
-  };
+  }
 
   virtual Status CreateDir(const std::string& name) {
     Status result;
@@ -475,7 +475,7 @@ class PosixEnv : public Env {
       result = IOError(name, errno);
     }
     return result;
-  };
+  }
 
   virtual Status DeleteDir(const std::string& name) {
     Status result;
@@ -483,7 +483,7 @@ class PosixEnv : public Env {
       result = IOError(name, errno);
     }
     return result;
-  };
+  }
 
   virtual Status GetFileSize(const std::string& fname, uint64_t* size) {
     Status s;
@@ -589,7 +589,7 @@ class PosixEnv : public Env {
   void PthreadCall(const char* label, int result) {
     if (result != 0) {
       fprintf(stderr, "pthread %s: %s\n", label, strerror(result));
-      exit(1);
+      abort();
     }
   }
 

util/hash.cc

@@ -6,6 +6,13 @@
 #include "util/coding.h"
 #include "util/hash.h"
 
+// The FALLTHROUGH_INTENDED macro can be used to annotate implicit fall-through
+// between switch labels. The real definition should be provided externally.
+// This one is a fallback version for unsupported compilers.
+#ifndef FALLTHROUGH_INTENDED
+#define FALLTHROUGH_INTENDED do { } while (0)
+#endif
+
 namespace leveldb {
 
 uint32_t Hash(const char* data, size_t n, uint32_t seed) {
@@ -28,10 +35,10 @@ uint32_t Hash(const char* data, size_t n, uint32_t seed) {
   switch (limit - data) {
     case 3:
       h += data[2] << 16;
-      // fall through
+      FALLTHROUGH_INTENDED;
     case 2:
       h += data[1] << 8;
-      // fall through
+      FALLTHROUGH_INTENDED;
     case 1:
       h += data[0];
       h *= m;