In all the places where log records are read, there was a check that

record.size() should not be less than 12.

This "magic number" seems to be the WriteBatch header (8 byte sequence
and 4 byte count).   Replaced all the places where "12" was used
by WriteBatchInternal::kHeader.

db/db_impl.cc

@@ -1186,7 +1186,7 @@ Status DBImpl::RecoverLogFiles(const std::vector<uint64_t>& log_numbers,
         continue_replay_log &&
         reader.ReadRecord(&record, &scratch, db_options_.wal_recovery_mode) &&
         status.ok()) {
-      if (record.size() < 12) {
+      if (record.size() < WriteBatchInternal::kHeader) {
         reporter.Corruption(record.size(),
                             Status::Corruption("log record too small"));
         continue;

db/repair.cc

@@ -265,7 +265,7 @@ class Repairer {
     mem->Ref();
     int counter = 0;
     while (reader.ReadRecord(&record, &scratch)) {
-      if (record.size() < 12) {
+      if (record.size() < WriteBatchInternal::kHeader) {
         reporter.Corruption(
             record.size(), Status::Corruption("log record too small"));
         continue;

db/transaction_log_impl.cc

@@ -107,7 +107,7 @@ void TransactionLogIteratorImpl::SeekToStartSequence(
     return;
   }
   while (RestrictedRead(&record, &scratch)) {
-    if (record.size() < 12) {
+    if (record.size() < WriteBatchInternal::kHeader) {
       reporter_.Corruption(
         record.size(), Status::Corruption("very small log record"));
       continue;
@@ -167,7 +167,7 @@ void TransactionLogIteratorImpl::NextImpl(bool internal) {
       currentLogReader_->UnmarkEOF();
     }
     while (RestrictedRead(&record, &scratch)) {
-      if (record.size() < 12) {
+      if (record.size() < WriteBatchInternal::kHeader) {
         reporter_.Corruption(
           record.size(), Status::Corruption("very small log record"));
         continue;

db/wal_manager.cc

@@ -455,7 +455,7 @@ Status WalManager::ReadFirstLine(const std::string& fname,
 
   if (reader.ReadRecord(&record, &scratch) &&
       (status.ok() || !db_options_.paranoid_checks)) {
-    if (record.size() < 12) {
+    if (record.size() < WriteBatchInternal::kHeader) {
       reporter.Corruption(record.size(),
                           Status::Corruption("log record too small"));
       // TODO read record's till the first no corrupt entry?

db/write_batch.cc

@@ -81,8 +81,6 @@ struct BatchContentClassifier : public WriteBatch::Handler {
 
 }  // anon namespace
 
-// WriteBatch header has an 8-byte sequence number followed by a 4-byte count.
-static const size_t kHeader = 12;
 
 struct SavePoint {
   size_t size;  // size of rep_
@@ -96,8 +94,8 @@ struct SavePoints {
 
 WriteBatch::WriteBatch(size_t reserved_bytes)
     : save_points_(nullptr), content_flags_(0), rep_() {
-  rep_.reserve((reserved_bytes > kHeader) ? reserved_bytes : kHeader);
-  rep_.resize(kHeader);
+  rep_.reserve((reserved_bytes > WriteBatchInternal::kHeader) ? reserved_bytes : WriteBatchInternal::kHeader);
+  rep_.resize(WriteBatchInternal::kHeader);
 }
 
 WriteBatch::WriteBatch(const std::string& rep)

db/write_batch_internal.h

@@ -63,6 +63,10 @@ class ColumnFamilyMemTablesDefault : public ColumnFamilyMemTables {
 // WriteBatch that we don't want in the public WriteBatch interface.
 class WriteBatchInternal {
  public:
+
+  // WriteBatch header has an 8-byte sequence number followed by a 4-byte count.
+  static const size_t kHeader = 12;
+
   // WriteBatch methods with column_family_id instead of ColumnFamilyHandle*
   static void Put(WriteBatch* batch, uint32_t column_family_id,
                   const Slice& key, const Slice& value);

tools/ldb_cmd.cc

@@ -1576,7 +1576,7 @@ void DumpWalFile(std::string wal_file, bool print_header, bool print_values,
     }
     while (reader.ReadRecord(&record, &scratch)) {
       row.str("");
-      if (record.size() < 12) {
+      if (record.size() < WriteBatchInternal::kHeader) {
         reporter.Corruption(record.size(),
                             Status::Corruption("log record too small"));
       } else {