Parameterize table magic number

Summary:

As we are having different types of tables and they all might share the same structure in block-based table:

[metaindex block]
[index block]
[Footer]

To be able to identify differnt types of tables, we need to parameterize the "magic number" in the `Footer`.

Test Plan:

make check

table/block_based_table_builder.cc

@@ -77,6 +77,14 @@ void LogPropertiesCollectionError(
 
 }  // anonymous namespace
 
+// kBlockedBasedTableMagicNumber was picked by running
+//    echo http://code.google.com/p/leveldb/ | sha1sum
+// and taking the leading 64 bits.
+// Please note that kBlockedBasedTableMagicNumber may also be accessed by
+// other .cc files so it have to be explicitly declared with "extern".
+extern const uint64_t kBlockedBasedTableMagicNumber
+    = 0xdb4775248b80fb57ull;
+
 struct BlockBasedTableBuilder::Rep {
   Options options;
   WritableFile* file;
@@ -503,7 +511,7 @@ Status BlockBasedTableBuilder::Finish() {
 
   // Write footer
   if (ok()) {
-    Footer footer;
+    Footer footer(kBlockedBasedTableMagicNumber);
     footer.set_metaindex_handle(metaindex_block_handle);
     footer.set_index_handle(index_block_handle);
     std::string footer_encoding;

table/block_based_table_builder.h

@@ -20,7 +20,6 @@ class BlockBuilder;
 class BlockHandle;
 class WritableFile;
 
-
 class BlockBasedTableBuilder : public TableBuilder {
  public:
   // Create a builder that will store the contents of the table it is

table/block_based_table_reader.cc

@@ -29,6 +29,8 @@
 
 namespace rocksdb {
 
+extern uint64_t kBlockedBasedTableMagicNumber;
+
 // The longest the prefix of the cache key used to identify blocks can be.
 // We are using the fact that we know for Posix files the unique ID is three
 // varints.
@@ -242,7 +244,7 @@ Status BlockBasedTable::Open(const Options& options,
     return Status::InvalidArgument("file is too short to be an sstable");
   }
 
-  Footer footer;
+  Footer footer(kBlockedBasedTableMagicNumber);
   s = footer.DecodeFrom(&footer_input);
   if (!s.ok()) return s;
 

table/format.h

@@ -50,7 +50,12 @@ class BlockHandle {
 // end of every table file.
 class Footer {
  public:
-  Footer() { }
+  // @table_magic_number serves two purposes:
+  //  1. Identify different types of the tables.
+  //  2. Help us to identify if a given file is a valid sst.
+  Footer(uint64_t table_magic_number) :
+      kTableMagicNumber(table_magic_number) {
+  }
 
   // The block handle for the metaindex block of the table
   const BlockHandle& metaindex_handle() const { return metaindex_handle_; }
@@ -77,13 +82,9 @@ class Footer {
  private:
   BlockHandle metaindex_handle_;
   BlockHandle index_handle_;
+  const uint64_t kTableMagicNumber;
 };
 
-// kTableMagicNumber was picked by running
-//    echo http://code.google.com/p/leveldb/ | sha1sum
-// and taking the leading 64 bits.
-static const uint64_t kTableMagicNumber = 0xdb4775248b80fb57ull;
-
 // 1-byte type + 32-bit crc
 static const size_t kBlockTrailerSize = 5;