Generalize footer reading from file

Summary:

Generalizing this process will help us to re-use the code for plain table

Test Plan:

ran ./table_test

table/block_based_table_builder.cc

@@ -77,12 +77,12 @@ void LogPropertiesCollectionError(
 
 }  // anonymous namespace
 
-// kBlockedBasedTableMagicNumber was picked by running
+// kBlockBasedTableMagicNumber 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
+// Please note that kBlockBasedTableMagicNumber may also be accessed by
 // other .cc files so it have to be explicitly declared with "extern".
-extern const uint64_t kBlockedBasedTableMagicNumber
+extern const uint64_t kBlockBasedTableMagicNumber
     = 0xdb4775248b80fb57ull;
 
 struct BlockBasedTableBuilder::Rep {
@@ -511,7 +511,7 @@ Status BlockBasedTableBuilder::Finish() {
 
   // Write footer
   if (ok()) {
-    Footer footer(kBlockedBasedTableMagicNumber);
+    Footer footer(kBlockBasedTableMagicNumber);
     footer.set_metaindex_handle(metaindex_block_handle);
     footer.set_index_handle(index_block_handle);
     std::string footer_encoding;

table/block_based_table_reader.cc

@@ -29,7 +29,7 @@
 
 namespace rocksdb {
 
-extern uint64_t kBlockedBasedTableMagicNumber;
+extern uint64_t kBlockBasedTableMagicNumber;
 
 // 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
@@ -228,24 +228,9 @@ Status BlockBasedTable::Open(const Options& options,
                              uint64_t size,
                              unique_ptr<TableReader>* table_reader) {
   table_reader->reset();
-  if (size < Footer::kEncodedLength) {
-    return Status::InvalidArgument("file is too short to be an sstable");
-  }
-
-  char footer_space[Footer::kEncodedLength];
-  Slice footer_input;
-  Status s = file->Read(size - Footer::kEncodedLength, Footer::kEncodedLength,
-                        &footer_input, footer_space);
-  if (!s.ok()) return s;
-
-  // Check that we actually read the whole footer from the file. It may be
-  // that size isn't correct.
-  if (footer_input.size() != Footer::kEncodedLength) {
-    return Status::InvalidArgument("file is too short to be an sstable");
-  }
 
-  Footer footer(kBlockedBasedTableMagicNumber);
-  s = footer.DecodeFrom(&footer_input);
+  Footer footer(kBlockBasedTableMagicNumber);
+  auto s = ReadFooterFromFile(file.get(), size, &footer);
   if (!s.ok()) return s;
 
   // We've successfully read the footer and the index block: we're

table/format.cc

@@ -73,6 +73,30 @@ Status Footer::DecodeFrom(Slice* input) {
   return result;
 }
 
+Status ReadFooterFromFile(RandomAccessFile* file,
+                          uint64_t file_size,
+                          Footer* footer) {
+  if (file_size < Footer::kEncodedLength) {
+    return Status::InvalidArgument("file is too short to be an sstable");
+  }
+
+  char footer_space[Footer::kEncodedLength];
+  Slice footer_input;
+  Status s = file->Read(file_size - Footer::kEncodedLength,
+                        Footer::kEncodedLength,
+                        &footer_input,
+                        footer_space);
+  if (!s.ok()) return s;
+
+  // Check that we actually read the whole footer from the file. It may be
+  // that size isn't correct.
+  if (footer_input.size() != Footer::kEncodedLength) {
+    return Status::InvalidArgument("file is too short to be an sstable");
+  }
+
+  return footer->DecodeFrom(&footer_input);
+}
+
 Status ReadBlockContents(RandomAccessFile* file,
                          const ReadOptions& options,
                          const BlockHandle& handle,

table/format.h

@@ -98,6 +98,11 @@ class Footer {
   const uint64_t kTableMagicNumber;
 };
 
+// Read the footer from file
+Status ReadFooterFromFile(RandomAccessFile* file,
+                          uint64_t file_size,
+                          Footer* footer);
+
 // 1-byte type + 32-bit crc
 static const size_t kBlockTrailerSize = 5;