Make an API to get database identity from the IDENTITY file

Summary: This would enable rocksdb users to get the db identity without depending on implementation details(storing that in IDENTITY file)

Test Plan: db/db_test (has identity checks)

Reviewers: dhruba, haobo, igor, kailiu

Reviewed By: dhruba

CC: leveldb

Differential Revision: https://reviews.facebook.net/D14463

Author: emayanke

db/db_impl.cc

@@ -3497,6 +3497,33 @@ void DBImpl::GetLiveFilesMetaData(std::vector<LiveFileMetaData> *metadata) {
   return versions_->GetLiveFilesMetaData(metadata);
 }
 
+Status DBImpl::GetDbIdentity(std::string& identity) {
+  std::string idfilename = IdentityFileName(dbname_);
+  unique_ptr<SequentialFile> idfile;
+  const EnvOptions soptions;
+  Status s = env_->NewSequentialFile(idfilename, &idfile, soptions);
+  if (!s.ok()) {
+    return s;
+  }
+  uint64_t file_size;
+  s = env_->GetFileSize(idfilename, &file_size);
+  if (!s.ok()) {
+    return s;
+  }
+  char buffer[file_size];
+  Slice id;
+  s = idfile->Read(file_size, &id, buffer);
+  if (!s.ok()) {
+    return s;
+  }
+  identity.assign(id.ToString());
+  // If last character is '\n' remove it from identity
+  if (identity.size() > 0 && identity.back() == '\n') {
+    identity.pop_back();
+  }
+  return s;
+}
+
 // Default implementations of convenience methods that subclasses of DB
 // can call if they wish
 Status DB::Put(const WriteOptions& opt, const Slice& key, const Slice& value) {

db/db_impl.h

@@ -85,6 +85,8 @@ class DBImpl : public DB {
   virtual void GetLiveFilesMetaData(
     std::vector<LiveFileMetaData> *metadata);
 
+  virtual Status GetDbIdentity(std::string& identity);
+
   // Extra methods (for testing) that are not in the public DB interface
 
   // Compact any files in the named level that overlap [*begin, *end]

db/db_test.cc

@@ -1731,31 +1731,23 @@ TEST(DBTest, ManifestRollOver) {
 
 TEST(DBTest, IdentityAcrossRestarts) {
   do {
-    std::string idfilename = IdentityFileName(dbname_);
-    unique_ptr<SequentialFile> idfile;
-    const EnvOptions soptions;
-    ASSERT_OK(env_->NewSequentialFile(idfilename, &idfile, soptions));
-    char buffer1[100];
-    Slice id1;
-    ASSERT_OK(idfile->Read(100, &id1, buffer1));
+    std::string id1;
+    ASSERT_OK(db_->GetDbIdentity(id1));
 
     Options options = CurrentOptions();
     Reopen(&options);
-    char buffer2[100];
-    Slice id2;
-    ASSERT_OK(env_->NewSequentialFile(idfilename, &idfile, soptions));
-    ASSERT_OK(idfile->Read(100, &id2, buffer2));
+    std::string id2;
+    ASSERT_OK(db_->GetDbIdentity(id2));
     // id1 should match id2 because identity was not regenerated
-    ASSERT_EQ(id1.ToString(), id2.ToString());
+    ASSERT_EQ(id1.compare(id2), 0);
 
+    std::string idfilename = IdentityFileName(dbname_);
     ASSERT_OK(env_->DeleteFile(idfilename));
     Reopen(&options);
-    char buffer3[100];
-    Slice id3;
-    ASSERT_OK(env_->NewSequentialFile(idfilename, &idfile, soptions));
-    ASSERT_OK(idfile->Read(100, &id3, buffer3));
-    // id1 should NOT match id2 because identity was regenerated
-    ASSERT_NE(id1.ToString(0), id3.ToString());
+    std::string id3;
+    ASSERT_OK(db_->GetDbIdentity(id3));
+    // id1 should NOT match id3 because identity was regenerated
+    ASSERT_NE(id1.compare(id3), 0);
   } while (ChangeCompactOptions());
 }
 

include/rocksdb/db.h

@@ -273,7 +273,7 @@ class DB {
   // Sets iter to an iterator that is positioned at a write-batch containing
   // seq_number. If the sequence number is non existent, it returns an iterator
   // at the first available seq_no after the requested seq_no
-  // Returns Status::Ok if iterator is valid
+  // Returns Status::OK if iterator is valid
   // Must set WAL_ttl_seconds or WAL_size_limit_MB to large values to
   // use this api, else the WAL files will get
   // cleared aggressively and the iterator might keep getting invalid before
@@ -292,6 +292,14 @@ class DB {
     std::vector<LiveFileMetaData> *metadata) {
   }
 
+  // Sets the globally unique ID created at database creation time by invoking
+  // Env::GenerateUniqueId(), in identity. Returns Status::OK if identity could
+  // be set properly
+  virtual Status GetDbIdentity(std::string& identity) {
+    identity.clear();
+    return Status::OK();
+  }
+
  private:
   // No copying allowed
   DB(const DB&);