Built-in support for generating unique IDs, bug fix (#8708)

Summary:
Env::GenerateUniqueId() works fine on Windows and on POSIX
where /proc/sys/kernel/random/uuid exists. Our other implementation is
flawed and easily produces collision in a new multi-threaded test.
As we rely more heavily on DB session ID uniqueness, this becomes a
serious issue.

This change combines several individually suitable entropy sources
for reliable generation of random unique IDs, with goal of uniqueness
and portability, not cryptographic strength nor maximum speed.

Specifically:
* Moves code for getting UUIDs from the OS to port::GenerateRfcUuid
rather than in Env implementation details. Callers are now told whether
the operation fails or succeeds.
* Adds an internal API GenerateRawUniqueId for generating high-quality
128-bit unique identifiers, by combining entropy from three "tracks":
  * Lots of info from default Env like time, process id, and hostname.
  * std::random_device
  * port::GenerateRfcUuid (when working)
* Built-in implementations of Env::GenerateUniqueId() will now always
produce an RFC 4122 UUID string, either from platform-specific API or
by converting the output of GenerateRawUniqueId.

DB session IDs now use GenerateRawUniqueId while DB IDs (not as
critical) try to use port::GenerateRfcUuid but fall back on
GenerateRawUniqueId with conversion to an RFC 4122 UUID.

GenerateRawUniqueId is declared and defined under env/ rather than util/
or even port/ because of the Env dependency.

Likely follow-up: enhance GenerateRawUniqueId to be faster after the
first call and to guarantee uniqueness within the lifetime of a single
process (imparting the same property onto DB session IDs).

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8708

Test Plan:
A new mini-stress test in env_test checks the various public
and internal APIs for uniqueness, including each track of
GenerateRawUniqueId individually. We can't hope to verify anywhere close
to 128 bits of entropy, but it can at least detect flaws as bad as the
old code. Serial execution of the new tests takes about 350 ms on
my machine.

Reviewed By: zhichao-cao, mrambacher

Differential Revision: D30563780

Pulled By: pdillinger

fbshipit-source-id: de4c9ff4b2f581cf784fcedb5f39f16e5185c364

CMakeLists.txt

@@ -713,6 +713,7 @@ set(SOURCES
         env/file_system_tracer.cc
         env/fs_remap.cc
         env/mock_env.cc
+        env/unique_id.cc
         file/delete_scheduler.cc
         file/file_prefetch_buffer.cc
         file/file_util.cc

HISTORY.md

@@ -3,6 +3,7 @@
 ### Bug Fixes
 * Allow secondary instance to refresh iterator. Assign read seq after referencing SuperVersion.
 * Fixed a bug of secondary instance's last_sequence going backward, and reads on the secondary fail to see recent updates from the primary.
+* Fixed a bug that could lead to duplicate DB ID or DB session ID in POSIX environments without /proc/sys/kernel/random/uuid.
 * Fix a race in DumpStats() with column family destruction due to not taking a Ref on each entry while iterating the ColumnFamilySet.
 
 ### New Features

TARGETS

@@ -225,6 +225,7 @@ cpp_library(
         "env/fs_remap.cc",
         "env/io_posix.cc",
         "env/mock_env.cc",
+        "env/unique_id.cc",
         "file/delete_scheduler.cc",
         "file/file_prefetch_buffer.cc",
         "file/file_util.cc",
@@ -545,6 +546,7 @@ cpp_library(
         "env/fs_remap.cc",
         "env/io_posix.cc",
         "env/mock_env.cc",
+        "env/unique_id.cc",
         "file/delete_scheduler.cc",
         "file/file_prefetch_buffer.cc",
         "file/file_util.cc",

db/db_impl/db_impl.cc

@@ -53,6 +53,7 @@
 #include "db/version_set.h"
 #include "db/write_batch_internal.h"
 #include "db/write_callback.h"
+#include "env/unique_id.h"
 #include "file/file_util.h"
 #include "file/filename.h"
 #include "file/random_access_file_reader.h"
@@ -3938,7 +3939,8 @@ Status DBImpl::GetDbIdentityFromIdentityFile(std::string* identity) const {
     return s;
   }
 
-  // If last character is '\n' remove it from identity
+  // If last character is '\n' remove it from identity. (Old implementations
+  // of Env::GenerateUniqueId() would include a trailing '\n'.)
   if (identity->size() > 0 && identity->back() == '\n') {
     identity->pop_back();
   }
@@ -3950,10 +3952,11 @@ Status DBImpl::GetDbSessionId(std::string& session_id) const {
   return Status::OK();
 }
 
-std::string DBImpl::GenerateDbSessionId(Env* env) {
-  // GenerateUniqueId() generates an identifier that has a negligible
-  // probability of being duplicated, ~128 bits of entropy
-  std::string uuid = env->GenerateUniqueId();
+std::string DBImpl::GenerateDbSessionId(Env*) {
+  // GenerateRawUniqueId() generates an identifier that has a negligible
+  // probability of being duplicated. It should have full 128 bits of entropy.
+  uint64_t a, b;
+  GenerateRawUniqueId(&a, &b);
 
   // Hash and reformat that down to a more compact format, 20 characters
   // in base-36 ([0-9A-Z]), which is ~103 bits of entropy, which is enough
@@ -3962,18 +3965,10 @@ std::string DBImpl::GenerateDbSessionId(Env* env) {
   // * Save ~ dozen bytes per SST file
   // * Shorter shared backup file names (some platforms have low limits)
   // * Visually distinct from DB id format
-  uint64_t a = NPHash64(uuid.data(), uuid.size(), 1234U);
-  uint64_t b = NPHash64(uuid.data(), uuid.size(), 5678U);
-  std::string db_session_id;
-  db_session_id.resize(20);
-  static const char* const base36 = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ";
-  size_t i = 0;
-  for (; i < 10U; ++i, a /= 36U) {
-    db_session_id[i] = base36[a % 36];
-  }
-  for (; i < 20U; ++i, b /= 36U) {
-    db_session_id[i] = base36[b % 36];
-  }
+  std::string db_session_id(20U, '\0');
+  char* buf = &db_session_id[0];
+  PutBaseChars<36>(&buf, 10, a, /*uppercase*/ true);
+  PutBaseChars<36>(&buf, 10, b, /*uppercase*/ true);
   return db_session_id;
 }
 

db/merge_test.cc

@@ -117,6 +117,9 @@ std::shared_ptr<DB> OpenDb(const std::string& dbname, const bool ttl = false,
 #endif  // !ROCKSDB_LITE
   EXPECT_OK(s);
   assert(s.ok());
+  // Allowed to call NowNanos during DB creation (in GenerateRawUniqueId() for
+  // session ID)
+  EnvMergeTest::now_nanos_count_ = 0;
   return std::shared_ptr<DB>(db);
 }
 
@@ -463,6 +466,8 @@ void testPartialMerge(Counters* counters, DB* db, size_t max_merge,
   ASSERT_OK(db->CompactRange(CompactRangeOptions(), nullptr, nullptr));
   ASSERT_EQ(tmp_sum, counters->assert_get("c"));
   ASSERT_EQ(num_partial_merge_calls, 0U);
+  // NowNanos was previously called in MergeHelper::FilterMerge(), which
+  // harmed performance.
   ASSERT_EQ(EnvMergeTest::now_nanos_count_, 0U);
 }
 

env/env.cc

@@ -12,6 +12,7 @@
 #include <thread>
 
 #include "env/composite_env_wrapper.h"
+#include "env/unique_id.h"
 #include "logging/env_logger.h"
 #include "memory/arena.h"
 #include "options/db_options.h"
@@ -21,6 +22,7 @@
 #include "rocksdb/system_clock.h"
 #include "rocksdb/utilities/object_registry.h"
 #include "util/autovector.h"
+#include "util/string_util.h"
 
 namespace ROCKSDB_NAMESPACE {
 namespace {
@@ -737,6 +739,46 @@ Status Env::GetHostNameString(std::string* result) {
   return s;
 }
 
+std::string Env::GenerateUniqueId() {
+  std::string result;
+  bool success = port::GenerateRfcUuid(&result);
+  if (!success) {
+    // Fall back on our own way of generating a unique ID and adapt it to
+    // RFC 4122 variant 1 version 4 (a random ID).
+    // https://en.wikipedia.org/wiki/Universally_unique_identifier
+    // We already tried GenerateRfcUuid so no need to try it again in
+    // GenerateRawUniqueId
+    constexpr bool exclude_port_uuid = true;
+    uint64_t upper, lower;
+    GenerateRawUniqueId(&upper, &lower, exclude_port_uuid);
+
+    // Set 4-bit version to 4
+    upper = (upper & (~uint64_t{0xf000})) | 0x4000;
+    // Set unary-encoded variant to 1 (0b10)
+    lower = (lower & (~(uint64_t{3} << 62))) | (uint64_t{2} << 62);
+
+    // Use 36 character format of RFC 4122
+    result.resize(36U);
+    char* buf = &result[0];
+    PutBaseChars<16>(&buf, 8, upper >> 32, /*!uppercase*/ false);
+    *(buf++) = '-';
+    PutBaseChars<16>(&buf, 4, upper >> 16, /*!uppercase*/ false);
+    *(buf++) = '-';
+    PutBaseChars<16>(&buf, 4, upper, /*!uppercase*/ false);
+    *(buf++) = '-';
+    PutBaseChars<16>(&buf, 4, lower >> 48, /*!uppercase*/ false);
+    *(buf++) = '-';
+    PutBaseChars<16>(&buf, 12, lower, /*!uppercase*/ false);
+    assert(buf == &result[36]);
+
+    // Verify variant 1 version 4
+    assert(result[14] == '4');
+    assert(result[19] == '8' || result[19] == '9' || result[19] == 'a' ||
+           result[19] == 'b');
+  }
+  return result;
+}
+
 SequentialFile::~SequentialFile() {
 }
 

env/env_posix.cc

@@ -469,32 +469,6 @@ void PosixEnv::WaitForJoin() {
 
 }  // namespace
 
-std::string Env::GenerateUniqueId() {
-  std::string uuid_file = "/proc/sys/kernel/random/uuid";
-  std::shared_ptr<FileSystem> fs = FileSystem::Default();
-
-  Status s = fs->FileExists(uuid_file, IOOptions(), nullptr);
-  if (s.ok()) {
-    std::string uuid;
-    s = ReadFileToString(fs.get(), uuid_file, &uuid);
-    if (s.ok()) {
-      return uuid;
-    }
-  }
-  // Could not read uuid_file - generate uuid using "nanos-random"
-  Random64 r(time(nullptr));
-  uint64_t random_uuid_portion =
-    r.Uniform(std::numeric_limits<uint64_t>::max());
-  uint64_t nanos_uuid_portion = NowNanos();
-  char uuid2[200];
-  snprintf(uuid2,
-           200,
-           "%lx-%lx",
-           (unsigned long)nanos_uuid_portion,
-           (unsigned long)random_uuid_portion);
-  return uuid2;
-}
-
 //
 // Default Posix Env
 //

env/env_test.cc

@@ -18,10 +18,10 @@
 
 #include <sys/types.h>
 
-#include <iostream>
-#include <unordered_set>
 #include <atomic>
 #include <list>
+#include <mutex>
+#include <unordered_set>
 
 #ifdef OS_LINUX
 #include <fcntl.h>
@@ -35,8 +35,10 @@
 #include <errno.h>
 #endif
 
+#include "db/db_impl/db_impl.h"
 #include "env/env_chroot.h"
 #include "env/env_encryption_ctr.h"
+#include "env/unique_id.h"
 #include "logging/log_buffer.h"
 #include "port/malloc.h"
 #include "port/port.h"
@@ -2458,6 +2460,190 @@ TEST_F(EncryptionProviderTest, LoadROT13Cipher) {
 
 #endif  // ROCKSDB_LITE
 
+namespace {
+
+constexpr size_t kThreads = 8;
+constexpr size_t kIdsPerThread = 1000;
+
+// This is a mini-stress test to check for duplicates in functions like
+// GenerateUniqueId()
+template <typename IdType, class Hash = std::hash<IdType>>
+struct NoDuplicateMiniStressTest {
+  std::unordered_set<IdType, Hash> ids;
+  std::mutex mutex;
+  Env* env;
+
+  NoDuplicateMiniStressTest() { env = Env::Default(); }
+
+  virtual ~NoDuplicateMiniStressTest() {}
+
+  void Run() {
+    std::array<std::thread, kThreads> threads;
+    for (size_t i = 0; i < kThreads; ++i) {
+      threads[i] = std::thread([&]() { ThreadFn(); });
+    }
+    for (auto& thread : threads) {
+      thread.join();
+    }
+    // All must be unique
+    ASSERT_EQ(ids.size(), kThreads * kIdsPerThread);
+  }
+
+  void ThreadFn() {
+    std::array<IdType, kIdsPerThread> my_ids;
+    // Generate in parallel threads as fast as possible
+    for (size_t i = 0; i < kIdsPerThread; ++i) {
+      my_ids[i] = Generate();
+    }
+    // Now collate
+    std::lock_guard<std::mutex> lock(mutex);
+    for (auto& id : my_ids) {
+      ids.insert(id);
+    }
+  }
+
+  virtual IdType Generate() = 0;
+};
+
+void VerifyRfcUuids(const std::unordered_set<std::string>& uuids) {
+  if (uuids.empty()) {
+    return;
+  }
+}
+
+using uint64_pair_t = std::pair<uint64_t, uint64_t>;
+struct HashUint64Pair {
+  std::size_t operator()(
+      std::pair<uint64_t, uint64_t> const& u) const noexcept {
+    // Assume suitable distribution already
+    return static_cast<size_t>(u.first ^ u.second);
+  }
+};
+
+}  // namespace
+
+TEST_F(EnvTest, GenerateUniqueId) {
+  struct MyStressTest : public NoDuplicateMiniStressTest<std::string> {
+    std::string Generate() override { return env->GenerateUniqueId(); }
+  };
+
+  MyStressTest t;
+  t.Run();
+
+  // Basically verify RFC-4122 format
+  for (auto& uuid : t.ids) {
+    ASSERT_EQ(36U, uuid.size());
+    ASSERT_EQ('-', uuid[8]);
+    ASSERT_EQ('-', uuid[13]);
+    ASSERT_EQ('-', uuid[18]);
+    ASSERT_EQ('-', uuid[23]);
+  }
+}
+
+TEST_F(EnvTest, GenerateDbSessionId) {
+  struct MyStressTest : public NoDuplicateMiniStressTest<std::string> {
+    std::string Generate() override { return DBImpl::GenerateDbSessionId(env); }
+  };
+
+  MyStressTest t;
+  t.Run();
+
+  // Basically verify session ID
+  for (auto& id : t.ids) {
+    ASSERT_EQ(20U, id.size());
+  }
+}
+
+constexpr bool kRequirePortGenerateRfcUuid =
+#if defined(OS_LINUX) || defined(OS_ANDROID) || defined(OS_WIN)
+    true;
+#else
+    false;
+#endif
+
+TEST_F(EnvTest, PortGenerateRfcUuid) {
+  if (!kRequirePortGenerateRfcUuid) {
+    ROCKSDB_GTEST_SKIP("Not supported/expected on this platform");
+    return;
+  }
+  struct MyStressTest : public NoDuplicateMiniStressTest<std::string> {
+    std::string Generate() override {
+      std::string u;
+      assert(port::GenerateRfcUuid(&u));
+      return u;
+    }
+  };
+
+  MyStressTest t;
+  t.Run();
+
+  // Extra verification on versions and variants
+  VerifyRfcUuids(t.ids);
+}
+
+// Test the atomic, linear generation of GenerateRawUuid
+TEST_F(EnvTest, GenerateRawUniqueId) {
+  struct MyStressTest
+      : public NoDuplicateMiniStressTest<uint64_pair_t, HashUint64Pair> {
+    uint64_pair_t Generate() override {
+      uint64_pair_t p;
+      GenerateRawUniqueId(&p.first, &p.second);
+      return p;
+    }
+  };
+
+  MyStressTest t;
+  t.Run();
+}
+
+// Test that each entropy source ("track") is at least adequate
+TEST_F(EnvTest, GenerateRawUniqueIdTrackPortUuidOnly) {
+  if (!kRequirePortGenerateRfcUuid) {
+    ROCKSDB_GTEST_SKIP("Not supported/expected on this platform");
+    return;
+  }
+
+  struct MyStressTest
+      : public NoDuplicateMiniStressTest<uint64_pair_t, HashUint64Pair> {
+    uint64_pair_t Generate() override {
+      uint64_pair_t p;
+      TEST_GenerateRawUniqueId(&p.first, &p.second, false, true, true);
+      return p;
+    }
+  };
+
+  MyStressTest t;
+  t.Run();
+}
+
+TEST_F(EnvTest, GenerateRawUniqueIdTrackEnvDetailsOnly) {
+  struct MyStressTest
+      : public NoDuplicateMiniStressTest<uint64_pair_t, HashUint64Pair> {
+    uint64_pair_t Generate() override {
+      uint64_pair_t p;
+      TEST_GenerateRawUniqueId(&p.first, &p.second, true, false, true);
+      return p;
+    }
+  };
+
+  MyStressTest t;
+  t.Run();
+}
+
+TEST_F(EnvTest, GenerateRawUniqueIdTrackRandomDeviceOnly) {
+  struct MyStressTest
+      : public NoDuplicateMiniStressTest<uint64_pair_t, HashUint64Pair> {
+    uint64_pair_t Generate() override {
+      uint64_pair_t p;
+      TEST_GenerateRawUniqueId(&p.first, &p.second, true, true, false);
+      return p;
+    }
+  };
+
+  MyStressTest t;
+  t.Run();
+}
+
 }  // namespace ROCKSDB_NAMESPACE
 
 int main(int argc, char** argv) {

env/unique_id.cc

@@ -0,0 +1,144 @@
+//  Copyright (c) Facebook, Inc. and its affiliates. All Rights Reserved.
+//  This source code is licensed under both the GPLv2 (found in the
+//  COPYING file in the root directory) and Apache 2.0 License
+//  (found in the LICENSE.Apache file in the root directory).
+
+#include "env/unique_id.h"
+
+#include <algorithm>
+#include <array>
+#include <cstring>
+#include <random>
+
+#include "port/port.h"
+#include "rocksdb/env.h"
+#include "rocksdb/version.h"
+#include "util/hash.h"
+
+namespace ROCKSDB_NAMESPACE {
+
+namespace {
+
+struct GenerateRawUniqueIdOpts {
+  Env* env = Env::Default();
+  bool exclude_port_uuid = false;
+  bool exclude_env_details = false;
+  bool exclude_random_device = false;
+};
+
+// Each of these "tracks" below should be sufficient for generating 128 bits
+// of entropy, after hashing the raw bytes. The tracks are separable for
+// testing purposes, but in production we combine as many tracks as possible
+// to ensure quality results even if some environments have degraded
+// capabilities or quality in some APIs.
+//
+// This approach has not been validated for use in cryptography. The goal is
+// generating globally unique values with high probability without coordination
+// between instances.
+//
+// Linux performance: EntropyTrackRandomDevice is much faster than
+// EntropyTrackEnvDetails, which is much faster than EntropyTrackPortUuid.
+
+struct EntropyTrackPortUuid {
+  std::array<char, 36> uuid;
+
+  void Populate(const GenerateRawUniqueIdOpts& opts) {
+    if (opts.exclude_port_uuid) {
+      return;
+    }
+    std::string s;
+    port::GenerateRfcUuid(&s);
+    if (s.size() >= uuid.size()) {
+      std::copy_n(s.begin(), uuid.size(), uuid.begin());
+    }
+  }
+};
+
+struct EntropyTrackEnvDetails {
+  std::array<char, 64> hostname_buf;
+  int64_t process_id;
+  uint64_t thread_id;
+  int64_t unix_time;
+  uint64_t nano_time;
+
+  void Populate(const GenerateRawUniqueIdOpts& opts) {
+    if (opts.exclude_env_details) {
+      return;
+    }
+    opts.env->GetHostName(hostname_buf.data(), hostname_buf.size())
+        .PermitUncheckedError();
+    process_id = port::GetProcessID();
+    thread_id = opts.env->GetThreadID();
+    opts.env->GetCurrentTime(&unix_time).PermitUncheckedError();
+    nano_time = opts.env->NowNanos();
+  }
+};
+
+struct EntropyTrackRandomDevice {
+  using RandType = std::random_device::result_type;
+  static constexpr size_t kNumRandVals =
+      /* generous bits */ 192U / (8U * sizeof(RandType));
+  std::array<RandType, kNumRandVals> rand_vals;
+
+  void Populate(const GenerateRawUniqueIdOpts& opts) {
+    if (opts.exclude_random_device) {
+      return;
+    }
+    std::random_device r;
+    for (auto& val : rand_vals) {
+      val = r();
+    }
+  }
+};
+
+struct Entropy {
+  uint64_t version_identifier;
+  EntropyTrackRandomDevice et1;
+  EntropyTrackEnvDetails et2;
+  EntropyTrackPortUuid et3;
+
+  void Populate(const GenerateRawUniqueIdOpts& opts) {
+    // If we change the format of what goes into the entropy inputs, it's
+    // conceivable there could be a physical collision in the hash input
+    // even though they are logically different. This value should change
+    // if there's a change to the "schema" here, including byte order.
+    version_identifier = (uint64_t{ROCKSDB_MAJOR} << 32) +
+                         (uint64_t{ROCKSDB_MINOR} << 16) +
+                         uint64_t{ROCKSDB_PATCH};
+    et1.Populate(opts);
+    et2.Populate(opts);
+    et3.Populate(opts);
+  }
+};
+
+void GenerateRawUniqueIdImpl(uint64_t* a, uint64_t* b,
+                             const GenerateRawUniqueIdOpts& opts) {
+  Entropy e;
+  std::memset(&e, 0, sizeof(e));
+  e.Populate(opts);
+  Hash2x64(reinterpret_cast<const char*>(&e), sizeof(e), a, b);
+}
+
+}  // namespace
+
+void GenerateRawUniqueId(uint64_t* a, uint64_t* b, bool exclude_port_uuid) {
+  GenerateRawUniqueIdOpts opts;
+  opts.exclude_port_uuid = exclude_port_uuid;
+  assert(!opts.exclude_env_details);
+  assert(!opts.exclude_random_device);
+  GenerateRawUniqueIdImpl(a, b, opts);
+}
+
+#ifndef NDEBUG
+void TEST_GenerateRawUniqueId(uint64_t* a, uint64_t* b, bool exclude_port_uuid,
+                              bool exclude_env_details,
+                              bool exclude_random_device) {
+  GenerateRawUniqueIdOpts opts;
+  opts.exclude_port_uuid = exclude_port_uuid;
+  opts.exclude_env_details = exclude_env_details;
+  opts.exclude_random_device = exclude_random_device;
+  GenerateRawUniqueIdImpl(a, b, opts);
+}
+#endif
+
+}  // namespace ROCKSDB_NAMESPACE

env/unique_id.h

@@ -0,0 +1,40 @@
+//  Copyright (c) Facebook, Inc. and its affiliates. All Rights Reserved.
+//  This source code is licensed under both the GPLv2 (found in the
+//  COPYING file in the root directory) and Apache 2.0 License
+//  (found in the LICENSE.Apache file in the root directory).
+
+// This file is for functions that extract novel entropy or sources of
+// uniqueness from the execution environment. (By contrast, random.h is
+// for algorithmic pseudorandomness.)
+//
+// These functions could eventually migrate to public APIs, such as in Env.
+
+#pragma once
+
+#include <cstdint>
+
+#include "rocksdb/rocksdb_namespace.h"
+
+namespace ROCKSDB_NAMESPACE {
+
+// Generates a new 128-bit identifier that is universally unique
+// (with high probability) for each call. The result is split into
+// two 64-bit pieces. This function has NOT been validated for use in
+// cryptography.
+//
+// This is used in generating DB session IDs and by Env::GenerateUniqueId
+// (used for DB IDENTITY) if the platform does not provide a generator of
+// RFC 4122 UUIDs or fails somehow. (Set exclude_port_uuid=true if this
+// function is used as a fallback for GenerateRfcUuid, because no need
+// trying it again.)
+void GenerateRawUniqueId(uint64_t* a, uint64_t* b,
+                         bool exclude_port_uuid = false);
+
+#ifndef NDEBUG
+// A version of above with options for challenge testing
+void TEST_GenerateRawUniqueId(uint64_t* a, uint64_t* b, bool exclude_port_uuid,
+                              bool exclude_env_details,
+                              bool exclude_random_device);
+#endif
+
+}  // namespace ROCKSDB_NAMESPACE

include/rocksdb/env.h

@@ -559,7 +559,10 @@ class Env {
   // Converts seconds-since-Jan-01-1970 to a printable string
   virtual std::string TimeToString(uint64_t time) = 0;
 
-  // Generates a unique id that can be used to identify a db
+  // Generates a human-readable unique ID that can be used to identify a DB.
+  // In built-in implementations, this is an RFC-4122 UUID string, but might
+  // not be in all implementations. Overriding is not recommended.
+  // NOTE: this has not be validated for use in cryptography
   virtual std::string GenerateUniqueId();
 
   // OptimizeForLogWrite will create a new EnvOptions object that is a copy of

port/port_posix.cc

@@ -25,6 +25,8 @@
 #include <unistd.h>
 
 #include <cstdlib>
+#include <fstream>
+#include <string>
 
 #include "util/string_util.h"
 
@@ -265,6 +267,18 @@ void SetCpuPriority(ThreadId id, CpuPriority priority) {
 
 int64_t GetProcessID() { return getpid(); }
 
+bool GenerateRfcUuid(std::string* output) {
+  output->clear();
+  std::ifstream f("/proc/sys/kernel/random/uuid");
+  std::getline(f, /*&*/ *output);
+  if (output->size() == 36) {
+    return true;
+  } else {
+    output->clear();
+    return false;
+  }
+}
+
 }  // namespace port
 }  // namespace ROCKSDB_NAMESPACE
 

port/port_posix.h

@@ -221,5 +221,9 @@ extern void SetCpuPriority(ThreadId id, CpuPriority priority);
 
 int64_t GetProcessID();
 
+// Uses platform APIs to generate a 36-character RFC-4122 UUID. Returns
+// true on success or false on failure.
+bool GenerateRfcUuid(std::string* output);
+
 } // namespace port
 }  // namespace ROCKSDB_NAMESPACE

port/win/env_win.cc

@@ -1404,25 +1404,6 @@ void WinEnv::IncBackgroundThreadsIfNeeded(int num, Env::Priority pri) {
 
 }  // namespace port
 
-std::string Env::GenerateUniqueId() {
-  std::string result;
-
-  UUID uuid;
-  UuidCreateSequential(&uuid);
-
-  RPC_CSTR rpc_str;
-  auto status = UuidToStringA(&uuid, &rpc_str);
-  (void)status;
-  assert(status == RPC_S_OK);
-
-  result = reinterpret_cast<char*>(rpc_str);
-
-  status = RpcStringFreeA(&rpc_str);
-  assert(status == RPC_S_OK);
-
-  return result;
-}
-
 std::shared_ptr<FileSystem> FileSystem::Default() {
   return port::WinFileSystem::Default();
 }

port/win/port_win.cc

@@ -11,18 +11,19 @@
 
 #include "port/win/port_win.h"
 
-#include <io.h>
-#include "port/port_dirent.h"
-#include "port/sys_time.h"
-
-#include <cstdlib>
-#include <stdio.h>
 #include <assert.h>
+#include <io.h>
+#include <rpc.h>
+#include <stdio.h>
 #include <string.h>
 
-#include <memory>
-#include <exception>
 #include <chrono>
+#include <cstdlib>
+#include <exception>
+#include <memory>
+
+#include "port/port_dirent.h"
+#include "port/sys_time.h"
 
 #ifdef ROCKSDB_WINDOWS_UTF8_FILENAMES
 // utf8 <-> utf16
@@ -279,6 +280,25 @@ void SetCpuPriority(ThreadId id, CpuPriority priority) {
 
 int64_t GetProcessID() { return GetCurrentProcessId(); }
 
+bool GenerateRfcUuid(std::string* output) {
+  UUID uuid;
+  UuidCreateSequential(&uuid);
+
+  RPC_CSTR rpc_str;
+  auto status = UuidToStringA(&uuid, &rpc_str);
+  if (status != RPC_S_OK) {
+    return false;
+  }
+
+  // rpc_str is nul-terminated
+  *output = reinterpret_cast<char*>(rpc_str);
+
+  status = RpcStringFreeA(&rpc_str);
+  assert(status == RPC_S_OK);
+
+  return true;
+}
+
 }  // namespace port
 }  // namespace ROCKSDB_NAMESPACE
 

port/win/port_win.h

@@ -349,6 +349,10 @@ extern void SetCpuPriority(ThreadId id, CpuPriority priority);
 
 int64_t GetProcessID();
 
+// Uses platform APIs to generate a 36-character RFC-4122 UUID. Returns
+// true on success or false on failure.
+bool GenerateRfcUuid(std::string* output);
+
 }  // namespace port
 
 

src.mk

@@ -94,6 +94,7 @@ LIB_SOURCES =                                                   \
   env/file_system_tracer.cc                                     \
   env/io_posix.cc                                               \
   env/mock_env.cc                                               \
+  env/unique_id.cc                                              \
   file/delete_scheduler.cc                                      \
   file/file_prefetch_buffer.cc                                  \
   file/file_util.cc                                             \

util/hash.cc

@@ -113,4 +113,11 @@ Unsigned128 Hash128(const char* data, size_t n) {
   return (Unsigned128{h.high64} << 64) | (h.low64);
 }
 
+void Hash2x64(const char* data, size_t n, uint64_t* high64, uint64_t* low64) {
+  // Same as seed = 0
+  auto h = XXH3_128bits(data, n);
+  *high64 = h.high64;
+  *low64 = h.low64;
+}
+
 }  // namespace ROCKSDB_NAMESPACE

util/hash.h

@@ -63,6 +63,10 @@ inline uint64_t NPHash64(const char* data, size_t n) {
 #endif
 }
 
+// Convenient and equivalent version of Hash128 without depending on 128-bit
+// scalars
+void Hash2x64(const char* data, size_t n, uint64_t* high64, uint64_t* low64);
+
 // Stable/persistent 32-bit hash. Moderate quality and high speed on
 // small inputs.
 // TODO: consider rename to Hash32

util/hash_test.cc

@@ -21,6 +21,7 @@ using ROCKSDB_NAMESPACE::EncodeFixed32;
 using ROCKSDB_NAMESPACE::GetSliceHash64;
 using ROCKSDB_NAMESPACE::Hash;
 using ROCKSDB_NAMESPACE::Hash128;
+using ROCKSDB_NAMESPACE::Hash2x64;
 using ROCKSDB_NAMESPACE::Hash64;
 using ROCKSDB_NAMESPACE::Lower32of64;
 using ROCKSDB_NAMESPACE::Lower64of128;
@@ -286,6 +287,12 @@ TEST(HashTest, Hash128Misc) {
       // Must be same as unseeded Hash128 and GetSliceHash128
       EXPECT_EQ(here, Hash128(str.data(), size));
       EXPECT_EQ(here, GetSliceHash128(Slice(str.data(), size)));
+      {
+        uint64_t hi, lo;
+        Hash2x64(str.data(), size, &hi, &lo);
+        EXPECT_EQ(Lower64of128(here), lo);
+        EXPECT_EQ(Upper64of128(here), hi);
+      }
 
       // Upper and Lower must reconstruct hash
       EXPECT_EQ(here,

util/string_util.h

@@ -42,6 +42,19 @@ extern void AppendEscapedStringTo(std::string* str, const Slice& value);
 // Return a string printout of "num"
 extern std::string NumberToString(uint64_t num);
 
+// Put n digits from v in base kBase to (*buf)[0] to (*buf)[n-1] and
+// advance *buf to the position after what was written.
+template <size_t kBase>
+inline void PutBaseChars(char** buf, size_t n, uint64_t v, bool uppercase) {
+  const char* digitChars = uppercase ? "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ"
+                                     : "0123456789abcdefghijklmnopqrstuvwxyz";
+  for (size_t i = n; i > 0; --i) {
+    (*buf)[i - 1] = digitChars[v % kBase];
+    v /= kBase;
+  }
+  *buf += n;
+}
+
 // Return a human-readable version of num.
 // for num >= 10.000, prints "xxK"
 // for num >= 10.000.000, prints "xxM"

utilities/env_librados.cc

@@ -861,25 +861,6 @@ librados::IoCtx* EnvLibrados::_GetIoctx(const std::string& fpath) {
 /************************************************************
                 public functions
 ************************************************************/
-/**
- * @brief generate unique id
- * @details Combine system time and random number.
- * @return [description]
- */
-std::string EnvLibrados::GenerateUniqueId() {
-  Random64 r(time(nullptr));
-  uint64_t random_uuid_portion =
-    r.Uniform(std::numeric_limits<uint64_t>::max());
-  uint64_t nanos_uuid_portion = NowNanos();
-  char uuid2[200];
-  snprintf(uuid2,
-           200,
-           "%16lx-%16lx",
-           (unsigned long)nanos_uuid_portion,
-           (unsigned long)random_uuid_portion);
-  return uuid2;
-}
-
 /**
  * @brief create a new sequential read file handler
  * @details it will check the existence of fname