Squashed 'src/leveldb/' changes from ab6c84e6f3..a7f9bdc611

diff --git a/CMakeLists.txt b/CMakeLists.txt
index cfd4faa3..78efde95 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -2,19 +2,25 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file. See the AUTHORS file for names of contributors.
 
-cmake_minimum_required(VERSION 3.9)
+cmake_minimum_required(VERSION 3.22)
 # Keep the version below in sync with the one in db.h
 project(leveldb VERSION 1.22.0 LANGUAGES C CXX)
 
-# This project can use C11, but will gracefully decay down to C89.
-set(CMAKE_C_STANDARD 11)
-set(CMAKE_C_STANDARD_REQUIRED OFF)
-set(CMAKE_C_EXTENSIONS OFF)
-
-# This project requires C++11.
-set(CMAKE_CXX_STANDARD 11)
-set(CMAKE_CXX_STANDARD_REQUIRED ON)
-set(CMAKE_CXX_EXTENSIONS OFF)
+# C standard can be overridden when this is used as a sub-project.
+if(NOT CMAKE_C_STANDARD)
+  # This project can use C11, but will gracefully decay down to C89.
+  set(CMAKE_C_STANDARD 11)
+  set(CMAKE_C_STANDARD_REQUIRED OFF)
+  set(CMAKE_C_EXTENSIONS OFF)
+endif(NOT CMAKE_C_STANDARD)
+
+# C++ standard can be overridden when this is used as a sub-project.
+if(NOT CMAKE_CXX_STANDARD)
+  # This project requires C++17.
+  set(CMAKE_CXX_STANDARD 17)
+  set(CMAKE_CXX_STANDARD_REQUIRED ON)
+  set(CMAKE_CXX_EXTENSIONS OFF)
+endif(NOT CMAKE_CXX_STANDARD)
 
 if (WIN32)
   set(LEVELDB_PLATFORM_NAME LEVELDB_PLATFORM_WINDOWS)
diff --git a/db/autocompact_test.cc b/db/autocompact_test.cc
index e6c97a05..f6e714c6 100644
--- a/db/autocompact_test.cc
+++ b/db/autocompact_test.cc
@@ -54,8 +54,8 @@ static const int kValueSize = 200 * 1024;
 static const int kTotalSize = 100 * 1024 * 1024;
 static const int kCount = kTotalSize / kValueSize;
 
-// Read through the first n keys repeatedly and check that they get
-// compacted (verified by checking the size of the key space).
+// Read through the first n keys repeatedly and check that reads do NOT
+// trigger compaction (seek compaction is disabled in this fork).
 void AutoCompactTest::DoReads(int n) {
   std::string value(kValueSize, 'x');
   DBImpl* dbi = reinterpret_cast<DBImpl*>(db_);
@@ -76,25 +76,23 @@ void AutoCompactTest::DoReads(int n) {
   const int64_t initial_size = Size(Key(0), Key(n));
   const int64_t initial_other_size = Size(Key(n), Key(kCount));
 
-  // Read until size drops significantly.
+  // Read repeatedly. The size of the read range must NOT shrink: with
+  // seek compaction disabled, reads never schedule a compaction.
   std::string limit_key = Key(n);
-  for (int read = 0; true; read++) {
-    ASSERT_LT(read, 100) << "Taking too long to compact";
+  for (int read = 0; read < 100; read++) {
     Iterator* iter = db_->NewIterator(ReadOptions());
     for (iter->SeekToFirst();
          iter->Valid() && iter->key().ToString() < limit_key; iter->Next()) {
       // Drop data
     }
     delete iter;
-    // Wait a little bit to allow any triggered compactions to complete.
-    Env::Default()->SleepForMicroseconds(1000000);
     uint64_t size = Size(Key(0), Key(n));
     fprintf(stderr, "iter %3d => %7.3f MB [other %7.3f MB]\n", read + 1,
             size / 1048576.0, Size(Key(n), Key(kCount)) / 1048576.0);
-    if (size <= initial_size / 10) {
-      break;
-    }
   }
+  // Give any background work a chance to run, even though none should.
+  Env::Default()->SleepForMicroseconds(1000000);
+  ASSERT_EQ(Size(Key(0), Key(n)), static_cast<uint64_t>(initial_size));
 
   // Verify that the size of the key space not touched by the reads
   // is pretty much unchanged.
diff --git a/db/db_test.cc b/db/db_test.cc
index 3c9f8942..16e8ecb9 100644
--- a/db/db_test.cc
+++ b/db/db_test.cc
@@ -735,15 +735,14 @@ TEST(DBTest, GetPicksCorrectFile) {
   } while (ChangeOptions());
 }
 
-TEST(DBTest, GetEncountersEmptyLevel) {
+TEST(DBTest, GetDoesNotTriggerSeekCompaction) {
   do {
     // Arrange for the following to happen:
     //   * sstable A in level 0
     //   * nothing in level 1
     //   * sstable B in level 2
-    // Then do enough Get() calls to arrange for an automatic compaction
-    // of sstable A.  A bug would cause the compaction to be marked as
-    // occurring at level 1 (instead of the correct level 0).
+    // Seek compaction is disabled in this fork, so repeated reads must
+    // not change the level layout. A manual compaction must still work.
 
     // Step 1: First place sstables in levels 0 and 2
     int compaction_count = 0;
@@ -761,14 +760,17 @@ TEST(DBTest, GetEncountersEmptyLevel) {
     ASSERT_EQ(NumTableFilesAtLevel(1), 0);
     ASSERT_EQ(NumTableFilesAtLevel(2), 1);
 
-    // Step 3: read a bunch of times
+    // Step 3: many read misses must not schedule any compaction.
     for (int i = 0; i < 1000; i++) {
       ASSERT_EQ("NOT_FOUND", Get("missing"));
     }
-
-    // Step 4: Wait for compaction to finish
     DelayMilliseconds(1000);
+    ASSERT_EQ(NumTableFilesAtLevel(0), 1);
+    ASSERT_EQ(NumTableFilesAtLevel(1), 0);
+    ASSERT_EQ(NumTableFilesAtLevel(2), 1);
 
+    // Step 4: a manual compaction still moves the L0 file down.
+    dbfull()->TEST_CompactRange(0, nullptr, nullptr);
     ASSERT_EQ(NumTableFilesAtLevel(0), 0);
   } while (ChangeOptions());
 }
diff --git a/db/version_set.cc b/db/version_set.cc
index cd07346e..2b56bbcd 100644
--- a/db/version_set.cc
+++ b/db/version_set.cc
@@ -400,16 +400,11 @@ Status Version::Get(const ReadOptions& options, const LookupKey& k,
   return state.found ? state.s : Status::NotFound(Slice());
 }
 
-bool Version::UpdateStats(const GetStats& stats) {
-  FileMetaData* f = stats.seek_file;
-  if (f != nullptr) {
-    f->allowed_seeks--;
-    if (f->allowed_seeks <= 0 && file_to_compact_ == nullptr) {
-      file_to_compact_ = f;
-      file_to_compact_level_ = stats.seek_file_level;
-      return true;
-    }
-  }
+bool Version::UpdateStats(const GetStats& /*stats*/) {
+  // Disable automatic compactions triggered by read seek counters.
+  // The heuristic was tuned for expensive random seeks and can create
+  // severe write amplification on large random-key databases.
+  // Size and manual compactions still run.
   return false;
 }
 
@@ -661,6 +656,8 @@ class VersionSet::Builder {
       // same as the compaction of 40KB of data.  We are a little
       // conservative and allow approximately one seek for every 16KB
       // of data before triggering a compaction.
+      //
+      // Note: seek compactions are disabled. See Version::UpdateStats.
       f->allowed_seeks = static_cast<int>((f->file_size / 16384U));
       if (f->allowed_seeks < 100) f->allowed_seeks = 100;
 
@@ -994,7 +991,7 @@ bool VersionSet::ReuseManifest(const std::string& dscname,
   }
   FileType manifest_type;
   uint64_t manifest_number;
-  uint64_t manifest_size;
+  uint64_t manifest_size = 0;
   if (!ParseFileName(dscbase, &manifest_number, &manifest_type) ||
       manifest_type != kDescriptorFile ||
       !env_->GetFileSize(dscname, &manifest_size).ok() ||
diff --git a/util/env_posix.cc b/util/env_posix.cc
index 8a33534e..00bfd598 100644
--- a/util/env_posix.cc
+++ b/util/env_posix.cc
@@ -42,8 +42,8 @@ namespace {
 // Set by EnvPosixTestHelper::SetReadOnlyMMapLimit() and MaxOpenFiles().
 int g_open_read_only_file_limit = -1;
 
-// Up to 4096 mmap regions for 64-bit binaries; none for 32-bit.
-constexpr const int kDefaultMmapLimit = (sizeof(void*) >= 8) ? 4096 : 0;
+// Up to 1000 mmap regions for 64-bit binaries; none for 32-bit.
+constexpr const int kDefaultMmapLimit = (sizeof(void*) >= 8) ? 1000 : 0;
 
 // Can be set using EnvPosixTestHelper::SetReadOnlyMMapLimit().
 int g_mmap_limit = kDefaultMmapLimit;
diff --git a/util/env_windows.cc b/util/env_windows.cc
index 8897e2aa..6ef27e45 100644
--- a/util/env_windows.cc
+++ b/util/env_windows.cc
@@ -81,10 +81,7 @@ class ScopedHandle {
 
   ScopedHandle& operator=(const ScopedHandle&) = delete;
 
-  ScopedHandle& operator=(ScopedHandle&& rhs) noexcept {
-    if (this != &rhs) handle_ = rhs.Release();
-    return *this;
-  }
+  ScopedHandle& operator=(ScopedHandle&& rhs) = delete;
 
   bool Close() {
     if (!is_valid()) {
diff --git a/util/hash.cc b/util/hash.cc
index 5432b618..6b277306 100644
--- a/util/hash.cc
+++ b/util/hash.cc
@@ -8,15 +8,6 @@
 
 #include "util/coding.h"
 
-// The FALLTHROUGH_INTENDED macro can be used to annotate implicit fall-through
-// between switch labels. The real definition should be provided externally.
-// This one is a fallback version for unsupported compilers.
-#ifndef FALLTHROUGH_INTENDED
-#define FALLTHROUGH_INTENDED \
-  do {                       \
-  } while (0)
-#endif
-
 namespace leveldb {
 
 uint32_t Hash(const char* data, size_t n, uint32_t seed) {
@@ -39,10 +30,10 @@ uint32_t Hash(const char* data, size_t n, uint32_t seed) {
   switch (limit - data) {
     case 3:
       h += static_cast<uint8_t>(data[2]) << 16;
-      FALLTHROUGH_INTENDED;
+      [[fallthrough]];
     case 2:
       h += static_cast<uint8_t>(data[1]) << 8;
-      FALLTHROUGH_INTENDED;
+      [[fallthrough]];
     case 1:
       h += static_cast<uint8_t>(data[0]);
       h *= m;