sql/sandboxed_vfs_file_impl.cc - chromium/src - Git at Google

 // Copyright 2025 The Chromium Authors
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.

 #ifdef UNSAFE_BUFFERS_BUILD
 // TODO(crbug.com/351564777): Remove this and convert code to safer constructs.
 #pragma allow_unsafe_buffers
 #endif

 #include "sql/sandboxed_vfs_file_impl.h"

 #include "base/notreached.h"
 #include "sql/sandboxed_vfs.h"

 namespace sql {

 SandboxedVfsFileImpl::SandboxedVfsFileImpl(base::File file,
                                            base::FilePath file_path,
                                            SandboxedVfsFileType file_type,
                                            SandboxedVfs* vfs)
     : SandboxedVfsFile(),
       file_(std::move(file)),
       sqlite_lock_mode_(SQLITE_LOCK_NONE),
       vfs_(vfs),
       file_type_(file_type),
       file_path_(std::move(file_path)) {}

 SandboxedVfsFileImpl::~SandboxedVfsFileImpl() = default;

 int SandboxedVfsFileImpl::Close() {
   file_.Close();
   delete this;
   return SQLITE_OK;
 }

 int SandboxedVfsFileImpl::Read(void* buffer, int size, sqlite3_int64 offset) {
   DCHECK(buffer);
   DCHECK_GE(size, 0);
   DCHECK_GE(offset, 0);

   // See http://www.sqlite.org/fileformat2.html#database_header
   constexpr int kSqliteDatabaseHeaderOffset = 0;
   constexpr int kSqliteDatabaseHeaderSize = 100;
   // SQLite's locking protocol only acquires locks on the database file. The
   // journal and the WAL file are always unlocked. Also, as an optimization,
   // SQLite first reads the database header without locking the file.
   DCHECK(sqlite_lock_mode_ > SQLITE_LOCK_NONE ||
          file_type_ != SandboxedVfsFileType::kDatabase ||
          (offset == kSqliteDatabaseHeaderOffset &&
           size == kSqliteDatabaseHeaderSize))
       << "Read from database file with lock mode " << sqlite_lock_mode_
       << "of size" << size << " at offset " << offset;

   char* data = reinterpret_cast<char*>(buffer);

   // If we supported mmap()ed files, we'd check for a memory mapping here,
   // and try to fill as much of the request as possible from the mmap()ed
   // region.

   int bytes_read = file_.Read(offset, data, size);
   DCHECK_LE(bytes_read, size);
   if (bytes_read == size) {
     return SQLITE_OK;
   }

   if (bytes_read < 0) {
     // SQLite first reads the database header without locking the file. On
     // Windows, this read will fail if there is an exclusive lock on the file,
     // even if the current process owns that lock.
     if (sqlite_lock_mode_ == SQLITE_LOCK_NONE) {
       // The unlocked read is considered an optimization. SQLite can continue
       // even if the read fails, as long as failure is communicated by zeroing
       // out the output buffer.
       std::memset(data, 0, size);
       return SQLITE_OK;
     }

     vfs_->SetLastError(base::File::GetLastFileError());
     return SQLITE_IOERR_READ;
   }

   // SQLite requires that we fill the unread bytes in the buffer with zeros.
   std::memset(data + bytes_read, 0, size - bytes_read);
   return SQLITE_IOERR_SHORT_READ;
 }

 int SandboxedVfsFileImpl::Write(const void* buffer,
                                 int size,
                                 sqlite3_int64 offset) {
   DCHECK(buffer);
   DCHECK_GE(size, 0);
   DCHECK_GE(offset, 0);

   // SQLite's locking protocol only acquires locks on the database file. The
   // journal and the WAL file are always unlocked.
   DCHECK(sqlite_lock_mode_ == SQLITE_LOCK_EXCLUSIVE ||
          file_type_ != SandboxedVfsFileType::kDatabase)
       << "Write to database file with lock mode " << sqlite_lock_mode_;

   const char* data = reinterpret_cast<const char*>(buffer);

   // If we supported mmap()ed files, we'd check for a memory mapping here,
   // and try to fill as much of the request as possible by copying to the
   // mmap()ed region.

   int bytes_written = file_.Write(offset, data, size);
   DCHECK_LE(bytes_written, size);
   if (bytes_written >= size) {
     return SQLITE_OK;
   }

   base::File::Error last_error = base::File::GetLastFileError();
   vfs_->SetLastError(last_error);
   if (last_error == base::File::Error::FILE_ERROR_NO_SPACE) {
     return SQLITE_FULL;
   }

   return SQLITE_IOERR_WRITE;
 }

 int SandboxedVfsFileImpl::Truncate(sqlite3_int64 size) {
   if (file_.SetLength(size)) {
     return SQLITE_OK;
   }

   return SQLITE_IOERR_TRUNCATE;
 }

 int SandboxedVfsFileImpl::Sync(int flags) {
   // NOTE: SQLite passes in (SQLITE_SYNC_NORMAL or SQLITE_SYNC_FULL),
   //       potentially OR-ed with SQLITE_SYNC_DATAONLY. Implementing these could
   //       lead to better performance.
   if (!file_.Flush()) {
     vfs_->SetLastError(base::File::GetLastFileError());
     return SQLITE_IOERR_FSYNC;
   }

   // The unix VFS also syncs the file's directory on the first xSync() call.
   // Chrome's LevelDB Env implementation does the same for specific files
   // (database manifests).
   //
   // For WebSQL, we would want to sync the directory at file open time, when the
   // file is opened for writing.

   return SQLITE_OK;
 }

 int SandboxedVfsFileImpl::FileSize(sqlite3_int64* result_size) {
   int64_t length = file_.GetLength();
   if (length < 0) {
     vfs_->SetLastError(base::File::GetLastFileError());
     return SQLITE_IOERR_FSTAT;
   }

   // SQLite's unix VFS reports 1-byte files as empty. This is documented as a
   // workaround for a fairly obscure bug. See unixFileSize() in os_unix.c.
   if (length == 1) {
     length = 0;
   }

   *result_size = length;
   return SQLITE_OK;
 }

 namespace {

 // True if our simplified implementation uses an exclusive lock for a mode.
 bool IsExclusiveLockMode(int sqlite_lock_mode) {
   switch (sqlite_lock_mode) {
     case SQLITE_LOCK_NONE:
     case SQLITE_LOCK_SHARED:
       return false;

     case SQLITE_LOCK_RESERVED:
     case SQLITE_LOCK_PENDING:
     case SQLITE_LOCK_EXCLUSIVE:
       return true;
   }

   NOTREACHED() << "Unsupported mode: " << sqlite_lock_mode;
 }

 }  // namespace

 int SandboxedVfsFileImpl::Lock(int mode) {
   DCHECK_GE(mode, sqlite_lock_mode_)
       << "SQLite asked the VFS to lock the file up to mode " << mode
       << " but the file is already locked at mode " << sqlite_lock_mode_;

 #if BUILDFLAG(IS_FUCHSIA)
   return SQLITE_IOERR_LOCK;
 #else
   base::File::LockMode file_lock_mode = base::File::LockMode::kExclusive;

   switch (mode) {
     case SQLITE_LOCK_NONE:
       return SQLITE_OK;

     case SQLITE_LOCK_SHARED:
       if (sqlite_lock_mode_ != SQLITE_LOCK_NONE) {
         return SQLITE_OK;
       }

       file_lock_mode = base::File::LockMode::kShared;
       break;

     case SQLITE_LOCK_RESERVED:
       // A SHARED lock is required before a RESERVED lock is acquired.
       DCHECK_EQ(sqlite_lock_mode_, SQLITE_LOCK_SHARED);
       file_lock_mode = base::File::LockMode::kExclusive;
       break;

     case SQLITE_LOCK_PENDING:
       NOTREACHED() << "SQLite never directly asks for PENDING locks";

     case SQLITE_LOCK_EXCLUSIVE:
       // A SHARED lock is required before an EXCLUSIVE lock is acquired.
       //
       // No higher level is required. In fact, SQLite upgrades the lock directly
       // from SHARED to EXCLUSIVE when rolling back a transaction, to avoid
       // having other readers queue up in the RESERVED state.
       DCHECK_GE(sqlite_lock_mode_, SQLITE_LOCK_SHARED);

       if (IsExclusiveLockMode(sqlite_lock_mode_)) {
         sqlite_lock_mode_ = mode;
         return SQLITE_OK;
       }
       file_lock_mode = base::File::LockMode::kExclusive;
       break;

     default:
       NOTREACHED() << "Unimplemented xLock() mode: " << mode;
   }

   DCHECK_EQ(IsExclusiveLockMode(mode),
             file_lock_mode == base::File::LockMode::kExclusive)
       << "Incorrect file_lock_mode logic for SQLite mode: " << mode;

   // On POSIX, it would be possible to upgrade atomically from a shared lock to
   // an exclusive lock. This implementation prioritizes the simplicity of no
   // platform-specific code over being faster in high contention cases.
   if (sqlite_lock_mode_ != SQLITE_LOCK_NONE) {
     base::File::Error error = file_.Unlock();
     if (error != base::File::FILE_OK) {
       vfs_->SetLastError(base::File::GetLastFileError());
       return SQLITE_IOERR_LOCK;
     }
     sqlite_lock_mode_ = SQLITE_LOCK_NONE;
   }

   base::File::Error error = file_.Lock(file_lock_mode);
   if (error != base::File::FILE_OK) {
     vfs_->SetLastError(base::File::GetLastFileError());
     return SQLITE_IOERR_LOCK;
   }

   sqlite_lock_mode_ = mode;
   return SQLITE_OK;
 #endif  // BUILDFLAG(IS_FUCHSIA)
 }

 int SandboxedVfsFileImpl::Unlock(int mode) {
   // The 2nd term in the DCHECK predicate is there because SQLite occasionally
   // attempts to unlock (to SQLITE_LOCK_NONE) a file that was already unlocked.
   // We're not aware of any other case of no-op VFS unlock calls.
   DCHECK(mode < sqlite_lock_mode_ ||
          (mode == sqlite_lock_mode_ && mode == SQLITE_LOCK_NONE))
       << "SQLite asked the VFS to unlock the file down to mode " << mode
       << " but the file is already at mode " << sqlite_lock_mode_;

   // No-op if we're already unlocked or at the requested mode.
   if (sqlite_lock_mode_ == mode || sqlite_lock_mode_ == SQLITE_LOCK_NONE) {
     return SQLITE_OK;
   }

 #if BUILDFLAG(IS_FUCHSIA)
   return SQLITE_IOERR_UNLOCK;
 #else
   // On POSIX, it is possible to downgrade atomically from an exclusive lock to
   // a shared lock. SQLite's unix VFS takes advantage of this. This
   // implementation prioritizes the simplicity of no platform-specific code over
   // being faster in high contention cases.
   base::File::Error error = file_.Unlock();
   if (error != base::File::FILE_OK) {
     vfs_->SetLastError(base::File::GetLastFileError());
     return SQLITE_IOERR_UNLOCK;
   }

   if (mode == SQLITE_LOCK_NONE) {
     sqlite_lock_mode_ = mode;
     return SQLITE_OK;
   }

   DCHECK_EQ(mode, SQLITE_LOCK_SHARED);
   error = file_.Lock(base::File::LockMode::kShared);
   if (error == base::File::FILE_OK) {
     sqlite_lock_mode_ = mode;
     return SQLITE_OK;
   }

   // Gave up the exclusive lock, but failed to get a shared lock.
   vfs_->SetLastError(base::File::GetLastFileError());
   sqlite_lock_mode_ = SQLITE_LOCK_NONE;
   return SQLITE_IOERR_UNLOCK;
 #endif  // BUILDFLAG(IS_FUCHSIA)
 }

 int SandboxedVfsFileImpl::CheckReservedLock(int* has_reserved_lock) {
   if (IsExclusiveLockMode(sqlite_lock_mode_)) {
     *has_reserved_lock = 1;
     return SQLITE_OK;
   }

   if (sqlite_lock_mode_ == SQLITE_LOCK_SHARED) {
     // Lock modes at or above RESERVED map to exclusive locks in our simplified
     // implementation. If this process has a shared lock, no other process can
     // have an exclusive lock.
     *has_reserved_lock = 0;
     return SQLITE_OK;
   }

 #if BUILDFLAG(IS_FUCHSIA)
   return SQLITE_IOERR_CHECKRESERVEDLOCK;
 #else
   // On POSIX, it's possible to query the existing lock state of a file. The
   // SQLite unix VFS takes advantage of this. On Windows, this isn't the case.
   // Follow the strategy of the Windows VFS, which checks by trying to get an
   // exclusive lock on the file.
   base::File::Error error = file_.Lock(base::File::LockMode::kShared);
   if (error != base::File::FILE_OK) {
     *has_reserved_lock = 1;
     return SQLITE_OK;
   }

   *has_reserved_lock = 0;
   if (file_.Unlock() == base::File::FILE_OK) {
     return SQLITE_OK;
   }

   // We acquired a shared lock that we can't get rid of.
   sqlite_lock_mode_ = SQLITE_LOCK_SHARED;
   return SQLITE_IOERR_CHECKRESERVEDLOCK;
 #endif  // BUILDFLAG(IS_FUCHSIA)
 }

 int SandboxedVfsFileImpl::FileControl(int opcode, void* data) {
   switch (opcode) {
     case SQLITE_FCNTL_MMAP_SIZE:
       // Implementing memory-mapping will require handling this correctly.
       return SQLITE_NOTFOUND;
     default:
       return SQLITE_NOTFOUND;
   }
 }

 int SandboxedVfsFileImpl::SectorSize() {
   return 0;
 }

 int SandboxedVfsFileImpl::DeviceCharacteristics() {
   // TODO(pwnall): Figure out if we can get away with returning 0 on Windows.
 #if BUILDFLAG(IS_WIN)
   return SQLITE_IOCAP_UNDELETABLE_WHEN_OPEN;
 #else
   // NOTE: SQLite's unix VFS attempts to detect the underlying filesystem and
   // sets some flags based on the result.
   return 0;
 #endif  // BUILDFLAG(IS_WIN)
 }

 int SandboxedVfsFileImpl::ShmMap(int page_index,
                                  int page_size,
                                  int extend_file_if_needed,
                                  void volatile** result) {
   DCHECK_GE(page_index, 0);
   DCHECK_GE(page_size, 0);
   DCHECK(result);

   // https://www.sqlite.org/wal.html#use_of_wal_without_shared_memory states
   // that SQLite only attempts to use shared memory "-shm" files for databases
   // in WAL mode that may be accessed by multiple processes (are not EXCLUSIVE).
   //
   // Chrome will not only use WAL mode on EXCLUSIVE databases.
   NOTREACHED() << "SQLite should not attempt to use shared memory";
 }

 int SandboxedVfsFileImpl::ShmLock(int offset, int size, int flags) {
   DCHECK_GE(offset, 0);
   DCHECK_GE(size, 0);

   // https://www.sqlite.org/wal.html#use_of_wal_without_shared_memory states
   // that SQLite only attempts to use shared memory "-shm" files for databases
   // in WAL mode that may be accessed by multiple processes (are not EXCLUSIVE).
   //
   // Chrome will not only use WAL mode on EXCLUSIVE databases.
   NOTREACHED() << "SQLite should not attempt to use shared memory";
 }

 void SandboxedVfsFileImpl::ShmBarrier() {
   // https://www.sqlite.org/wal.html#use_of_wal_without_shared_memory states
   // that SQLite only attempts to use shared memory "-shm" files for databases
   // in WAL mode that may be accessed by multiple processes (are not EXCLUSIVE).
   //
   // Chrome will not only use WAL mode on EXCLUSIVE databases.
   NOTREACHED() << "SQLite should not attempt to use shared memory";
 }

 int SandboxedVfsFileImpl::ShmUnmap(int also_delete_file) {
   // https://www.sqlite.org/wal.html#use_of_wal_without_shared_memory states
   // that SQLite only attempts to use shared memory "-shm" files for databases
   // in WAL mode that may be accessed by multiple processes (are not EXCLUSIVE).
   //
   // Chrome will not only use WAL mode on EXCLUSIVE databases.
   NOTREACHED() << "SQLite should not attempt to use shared memory";
 }

 int SandboxedVfsFileImpl::Fetch(sqlite3_int64 offset, int size, void** result) {
   DCHECK_GE(offset, 0);
   DCHECK_GE(size, 0);
   DCHECK(result);

   // NOTE: This would be needed for mmap()ed file support.
   *result = nullptr;
   return SQLITE_IOERR;
 }

 int SandboxedVfsFileImpl::Unfetch(sqlite3_int64 offset, void* fetch_result) {
   DCHECK_GE(offset, 0);
   DCHECK(fetch_result);

   // NOTE: This would be needed for mmap()ed file support.
   return SQLITE_IOERR;
 }

 }  // namespace sql
	// Copyright 2025 The Chromium Authors
	// Use of this source code is governed by a BSD-style license that can be
	// found in the LICENSE file.

	#ifdef UNSAFE_BUFFERS_BUILD
	// TODO(crbug.com/351564777): Remove this and convert code to safer constructs.
	#pragma allow_unsafe_buffers
	#endif

	#include "sql/sandboxed_vfs_file_impl.h"

	#include "base/notreached.h"
	#include "sql/sandboxed_vfs.h"

	namespace sql {

	SandboxedVfsFileImpl::SandboxedVfsFileImpl(base::File file,
	base::FilePath file_path,
	SandboxedVfsFileType file_type,
	SandboxedVfs* vfs)
	: SandboxedVfsFile(),
	file_(std::move(file)),
	sqlite_lock_mode_(SQLITE_LOCK_NONE),
	vfs_(vfs),
	file_type_(file_type),
	file_path_(std::move(file_path)) {}

	SandboxedVfsFileImpl::~SandboxedVfsFileImpl() = default;

	int SandboxedVfsFileImpl::Close() {
	file_.Close();
	delete this;
	return SQLITE_OK;
	}

	int SandboxedVfsFileImpl::Read(void* buffer, int size, sqlite3_int64 offset) {
	DCHECK(buffer);
	DCHECK_GE(size, 0);
	DCHECK_GE(offset, 0);

	// See http://www.sqlite.org/fileformat2.html#database_header
	constexpr int kSqliteDatabaseHeaderOffset = 0;
	constexpr int kSqliteDatabaseHeaderSize = 100;
	// SQLite's locking protocol only acquires locks on the database file. The
	// journal and the WAL file are always unlocked. Also, as an optimization,
	// SQLite first reads the database header without locking the file.
	DCHECK(sqlite_lock_mode_ > SQLITE_LOCK_NONE \|\|
	file_type_ != SandboxedVfsFileType::kDatabase \|\|
	(offset == kSqliteDatabaseHeaderOffset &&
	size == kSqliteDatabaseHeaderSize))
	<< "Read from database file with lock mode " << sqlite_lock_mode_
	<< "of size" << size << " at offset " << offset;

	char* data = reinterpret_cast<char*>(buffer);

	// If we supported mmap()ed files, we'd check for a memory mapping here,
	// and try to fill as much of the request as possible from the mmap()ed
	// region.

	int bytes_read = file_.Read(offset, data, size);
	DCHECK_LE(bytes_read, size);
	if (bytes_read == size) {
	return SQLITE_OK;
	}

	if (bytes_read < 0) {
	// SQLite first reads the database header without locking the file. On
	// Windows, this read will fail if there is an exclusive lock on the file,
	// even if the current process owns that lock.
	if (sqlite_lock_mode_ == SQLITE_LOCK_NONE) {
	// The unlocked read is considered an optimization. SQLite can continue
	// even if the read fails, as long as failure is communicated by zeroing
	// out the output buffer.
	std::memset(data, 0, size);
	return SQLITE_OK;
	}

	vfs_->SetLastError(base::File::GetLastFileError());
	return SQLITE_IOERR_READ;
	}

	// SQLite requires that we fill the unread bytes in the buffer with zeros.
	std::memset(data + bytes_read, 0, size - bytes_read);
	return SQLITE_IOERR_SHORT_READ;
	}

	int SandboxedVfsFileImpl::Write(const void* buffer,
	int size,
	sqlite3_int64 offset) {
	DCHECK(buffer);
	DCHECK_GE(size, 0);
	DCHECK_GE(offset, 0);

	// SQLite's locking protocol only acquires locks on the database file. The
	// journal and the WAL file are always unlocked.
	DCHECK(sqlite_lock_mode_ == SQLITE_LOCK_EXCLUSIVE \|\|
	file_type_ != SandboxedVfsFileType::kDatabase)
	<< "Write to database file with lock mode " << sqlite_lock_mode_;

	const char* data = reinterpret_cast<const char*>(buffer);

	// If we supported mmap()ed files, we'd check for a memory mapping here,
	// and try to fill as much of the request as possible by copying to the
	// mmap()ed region.

	int bytes_written = file_.Write(offset, data, size);
	DCHECK_LE(bytes_written, size);
	if (bytes_written >= size) {
	return SQLITE_OK;
	}

	base::File::Error last_error = base::File::GetLastFileError();
	vfs_->SetLastError(last_error);
	if (last_error == base::File::Error::FILE_ERROR_NO_SPACE) {
	return SQLITE_FULL;
	}

	return SQLITE_IOERR_WRITE;
	}

	int SandboxedVfsFileImpl::Truncate(sqlite3_int64 size) {
	if (file_.SetLength(size)) {
	return SQLITE_OK;
	}

	return SQLITE_IOERR_TRUNCATE;
	}

	int SandboxedVfsFileImpl::Sync(int flags) {
	// NOTE: SQLite passes in (SQLITE_SYNC_NORMAL or SQLITE_SYNC_FULL),
	// potentially OR-ed with SQLITE_SYNC_DATAONLY. Implementing these could
	// lead to better performance.
	if (!file_.Flush()) {
	vfs_->SetLastError(base::File::GetLastFileError());
	return SQLITE_IOERR_FSYNC;
	}

	// The unix VFS also syncs the file's directory on the first xSync() call.
	// Chrome's LevelDB Env implementation does the same for specific files
	// (database manifests).
	//
	// For WebSQL, we would want to sync the directory at file open time, when the
	// file is opened for writing.

	return SQLITE_OK;
	}

	int SandboxedVfsFileImpl::FileSize(sqlite3_int64* result_size) {
	int64_t length = file_.GetLength();
	if (length < 0) {
	vfs_->SetLastError(base::File::GetLastFileError());
	return SQLITE_IOERR_FSTAT;
	}

	// SQLite's unix VFS reports 1-byte files as empty. This is documented as a
	// workaround for a fairly obscure bug. See unixFileSize() in os_unix.c.
	if (length == 1) {
	length = 0;
	}

	*result_size = length;
	return SQLITE_OK;
	}

	namespace {

	// True if our simplified implementation uses an exclusive lock for a mode.
	bool IsExclusiveLockMode(int sqlite_lock_mode) {
	switch (sqlite_lock_mode) {
	case SQLITE_LOCK_NONE:
	case SQLITE_LOCK_SHARED:
	return false;

	case SQLITE_LOCK_RESERVED:
	case SQLITE_LOCK_PENDING:
	case SQLITE_LOCK_EXCLUSIVE:
	return true;
	}

	NOTREACHED() << "Unsupported mode: " << sqlite_lock_mode;
	}

	} // namespace

	int SandboxedVfsFileImpl::Lock(int mode) {
	DCHECK_GE(mode, sqlite_lock_mode_)
	<< "SQLite asked the VFS to lock the file up to mode " << mode
	<< " but the file is already locked at mode " << sqlite_lock_mode_;

	#if BUILDFLAG(IS_FUCHSIA)
	return SQLITE_IOERR_LOCK;
	#else
	base::File::LockMode file_lock_mode = base::File::LockMode::kExclusive;

	switch (mode) {
	case SQLITE_LOCK_NONE:
	return SQLITE_OK;

	case SQLITE_LOCK_SHARED:
	if (sqlite_lock_mode_ != SQLITE_LOCK_NONE) {
	return SQLITE_OK;
	}

	file_lock_mode = base::File::LockMode::kShared;
	break;

	case SQLITE_LOCK_RESERVED:
	// A SHARED lock is required before a RESERVED lock is acquired.
	DCHECK_EQ(sqlite_lock_mode_, SQLITE_LOCK_SHARED);
	file_lock_mode = base::File::LockMode::kExclusive;
	break;

	case SQLITE_LOCK_PENDING:
	NOTREACHED() << "SQLite never directly asks for PENDING locks";

	case SQLITE_LOCK_EXCLUSIVE:
	// A SHARED lock is required before an EXCLUSIVE lock is acquired.
	//
	// No higher level is required. In fact, SQLite upgrades the lock directly
	// from SHARED to EXCLUSIVE when rolling back a transaction, to avoid
	// having other readers queue up in the RESERVED state.
	DCHECK_GE(sqlite_lock_mode_, SQLITE_LOCK_SHARED);

	if (IsExclusiveLockMode(sqlite_lock_mode_)) {
	sqlite_lock_mode_ = mode;
	return SQLITE_OK;
	}
	file_lock_mode = base::File::LockMode::kExclusive;
	break;

	default:
	NOTREACHED() << "Unimplemented xLock() mode: " << mode;
	}

	DCHECK_EQ(IsExclusiveLockMode(mode),
	file_lock_mode == base::File::LockMode::kExclusive)
	<< "Incorrect file_lock_mode logic for SQLite mode: " << mode;

	// On POSIX, it would be possible to upgrade atomically from a shared lock to
	// an exclusive lock. This implementation prioritizes the simplicity of no
	// platform-specific code over being faster in high contention cases.
	if (sqlite_lock_mode_ != SQLITE_LOCK_NONE) {
	base::File::Error error = file_.Unlock();
	if (error != base::File::FILE_OK) {
	vfs_->SetLastError(base::File::GetLastFileError());
	return SQLITE_IOERR_LOCK;
	}
	sqlite_lock_mode_ = SQLITE_LOCK_NONE;
	}

	base::File::Error error = file_.Lock(file_lock_mode);
	if (error != base::File::FILE_OK) {
	vfs_->SetLastError(base::File::GetLastFileError());
	return SQLITE_IOERR_LOCK;
	}

	sqlite_lock_mode_ = mode;
	return SQLITE_OK;
	#endif // BUILDFLAG(IS_FUCHSIA)
	}

	int SandboxedVfsFileImpl::Unlock(int mode) {
	// The 2nd term in the DCHECK predicate is there because SQLite occasionally
	// attempts to unlock (to SQLITE_LOCK_NONE) a file that was already unlocked.
	// We're not aware of any other case of no-op VFS unlock calls.
	DCHECK(mode < sqlite_lock_mode_ \|\|
	(mode == sqlite_lock_mode_ && mode == SQLITE_LOCK_NONE))
	<< "SQLite asked the VFS to unlock the file down to mode " << mode
	<< " but the file is already at mode " << sqlite_lock_mode_;

	// No-op if we're already unlocked or at the requested mode.
	if (sqlite_lock_mode_ == mode \|\| sqlite_lock_mode_ == SQLITE_LOCK_NONE) {
	return SQLITE_OK;
	}

	#if BUILDFLAG(IS_FUCHSIA)
	return SQLITE_IOERR_UNLOCK;
	#else
	// On POSIX, it is possible to downgrade atomically from an exclusive lock to
	// a shared lock. SQLite's unix VFS takes advantage of this. This
	// implementation prioritizes the simplicity of no platform-specific code over
	// being faster in high contention cases.
	base::File::Error error = file_.Unlock();
	if (error != base::File::FILE_OK) {
	vfs_->SetLastError(base::File::GetLastFileError());
	return SQLITE_IOERR_UNLOCK;
	}

	if (mode == SQLITE_LOCK_NONE) {
	sqlite_lock_mode_ = mode;
	return SQLITE_OK;
	}

	DCHECK_EQ(mode, SQLITE_LOCK_SHARED);
	error = file_.Lock(base::File::LockMode::kShared);
	if (error == base::File::FILE_OK) {
	sqlite_lock_mode_ = mode;
	return SQLITE_OK;
	}

	// Gave up the exclusive lock, but failed to get a shared lock.
	vfs_->SetLastError(base::File::GetLastFileError());
	sqlite_lock_mode_ = SQLITE_LOCK_NONE;
	return SQLITE_IOERR_UNLOCK;
	#endif // BUILDFLAG(IS_FUCHSIA)
	}

	int SandboxedVfsFileImpl::CheckReservedLock(int* has_reserved_lock) {
	if (IsExclusiveLockMode(sqlite_lock_mode_)) {
	*has_reserved_lock = 1;
	return SQLITE_OK;
	}

	if (sqlite_lock_mode_ == SQLITE_LOCK_SHARED) {
	// Lock modes at or above RESERVED map to exclusive locks in our simplified
	// implementation. If this process has a shared lock, no other process can
	// have an exclusive lock.
	*has_reserved_lock = 0;
	return SQLITE_OK;
	}

	#if BUILDFLAG(IS_FUCHSIA)
	return SQLITE_IOERR_CHECKRESERVEDLOCK;
	#else
	// On POSIX, it's possible to query the existing lock state of a file. The
	// SQLite unix VFS takes advantage of this. On Windows, this isn't the case.
	// Follow the strategy of the Windows VFS, which checks by trying to get an
	// exclusive lock on the file.
	base::File::Error error = file_.Lock(base::File::LockMode::kShared);
	if (error != base::File::FILE_OK) {
	*has_reserved_lock = 1;
	return SQLITE_OK;
	}

	*has_reserved_lock = 0;
	if (file_.Unlock() == base::File::FILE_OK) {
	return SQLITE_OK;
	}

	// We acquired a shared lock that we can't get rid of.
	sqlite_lock_mode_ = SQLITE_LOCK_SHARED;
	return SQLITE_IOERR_CHECKRESERVEDLOCK;
	#endif // BUILDFLAG(IS_FUCHSIA)
	}

	int SandboxedVfsFileImpl::FileControl(int opcode, void* data) {
	switch (opcode) {
	case SQLITE_FCNTL_MMAP_SIZE:
	// Implementing memory-mapping will require handling this correctly.
	return SQLITE_NOTFOUND;
	default:
	return SQLITE_NOTFOUND;
	}
	}

	int SandboxedVfsFileImpl::SectorSize() {
	return 0;
	}

	int SandboxedVfsFileImpl::DeviceCharacteristics() {
	// TODO(pwnall): Figure out if we can get away with returning 0 on Windows.
	#if BUILDFLAG(IS_WIN)
	return SQLITE_IOCAP_UNDELETABLE_WHEN_OPEN;
	#else
	// NOTE: SQLite's unix VFS attempts to detect the underlying filesystem and
	// sets some flags based on the result.
	return 0;
	#endif // BUILDFLAG(IS_WIN)
	}

	int SandboxedVfsFileImpl::ShmMap(int page_index,
	int page_size,
	int extend_file_if_needed,
	void volatile** result) {
	DCHECK_GE(page_index, 0);
	DCHECK_GE(page_size, 0);
	DCHECK(result);

	// https://www.sqlite.org/wal.html#use_of_wal_without_shared_memory states
	// that SQLite only attempts to use shared memory "-shm" files for databases
	// in WAL mode that may be accessed by multiple processes (are not EXCLUSIVE).
	//
	// Chrome will not only use WAL mode on EXCLUSIVE databases.
	NOTREACHED() << "SQLite should not attempt to use shared memory";
	}

	int SandboxedVfsFileImpl::ShmLock(int offset, int size, int flags) {
	DCHECK_GE(offset, 0);
	DCHECK_GE(size, 0);

	// https://www.sqlite.org/wal.html#use_of_wal_without_shared_memory states
	// that SQLite only attempts to use shared memory "-shm" files for databases
	// in WAL mode that may be accessed by multiple processes (are not EXCLUSIVE).
	//
	// Chrome will not only use WAL mode on EXCLUSIVE databases.
	NOTREACHED() << "SQLite should not attempt to use shared memory";
	}

	void SandboxedVfsFileImpl::ShmBarrier() {
	// https://www.sqlite.org/wal.html#use_of_wal_without_shared_memory states
	// that SQLite only attempts to use shared memory "-shm" files for databases
	// in WAL mode that may be accessed by multiple processes (are not EXCLUSIVE).
	//
	// Chrome will not only use WAL mode on EXCLUSIVE databases.
	NOTREACHED() << "SQLite should not attempt to use shared memory";
	}

	int SandboxedVfsFileImpl::ShmUnmap(int also_delete_file) {
	// https://www.sqlite.org/wal.html#use_of_wal_without_shared_memory states
	// that SQLite only attempts to use shared memory "-shm" files for databases
	// in WAL mode that may be accessed by multiple processes (are not EXCLUSIVE).
	//
	// Chrome will not only use WAL mode on EXCLUSIVE databases.
	NOTREACHED() << "SQLite should not attempt to use shared memory";
	}

	int SandboxedVfsFileImpl::Fetch(sqlite3_int64 offset, int size, void** result) {
	DCHECK_GE(offset, 0);
	DCHECK_GE(size, 0);
	DCHECK(result);

	// NOTE: This would be needed for mmap()ed file support.
	*result = nullptr;
	return SQLITE_IOERR;
	}

	int SandboxedVfsFileImpl::Unfetch(sqlite3_int64 offset, void* fetch_result) {
	DCHECK_GE(offset, 0);
	DCHECK(fetch_result);

	// NOTE: This would be needed for mmap()ed file support.
	return SQLITE_IOERR;
	}

	} // namespace sql