LevelDB-course-Lab

History

costan 296de8d5b8 leveldb: Fix PosixWritableFile::Sync() on Apple systems. Apple doesn't follow POSIX specifications for fsync(). Instead, fsync() guarantees to flush the buffer cache to the device, which means the data will survive kernel panics, but may not survive power outages. Applications that need stronger guarantees (like databases) need to use fcntl(F_FULLFSYNC). This CL switches PosixWritableFile::Sync() to get the stronger guarantees on Apple systems. The improved implementation follows the same principles as SQLite [1] and node.js [2]. Research for the fcntl() to fsync() fallback strategy: Apple's released source code at https://opensource.apple.com/ shows at least three different error codes being returned when a filesystem does not support F_FULLFSYNC. fcntl() is implemented in xnu-4903.221.2 in bsd/kern/kern_descrip.c, where it delegates to fcntl_nocancel(). The documentation for fcntl_nocancel() mentions error codes for some operations, but does not include F_FULLFSYNC. The F_FULLSYNC branch in fcntl_nocancel() calls VNOP_IOCTL(_, F_FULLSYNC, NULL, 0, _), whose return value sets the error code. VNOP_IOCTL() is implemented in bsd/vfs/kpi_vfs.c and calls the ioctl function in the vnode's operation vector. The per-filesystem function names follow the pattern _vnop_ioctl() for all the instances in opensource code: {hfs,msdosfs,nfs,ntfs,smbfs,webdav,zfs}_vnop_ioctl(). hfs-407.30.1, msdosfs-229.200.3, and nfs in xnu-4903.221.2 handle F_FULLFSYNC. ntfs-94.200.1 and smb-759.40.1 do not handle F_FULLFSYNC, and the default branch returns ENOSUP. webdav-380.200.1 also does not handle F_FULLFSYNC, but the default branch returns EINVAL. zfs-59 also does not handle F_FULLSYNC, and its default branch returns ENOTTY. From a different angle, Apple's ntfs-94.200.1 includes utility code that uses fcntl(F_FULLFSYNC) and falls back to fsync() just like we do, supporting the hypothesis that there is no good way to detect lack of F_FULLFSYNC support. Also, Apple's fcntl() man page [3] does not mention a way to detect lack of F_FULLFSYNC support. [1] https://www.sqlite.org/src/doc/trunk/src/os_unix.c [2] https://github.com/libuv/libuv/blob/master/src/unix/fs.c [3] https://developer.apple.com/library/archive/documentatiVon/System/Conceptual/ManPages_iPhoneOS/man2/fcntl.2.html Tested: https://travis-ci.org/pwnall/leveldb/builds/477318498 TAP global presubmit ------------- Created by MOE: https://github.com/google/moe MOE_MIGRATED_REVID=228593729			6 years ago
..
win	reverting disastrous MOE commit, returning to r21	删除	13 years ago
README	Replace port_posix with port_stdcxx.		6 years ago
atomic_pointer.h	Take <atomic> for granted in port/atomic_pointer.h.		6 years ago
port.h	Replace port_posix with port_stdcxx.		6 years ago
port_config.h.in	leveldb: Fix PosixWritableFile::Sync() on Apple systems.		6 years ago
port_example.h	Remove InitOnce from the port API.		6 years ago
port_stdcxx.h	Remove InitOnce from the port API.		6 years ago
thread_annotations.h	Enable thread safety annotations in open source version.		6 years ago

README

This directory contains interfaces and implementations that isolate the
rest of the package from platform details.

Code in the rest of the package includes "port.h" from this directory.
"port.h" in turn includes a platform specific "port_<platform>.h" file
that provides the platform specific implementation.

See port_stdcxx.h for an example of what must be provided in a platform
specific header file.