4b5a64f467
FILE_FLAG_BACKUP_SEMANTICS is necessary to open directories on Windows, and to enable backup applications do extended operations on files if they hold the SE_BACKUP_NAME and SE_RESTORE_NAME privileges. os.OpenFile currently sets FILE_FLAG_BACKUP_SEMANTICS for all supported cases except when the file is opened with O_WRONLY | O_RDWR (that is, access mode 3). This access mode doesn't correspond to any of the standard POSIX access modes, but some OSes special case it to mean different things. For example, on Linux, O_WRONLY | O_RDWR means check for read and write permission on the file and return a file descriptor that can't be used for reading or writing. On Windows, os.OpenFile has historically mapped O_WRONLY | O_RDWR to a 0 access mode, which Windows internally interprets as FILE_READ_ATTRIBUTES. Additionally, it doesn't prepare the file for I/O, given that the read attributes permission doesn't allow reading or writing (not that this is similar to what happens on Linux). This makes opening the file around 50% faster, and one can still use the handle to stat it, so some projects have been using this behavior to open files without I/O access. This CL updates os.OpenFile so that directories can also be opened without I/O access. This effectively closes #23312, as all the remaining cases where we don't set FILE_FLAG_BACKUP_SEMANTICS imply opening with O_WRONLY or O_RDWR, and that's not allowed by Unix's open. Closes #23312. Change-Id: I77c4f55e1ca377789aef75bd8a9bce2b7499f91d Reviewed-on: https://go-review.googlesource.com/c/go/+/673035 Reviewed-by: Damien Neil <dneil@google.com> Reviewed-by: Michael Knyszek <mknyszek@google.com> LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> |
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