Hi Jeffrey,
looks like writing our mails overlapped: https://cygwin.com/pipermail/cygwin/2024-March/255622.html On Mar 6 13:55, Jeffrey Altman via Cygwin wrote: > On 3/6/2024 12:19 PM, Corinna Vinschen via Cygwin wrote: > > We can add an explicit call to > > > > RtlSetProcessPlaceholderCompatibilityMode (PHCM_EXPOSE_PLACEHOLDERS); > > > > and we can recognize the IO_REPARSE_TAG_FILE_PLACEHOLDER and > > IO_REPARSE_TAG_CLOUD_* tags during symlink evaluation, but even then > > we still have to know what the reparse point buffer actually contains. > > > > Given that the content of reparse points with these reparse tags are > > undocumented, some people using cloud services should examine these > > reparse points so we can add some suitable code to Cygwin. > > > > > > Corinna > I'm not an expert in this area by any means but here are my recollections > from when Microsoft presented in-person on cloud placeholders to filter and > filesystem developers many years ago. > > Files and directories that are placeholders should have either the > FILE_ATTRIBUTE_RECALL_ON_DATA_ACCESS or FILE_ATTRIBUTE_RECALL_ON_OPEN file > attributes set. When these attributes are set, applications and mini filters > are advised not to "read" or "open" the files or directories unless they > absolutely need to Per https://learn.microsoft.com/en-us/windows/win32/fileio/file-attribute-constants FILE_ATTRIBUTE_RECALL_ON_OPEN only appears in directory listing classes, but not in standard FILE_BASIC_INFORMATION and alike. That's a bit of a problem considering how we check files during path conversion. The MSDN article doesn't state the same for FILE_ATTRIBUTE_RECALL_ON_DATA_ACCESS, which is good, I think. But I'm a bit puzzled then in terms of FILE_ATTRIBUTE_OFFLINE. Is it not used for OneDrive files? > [...] > I'm not sure that exposing the object as a symlink is a good idea. Yeah, that's what I realized as well, see my aforementioned mail. > Perhaps the question that needs to be asked is whether there are opens that > can be skipped if an object is known to not be locally present (either of > the FILE_ATTRIBUTE flags are set)? This may be the way to go, see my mail. It wouldn't be much of a problem to check all attribute bits, i.e. FILE_ATTRIBUTE_OFFLINE, FILE_ATTRIBUTE_RECALL_ON_DATA_ACCESS and FILE_ATTRIBUTE_RECALL_ON_OPEN. Maybe that's what we should do. Thanks, Corinna -- Problem reports: https://cygwin.com/problems.html FAQ: https://cygwin.com/faq/ Documentation: https://cygwin.com/docs.html Unsubscribe info: https://cygwin.com/ml/#unsubscribe-simple
