I am trying to detect when Dropbox is busy updating a file in a user's Dropbox in Mac OS X. By running stat(1) or stat(2) on a file, I see that the user defined flags for file (st_flags) are normally 0x40, but while Dropbox is updating the file they change to 0x00 for a couple seconds.
Poking around my Desktop and other folders, I see that ~95% of files have flags value 0x00 but about 5% have value 0x40. So it may be not just a Dropbox implementation detail. I cannot discern any pattern for predicting which files have 0x40 flags. Does anyone know what these values mean? Who is the "user" that is defining them?
Well even though Martin essentially answered this with a very educated guess, I'm entering this as an answer because it's too long to enter as a comment.
Here is the evidence…
• Indeed, 10.7 is when Auto Save and Versions was introduced, so that's why you don't see UF_TRACKED in stat.h in 10.6.
• I tried my experiment on a Mac running Mac OS X 10.7 and it behaves the same as in 10.8.
• There is a pattern: Document files created by some applications, which after examining a few appear to be those which have adopted Auto Save and Versions, are the ones which have the UF_TRACKED = 0x40.
• Another experiment. I renamed the revision daemon executable in Mac OS X,
/System/Library/PrivateFrameworks/GenerationalStorage.framework/Versions/A/Support/revisiond
then restarted the Mac, and monitored the UF_TRACKED state of a document file in Dropbox which had 0x40. I then changed the file on another Mac, so that Dropbox would push it to this Mac with the revision daemon disabled. Result: The UF_TRACKED state of the file changed from 0x40 to 0x00, but this time it did not change back to 0x40 after 2 seconds.
• It did change to 0x40 30 seconds later, after I restored the revision daemon to its original name, and it relaunched. (Apparently revisiond is started by launchd with a KeepAlive attribute.)
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So the evidence is overwhelming that Martin's guess is correct. It is Apple's revision daemon and not Dropbox which is setting UF_TRACKED to 0x40. The meaning of this bit is that its document revision is being tracked by Lion Auto Save and Versions.
The flags can be set with the chflags
command line tool or the chflags()
system call (see man 2 chflags). The values can be found in "/usr/include/sys/stat.h".
UF_TRACKED
seems to be a bit special. It is documented in "sys/stat.h"
#define UF_TRACKED 0x00000040 /* file renames and deletes are tracked */
but not in the "chflags" man-page.
Unfortunately, I cannot tell you what "tracked" precisely means, but perhaps this helps already to find more information.
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