comment about reverting sync change

Author: williamstein

src/packages/sync-fs/README.md

@@ -10,7 +10,7 @@ The code here helps with periodically syncing a compute server and the project,
 where the compute server uses unionfs\-fuse combined with websocketfs, and the
 project uses a non\-FUSE fast local filesystem \(e.g., ext4 or zfs\). This
 algorithm will result in the file systems being equal if there is no activity for
-a few seconds. ItϨ's meant to provide a "last on to change the path wins", BUT
+a few seconds. It's meant to provide a "last one to change the file wins", BUT
 with a tolerance of maybe ~10 seconds, especially for deletes.
 
 This does not use inotify or any other file watching because cocalc runs on
@@ -22,6 +22,9 @@ tree is way too much. Instead, when doing sync, we will just walk the tree.
 Running 'find' as a subcommand seems optimal, taking a few KB memory and about
 1s for several hundred thousand files.
 
+
+- TODO: This sync protocol does NOT deal with file permissions, e.g., changing a file to be executable when it wasn't, since that doesn't update the mtime.  See https://github.com/sagemathinc/cocalc/issues/7342
+
 ## ALGORITHM
 
 The actual sync works as follows. For now, we will do this periodically, possibly triggered
@@ -128,3 +131,5 @@ OK, so our protocol instead is that if the time is off by at least 10s \(say\),
 
 - mtime versus ctime.  We do not use ctime at all. We do use mtime, but it is used to decide in which direction to sync files when there is a conflict.  It is NOT used as a threshold for whether or not to copy files at all.  E.g., if you have an old file `a.c` and type `cp -a a.c a2.c` on the compute server, then `a2.c` does still get copied back to the project.
 
+- mtime versus ctime, part 2: To quote the internet: "You cannot change the ctime by ordinary means. This is by design: the ctime is always updated to the current when you change any of the file's metadata, and there is no way to impose a different ctime." -- https://unix.stackexchange.com/questions/36021/how-can-i-change-change-date-of-file/36105#36105
+