Added GIT_ANNEX_VECTOR_CLOCK environment variable

Can be used to override the default timestamps used in log files in the
git-annex branch. This is a dangerous environment variable; use with
caution.

Note that this only affects writing to the logs on the git-annex branch.
It is not used for metadata in git commits (other env vars can be set for
that).

There are many other places where timestamps are still used, that don't
get committed to git, but do touch disk. Including regular timestamps
of files, and timestamps embedded in some files in .git/annex/, including
the last fsck timestamp and timestamps in transfer log files.

A good way to find such things in git-annex is to get for getPOSIXTime and
getCurrentTime, although some of the results are of course false positives
that never hit disk (unless git-annex gets swapped out..)

So this commit does NOT necessarily make git-annex comply with some HIPPA
privacy regulations; it's up to the user to determine if they can use it in
a way compliant with such regulations.

Benchmarking: It takes 0.00114 milliseconds to call getEnv
"GIT_ANNEX_VECTOR_CLOCK" when that env var is not set. So, 100 thousand log
files can be written with an added overhead of only 0.114 seconds. That
should be by far swamped by the actual overhead of writing the log files
and making the commit containing them.

This commit was supported by the NSF-funded DataLad project.

1 files changed, 13 insertions, 0 deletions

diff --git a/doc/git-annex.mdwn b/doc/git-annex.mdwn
index 99f6c9076..c7d0f10da 100644
--- a/doc/git-annex.mdwn
+++ b/doc/git-annex.mdwn
@@ -1467,6 +1467,19 @@ These environment variables are used by git-annex when set:
   Usually it's better to configure any desired options through your
   ~/.ssh/config file, or by setting `annex.ssh-options`.
 
+* `GIT_ANNEX_VECTOR_CLOCK`
+
+  Normally git-annex timestamps lines in the log files committed to the
+  git-annex branch. Setting this environment variable to a number
+  will make git-annex use that rather than the current number of seconds
+  since the UNIX epoch. Note that decimal seconds are supported.
+  
+  This is only provided for advanced users who either have a better way to
+  tell which commit is current than the local clock, or who need to avoid
+  embedding timestamps for policy reasons. Misuse of this environment
+  variable can confuse git-annex's book-keeping, sometimes in ways that
+  `git annex fsck` is unable to repair.
+
 Some special remotes use additional environment variables
 for authentication etc. For example, `AWS_ACCESS_KEY_ID`
 and `GIT_ANNEX_P2P_AUTHTOKEN`. See special remote documentation.