Re: newbie questions about git design and features (some wrt hg)

Theodore Tso wrote:

 -- Macro: int O_APPEND
     The bit that enables append mode for the file.  If set, then all
     `write' operations write the data at the end of the file, extending
     it, regardless of the current file position.  This is the only
     reliable way to append to a file.  In append mode, you are
     guaranteed that the data you write will always go to the current
     end of the file, regardless of other processes writing to the
     file.  Conversely, if you simply set the file position to the end
     of file and write, then another process can extend the file after
     you set the file position but before you write, resulting in your
     data appearing someplace before the real end of file.

I don't quote understand how that would help hg (Mercurial) to have operations like commit, pull/fetch or push atomic, i.e. all or nothing. In hg you have to update individual files (blobs buckets) storing delta and perhaps full version, update manifest file (flat tree) and update changelog (commit): what happens if for example there are two concurrent operations trying to update repository, e.g. two push operations in parallel (from two different developers), or fetch from cron and commit? What happens if operation is interrupted (e.g. lost connection to network during fetch)?

In git both situations result in some prune-able and fsck-visible crud in repository, but repository stays uncorrupted, and all operations are atomic (all or nothing).