Normally, chg determines which
hg executable to use by first consulting the
$HG environment variables, and if neither are present defaults
hg found in the user's
$PATH. If built with the
flag, chg will instead assume that there exists an
hg executable in the same
directory as the
chg binary and attempt to use that.
This can cause problems in situations where there are multiple actively-used
Mercurial installations on the same system. When a
chg client connects to a
running command server, the server process performs some basic validation to
determine whether a new command server needs to be spawned. These checks include
things like checking certain "sensitive" environment variables and config
sections, as well as checking whether the mtime of the extensions, hg's
__version__.py module, and the Python interpreter have changed.
Crucially, the command server doesn't explicitly check whether the executable it
is running from matches the executable that the
chg client would have
otherwise invoked had there been no existing command server process. Without
HGPATHREL, this still gets implicitly checked during the validation step,
because the only way to specify an alternate hg executable (apart from
is via the
$HG environment variables, both of which are checked.
HGPATHREL, however, the command server has no way of knowing which hg
executable the client would have run. This means that a client located at
/version_B/bin/chg will happily connect to a command server running
/version_A/bin/hg instead of
/version_B/bin/hg as expected. A simple
solution is to have the client set
$CHGHG itself, which then allows the
command server's environment validation to work as intended.
I have tested this manually using two locally built hg installations and it
seems to work with no ill effects. That said, I'm not sure how to write an
automated test for this since the
chg available to the tests isn't even built
HGPATHREL compiler flag to begin with.