transaction: detect an attempt to truncate-to-extend on playback, raise error On some networked filesystems, writes can have delayed finalization/confirmation and write races can occur such that a remote modification will "win" and modifications will be lost. There is no functionality for providing this feedback to userspace programs (in fact, there's not even functionality for providing this information to the Linux kernel...), so these programs may see the files suddenly change. We've noticed that there have been cases where Mercurial has detected something has gone wrong and attempts to abort (rolling back the transaction), which is good. However, when rolling back the transaction, for the append-only files, we attempt to "truncate" the file back to the size it was in before the hg transaction started, but end up *extending* it. This may be harmless, but if this happens to the 00changelog.i file, we get a bunch of nulls on the end of the file and this causes hg to become *really* confused. :) If we detect that some modification of the file outside of this Mercurial process has caused the file to be smaller than the size we are attempting to truncate to, let's just exit and stop trying to clean up the repository - continuing will likely just cause more damage. Differential Revision: https://phab.mercurial-scm.org/D6867

osutil: tolerate Py_GetArgcArgv not being set up properly Differential Revision: https://phab.mercurial-scm.org/D6866

osutil: allow disabling setprocname via a define passed to the compiler In some situations, we run a custom python launcher that appears to not set up Py_GetArgcArgv correctly. We then proceed to promptly crash when we attempt to dereference NULL. Being able to completely disable setprocname is beneficial in these situations, since we won't even attempt to use it, even if the case that causes the crash is fixed. Right now, if I compile osutil.so with -DSETPROCNAME_USE_NONE, the compilation fails on python3 due to SETPROCNAME_USE_NONE redefinition. I could possibly work around that, but it's likely helpful to have a way of disabling this completely without it being brittle (i.e. if python3 ever gains the ability to perform this operation). Differential Revision: https://phab.mercurial-scm.org/D6865