It looks like lockfile implementation did still suffer from a race
condition. Only when creating the file with O_EXCL, file creation fails
if the file does already exist. One could use O_EXCL, but Windows
might use another flag to achieve the same. Moreover, O_EXCL doesn't
work as expected if the files are accessed over NFS.
In contrast, mkdir() is atomic in every of the mentioned cases.
Aside from using a lockdirectory, this patch also backups the current
wallet.dat contents to wallet.bck in case writing a possibly hefty
amount of JSON to the new wallet.dat fails. It is hoped that at least
one of the .bck and .dat files has usable contents in case failure.
Severus Sneep
8227ce941a