Thanks everyone - great responses! Answers all my questions. On Fri, Jun 1, 2018 at 1:00 PM, Eric Blake <ebl...@redhat.com> wrote:
> On 06/01/2018 01:45 PM, John Calcote wrote: > >> I recently ran across some sample code in section 19.4 of the Autoconf >> manual (modified slightly to reduce example): >> >> $(TESTSUITE): $(srcdir)/testsuite.at $(srcdir)/package.m4 >> autom4te --language=autotest -I '$(srcdir)' -o $@.tmp $@.at >> mv $@.tmp $@ >> >> This question isn't about autotest, but rather about the two commands in >> this rule - why generate the output into $@.tmp and then mv $@.tmp into >> $@? >> Is there some power mv has over autom4te that allows it better access to >> the target under some conditions? >> > > Atomicity. autom4te generates its output piecemeal (while it is running, > you can see an incomplete version of $@.tmp); such an incomplete file will > probably fail miserably but in an unpredictable manner when run as a shell > script. As an example of the damage possible with an incomplete script? > Suppose you are generating a shell script that states: > > rm -rf *.tmp > > but due to buffering constraints on stdio, the kernel happens to have > flushed "rm -rf *" to disk (perhaps because it hit a 64k boundary, or so), > but still has ".tmp" buffered up for a later write. In the common case, > executing an incomplete script will hopefully cause a syntax error; but in > cases like I just described, it can cause data loss. > > Thus, we generate into a temporary file (which no one will read in an > intermediate state), then use mv to the final location (which has atomic > semantics - anyone open()ing the file will see either the old inode which > was presumably complete, or the new inode that we just completed), so that > the target file is always a valid and complete file for reading or > execution. > > In fact, the notion of generate to a temporary then move into place is a > rather common idiom. > > -- > Eric Blake, Principal Software Engineer > Red Hat, Inc. +1-919-301-3266 > Virtualization: qemu.org | libvirt.org > _______________________________________________ Autoconf mailing list Autoconf@gnu.org https://lists.gnu.org/mailman/listinfo/autoconf
