Perl for Web Site Management

Race Conditions

Now let’s jump out of the make_page.cgi script for a moment and see what I did to make_cf.plx to make it play nice in the presence of other copies of itself running at the same time. Before doing so, though, let’s talk for a moment about what might have gone wrong if I didn’t take those precautions.

The make_cf.plx script, you will remember, uses File::Find to descend recursively through the directories of the CyberFair site, reading in all the HTML files and storing their metainformation in a special data structure. Then it traverses that data structure, rewriting all the files to reflect the latest version of the site’s design template and creating links to the other pages in the site.

Now, think for a moment about the following situation: one copy of make_cf.plx starts up, reads all the metainformation, and then begins rewriting pages. At that moment, a second copy starts up and begins reading metainformation. The first copy opens a file for writing, thereby clobbering everything that was previously in it. A moment later, it writes out the new version of that file, with everything updated nicely. But during that very short interval between the file being clobbered and the new version being completely written, the second copy of make_cf.plx comes along and reads the file. Maybe it gets no data because it reads the file while it is still clobbered. Maybe it gets corrupted data because it reads the file when it is only half-written. Either way will result in errors ...