[pca] multi-system download concurrency

[Martin Paul]

Dani Calloway wrote:
> I wish to use PCA to cache patches for a large number of (not
> necessarily homogeneous) systems at once, all of which are behind a
> firewall which does not allow access to the "outside".  So I currently
> use PCA's excellent --from-file feature and run the --download one
> system at a time.

In such an environment a local caching proxy (pca-proxy.cgi) might be a 
real winner. If the restricted machines can access another local machine 
(which can access sunsolve), you wouldn't even need from-files anymore.

> So, the question is whether PCA is threadsafe. That is, can I fork off
> four or five or ten instances at once without worrying that the patch
> downloads will become corrupted, should more than one instance of PCA
> attempt to download the same file at the same time?

Yes, this should work. pca uses lock files to make sure only one 
instance of pca is downloading files into the same directory.

> Also, somewhat unrelated, should a patch become corrupt, is there a
> mechanism for detecting that (GPG signature or SHA1 or similar)?

No. Sun doesn't provide checksums etc. for the zip files. I have thought 
about using the jar version of the files, but had some problems 
(crashes) when trying to verify these files, so it didn't seem stable 
enough to be used within pca for me. Still, you usually will notice that 
a file is corrupt when it fails to unzip during patch installation. pca 
will check the return code of "unzip" to see if the zip file is correct 
and complete.

Martin.