[XrdTpc] Fix order of chaining when parsing xrootd.fslib.

[bbockelm]

Add comments to prevent future confusion over how this line is parsed.

Prior to this, a xrootd.fslib line that specifies a plugin chain:

xrootd.fslib /usr/lib64/libXrdMultiuser.so default

would not actually create the base default plugin.

[bbockelm]

@simonmichal - I think Fabrizio might be on vacation. Would you be able to review this PR?

[ffurano]

Hi, I am around, yet a bit spotty.
This PR seems to be a fix to XrdTPC to make it able to load correctly a chain of Sfs plugins. The fix seems small and clean in a much larger environment. I may understand what this is for, however I miss a properly malicious way to do a meaningful test involving the other libs that are mentioned.
I would accept it just because it makes sense to me.

[abh3]

Hi Fabrizio, Not to worry. It got merged as it, as you said, was localized and seemed clean. Andy

…

On Wed, 8 Aug 2018, Fabrizio Furano wrote: Hi, I am around, yet a bit spotty. This PR seems to be a fix to XrdTPC to make it able to load correctly a chain of Sfs plugins. The fix seems small and clean in a much larger environment. I may understand what this is for, however I miss a properly malicious way to do a meaningful test involving the other libs that are mentioned. I would accept it just because it makes sense to me. -- You are receiving this because you are subscribed to this thread. Reply to this email directly or view it on GitHub: #783 (comment)

[bbockelm]

Hi,

Yup - this addresses a bug that we discovered when testing the multiuser plugin at Florida. To reproduce, add:

http.exthandler xrdtpc libXrdHttpTPC.so
xrootd.fslib libXrdMultiuser.so default

I think this would also reproduce:

http.exthandler xrdtpc libXrdHttpTPC.so
xrootd.fslib libXrdThrottle.so default

(i.e., chain with the throttle plugin but do not use the xrootd.fslib throttle default abbreviation)

Regardless, thanks for the merge!

Brian