[pfc] Http Cache-Control Validation

[alja]

This code adds cache-control check using File::Fctnl and FileSystem::Query.
The change is related to pelican-client code PelicanPlatform/xrdcl-pelican#59

[alja]

@osschar @bbockelm
The validation has to be optional.

Should I introduce pfc.cache-validation parameter?
or
pfc.http-cache-control-validation ?

[alja]

Test

Check cache control values with curl -I option from origin

alja@green:~/xrd-dev/test$ curl -I http://xrd-cache-1.t2.ucsd.edu/alja/sap.png

HTTP/1.1 200 OK

Date: Wed, 02 Apr 2025 20:44:28 GMT

Server: Apache/2.4.57 (AlmaLinux) OpenSSL/3.0.7 mod_perl/2.0.12 Perl/v5.32.1

Last-Modified: Wed, 02 Apr 2025 19:00:04 GMT

ETag: "8a6-631d042d52fef"

Accept-Ranges: bytes

Content-Length: 2214

Cache-Control: max-age=2592000, must-revalidate, public

Expires: Fri, 02 May 2025 20:44:28 GMT

Content-Type: image/png

Get the file in cache with proxy forwarding

curl http://localhost:7777//http://xrd-cache-1.t2.ucsd.edu/alja/dada.txt

Check saved cache-control attributes as file system xattr in the cinfo file

alja@green:~/xrd-dev$ getfattr -d  data/alja/xstat/sap.png.cinfo
file: data/alja/xstat/sap.png.cinfo
user.pfc.cache-control="{\"ETag\":\"8a6-6342d5a885963\",\"expire\":1749655301,\"revalidate\":true}"
user.pfc.fsize=0spggAAAAAAAA=

[osschar]

First pass, I was also remembering things as I went so maybe some comments are silly.

[osschar]

this documentation needs to be changed, it is mostly copy of Fstat.
i'm not sure if saying that it defaults to passthrough makes sense ... we need to see if there will be some Fcntl calls that need to be handled by the cache / open-file / IO.

[osschar]

why does it have to go to-string and then from-string? it's internal XrdCl enum/int.