PostgreSQL Service Name Enhancement - Wildcard support for LDAP/DNS lookup

Hi,

I am looking to patch the LDAP Service Name feature and would like some feedback prior to doing so. In general, while I personally view this as a bug fix, it could also easily be considered to simply be an enhancement to current functionality and therefore the below is written as a proposed enhancement.

Note: This will also potentially serve as my first patch submission, so this will be a "smoke test" of my direct interaction with the mailing list as a Goldman Sachs Employee for proposal/feedback as well as following up with a potential patch/participation in commit processes, etc. As such my replies and related code submissions may be delayed for processes to happen on my end, with my associated apologies in advance. I hope to have this process completed for contribution to the 9.5 release.

################## Currently:

* Service names are provided as an available abstraction mechanism for a host:port (and user, dbname, ssl requirements, etc.) information for connection parameters via libpq.

* If service names are used, they must each individually have an entry in pg_service.conf.

* In a service name entry, host:port entries, hosts can either be directly provided or their values may be referenced via an LDAP location that provides equivalent name/value pairs for connection information.

The service name feature facilitates the use of dynamic/managed host entries or allows for active connection configuration management with the goal in either case of separating such overhead from application or other process configuration.

Therefore:

* Dynamic DNS entries require a fixed port or active management of the file (DNS resolution can happen elsewhere)

* Non-Dynamic DNS entries require active management of pg_service.conf, but may still be desirable for configuration management reasons - version control?

* LDAP entries are largely static, but do require active management of pg_service.conf if new data servers are to be added (or old ones decommissioned)

In all cases described above, a newly provisioned data server requires a new service name entry, which requires the file to be actively managed. This mitigates the value of using LDAP or Dynamic DNS entries (with fixed ports in the DNS case) since an active distribution mechanism must still be in place, especially for deployments of many servers and perhaps this useful mechanism is used less often as a result.

################## Proposal:

* Have a wildcard entry that allows string substitution of the service name into either the host field or an LDAP record.

I do not want to over complicate the existing configuration file or parsing logic as it is pretty lightweight currently. As such, I am looking for feedback related to if it would be acceptable to have a "wildcard" entry for service names as well as what that wildcard should be. I do *not* propose that complex string substitution, including regular expressions, etc. be utilized for service name matching at this time.

Per 31.16 in the 9.4 Documentation, a valid service entry is of the following format:

# comment

[mydb]

host=somehost.domain.com

port=5433

user=admin

Would specifying a special value for the service name, perhaps [%], be an acceptable implementation of this enhancement/fix to my above concerns?

Example:

# comment

[%]

host=%.domain.com

port=5433

user=admin

...or more interestingly, per 31.17:

# comment

[%]

ldap=ldap://ldap.mycompany.com/dc=mycompany,dc=com?uniqueMember?one?(cn=%)

The location of [%] and therefore order of the service names would still be honored. I believe this allows for useful functionality:

* LDAP wildcard listed first - libpq can always try an LDAP lookup and proceed with further service names if one is not found in LDAP location

* LDAP wildcard listed last - only look up LDAP entries if a service name doesn't match prior service names

* LDAP wildcard in the middle - combination of first 2 behaviors

* DNS wildcard listed last - try connecting with service name in well-formed dns-based host entry (this likely can't be first as it would always match)

* Combination of the LDAP/keyword = value entries per 31.17:

From the existing documentation: "Processing of pg_service.conf is terminated after a successful LDAP lookup, but is continued if the LDAP server cannot be contacted. This is to provide a fallback with further LDAP URL lines that point to different LDAP servers, classical keyword = value pairs, or default connection options. If you would rather get an error message in this case, add a syntactically incorrect line after the LDAP URL."

The "%" in the ldap, host entry would replace with the service name in use - also possible to make it less likely to match a future valid "%" by providing it as [%] in the string, though I don't know why a % would reasonably show up in an ldap or host record.

I have not made the patch yet, but it looks like this change is isolated to the parseServiceFile subroutine, currently starting on line 3867 in https://github.com/postgres/postgres/blob/master/src/interfaces/libpq/fe-connect.c (and associated documentation).

################## Scope:

Note that this discussion is scoped only for LDAP directory services and not for LDAP authentication/authorization. Again, I do *not* propose that complex string substitution, including regular expressions, etc. be utilized for service name matching at this time. Just looking for a simple [%] entry, which could be extended for flexibility in a consistent way, if people find this general case useful.

I am looking forward to feedback and our first interaction on the distribution list.

Thanks,

Bryan