RFR: 8262520: Add CLHSDB command to connect to debug server

[Chris Plummer]

On Mon, 1 Mar 2021 23:55:18 GMT, Yasumasa Suenaga <ysuenaga at openjdk.org> wrote:

>> You'll need a CSR for this.
>> 
>> Will `jhsdb clhsdb --connect debugserver` work? If not, it should to be consistent with the other commands.
>> 
>> Does `detach` undo the `connect`? It's not very symetric if we do since we already had `attach` + `detach` and now you added `connect` + `detach`. Since you can `attach` to a pid or a core file, maybe instead of adding `connect` you should just put the debugd support in `attach`.
>
>> You'll need a CSR for this.
> 
> Ok, I will add it after this discussion ( `coonect` or `attach` )
> 
>> Will `jhsdb clhsdb --connect debugserver` work? If not, it should to be consistent with the other commands.
> 
> It does not work. CLHSDB and HSDB do not accept '--connect'. I plan to add it after this PR.
> 
>> Does `detach` undo the `connect`?
> 
> Yes.
> 
>> It's not very symetric if we do since we already had `attach` + `detach` and now you added `connect` + `detach`. Since you can `attach` to a pid or a core file, maybe instead of adding `connect` you should just put the debugd support in `attach`.
> 
> I thought `connect` command is easy to understand because it is similar with `--connect` option in jhsdb, and also the change is smaller than adding to `attach` because `attach` already has 1 string argument (PID).
> I'm not familiar of network, but I concern hostname which is configured by numeric chars only - I'm not sure it is allowed, but [RFC 1123](https://tools.ietf.org/html/rfc1123) allows digit character at the first.
> 
>       The syntax of a legal Internet host name was specified in RFC-952
>       [DNS:4].  One aspect of host name syntax is hereby changed: the
>       restriction on the first character is relaxed to allow either a
>       letter or a digit.  Host software MUST support this more liberal
>       syntax.
> 
> If digit hostname (e.g. 1234) is allowed, we cannot add debugd support to `attach` because we cannot distinguish it is PID or hostname. If so, it is reasonable to add `connect` command for this purpose.

`jhsdb` has `--pid`, `--core`, and `--connect` as the 3 ways to "attach" for lack of a better word. It would have been nice if the clhsdb command did a better job of copying these three command line arguments. You are suggesting that adding a `connect` command would have similarity to the `--connect` option, but unfortunately the `attach` command has no such similarity with the command line arguments `--pid` and `--core`, so I'm not so sure doing so with `connect` is actually helping in that regard, especially when "connect" and "attach" pretty much mean the same thing.

As for numeric host names, yes, that is possible, but then you could also have a numeric core file name. If we really want to avoid the numeric host name problem, perhaps something like `attachd` would be better than `connect`, but it seems any choice we make will have it's drawbacks due the the baggage of existing commands and options.

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PR: https://git.openjdk.java.net/jdk/pull/2773