Re: [PATCH] Allow Postgres to pick an unused port to listen

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От Yurii Rashkovskii
Тема Re: [PATCH] Allow Postgres to pick an unused port to listen
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Msg-id CA+RLCQwYw-Er-E_RGNCDfA514w+1YL8HGhNstxX=y1gLAABFdA@mail.gmail.com
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Ответ на Re: [PATCH] Allow Postgres to pick an unused port to listen  (Tom Lane <tgl@sss.pgh.pa.us>)
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Hi Tom,

Thank you for your feedback. Below are my comments.

On Wed, Mar 29, 2023 at 6:55 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
Yurii Rashkovskii <yrashk@gmail.com> writes:
> I would like to suggest a patch against master (although it may be worth
> backporting it) that makes it possible to listen on any unused port.

I think this is a bad idea, mainly because this:

> Instead, with this patch, one can specify `port` as `0` (the "wildcard"
> port) and retrieve the assigned port from postmaster.pid

is a horrid way to find out what was picked, and yet there could
be no other.

Can you elaborate on why reading postmaster.pid is a horrid way to discover the port, given that it is a pretty simple format with a fixed line number for the port?


Our existing design for this sort of thing is to let the testing
framework choose the port, and I don't really see what's wrong
with that approach.  Yes, I know it's theoretically subject to
race conditions, but that hasn't seemed to be a problem in
practice.  It's especially not a problem given that modern
testing practice tends to not open any TCP port at all, just
a Unix socket in a test-private directory, so that port
conflicts are a non-issue.

I keep running into this race condition nearly daily, which is why I proposed to address it with this patch. Yes, I know that one can get around this with UNIX sockets,
but they have limited capabilities (not accessible outside of the local machine, to begin with). Here's a real-world example of why I need to be able to use TCP ports:

I have an extension that allows managing the lifecycle of [Docker/OCI] containers, and it passes Postgres connection details to these containers as environment variables. 
These containers can now connect to Postgres using any program that can communicate using the wire protocol. I test this functionality in an automated test that is executed
concurrently with others. Testing that the extension can supply the correct connection information to the containers is important.

If we forget the importance of testing this specific part of the functionality for a bit, the rest of my issue can be _theoretically_ resolved by passing the UNIX socket in `PGHOST` instead.

However, it won't work in a typical Docker Desktop for macOS setup as it utilizes a virtual machine, and therefore, I can't simply use a UNIX socket between them.

Here's an example:

1. Create a UNIX socket listener:

```
socat unix-l:test.sock,fork system:'echo hello'
```

2. Verify that it works locally:

```
$ socat test.sock -
hello
```

3. Now, while being on macOS and using Docker Desktop, let Docker mount the directory with the socket and try to connect it from there:

``` 
$ docker run -v /path/to/sock/dir:/sock -ti ubuntu bash
# apt update && apt install -y socat
# socat /sock/test.sock -
2023/03/29 23:34:48 socat[451] E connect(5, AF=1 "/sock/test.sock", 17): Connection refused
```

I get that the UNIX socket around works for many cases, but it does not work for mine. Hence the proposal. Allowing a (fairly common) practice of a wildcard port with the discovery of it via
postmaster.pid resolves all the above concerns without having to resort to a rather race-condition-prone way to pick a port (or a complicated way to do so with proper locking). 

--

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