Currently when connection polling fails in WSAPoll/poll the library throws a generic std::runtime_error. This happen in various cases including

When the client keeps an open connection to the PostgreSQL server and the server is restarted. This causes the server to close the connection on its side, so when the client tries to read from the socket it gets POLLHUP (0x0002) or when it tries to write to the socket it gets POLLHUP | POLLERR (0x0003).
When there is a network error, e.g. "no route to host", attempts to write to the socket cause POLLHUP | POLLERR (0x0003).

It makes sense to differentiate network errors like the above and internal errors caused by other reasons. For network errors some kind of retry mechanism could be used, while internal errors should probably cause program termination and/or a crash dump.

This pull request adds a separate network error type which is used when polling yields unexpected event flags, thus allowing differentiation between network error and internal errors.

Codecov Report

Base: 99.65% // Head: 99.65% // No change to project coverage :thumbsup:

Coverage data is based on head (72491b4) compared to base (eba92ef). Patch has no changes to coverable lines.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #67 +/- ## ======================================= Coverage 99.65% 99.65% ======================================= Files 68 68 Lines 2297 2297 ======================================= Hits 2289 2289 Misses 8 8 ``` | [Impacted Files](https://codecov.io/gh/taocpp/taopq/pull/67?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=taocpp) | Coverage Δ | | |---|---|---| | [src/lib/pq/connection.cpp](https://codecov.io/gh/taocpp/taopq/pull/67/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=taocpp#diff-c3JjL2xpYi9wcS9jb25uZWN0aW9uLmNwcA==) | `98.66% <ø> (ø)` | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=taocpp). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=taocpp)

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taocpp / taopq

Use a separate network error type when polling fails because of a network error #67

Codecov Report