pgdb: cover transactions with inTransaction function to simplify error handling

[rumyantseva]

Description

Note: This PR might be hard to review, as a lot of lines of the pgdb package were changed. Let me know how I can help you with reviewing this PR.

There is no special issue for this fix, but it's done as a part of ref #372.

inTransaction

As now we use transactions in many places, handling potential errors (when a transaction begins, commits or rollbacks) takes quite a lot of lines of code (:D) and requires some accuracy.

To simplify this approach, this PR offers an inTransaction function to "wrap" any function with a transaction.

Querier

pgxtype.Querier is introduced instead of pgx.Tx to unify function signatures.

Dealing with errors

As some of the pgdb errors might be wrapped, the comparison of errors was refactored to errors.Is and errors.As instead of ==.

Not in scope

This PR only offers a refactoring for the existing transactions, adding new transactions is not in the scope of this PR.

Readiness checklist

• I set assignee, reviewers, labels, project and sprint.
• I added tests for new functionality or bugfixes.
• I ran task all, and it passed.
• I added/updated documentation for exported and unexported functions, variables, types, etc.
• I checked complex documentation rendering with bin/godoc -index -play -timestamps -http=127.0.0.1:6060.

[codecov]

Codecov Report

Merging #833 (8e003c9) into main (22dcc23) will increase coverage by 0.26%.
The diff coverage is 64.62%.

@@            Coverage Diff             @@
##             main     #833      +/-   ##
==========================================
+ Coverage   59.10%   59.37%   +0.26%     
==========================================
  Files         218      218              
  Lines        9855     9833      -22     
==========================================
+ Hits         5825     5838      +13     
+ Misses       3320     3296      -24     
+ Partials      710      699      -11     

Impacted Files	Coverage Δ
internal/util/testutil/pg.go	40.74% <0.00%> (ø)
internal/handlers/pg/pgdb/pool.go	64.25% <63.88%> (+6.37%)	⬆️
internal/handlers/pg/pgdb/settings.go	37.20% <73.68%> (ø)
internal/handlers/pg/msg_create.go	69.35% <75.00%> (ø)
internal/handlers/pg/msg_drop.go	61.76% <100.00%> (ø)
internal/handlers/pg/msg_dropdatabase.go	62.06% <100.00%> (ø)
internal/handlers/pg/msg_dbstats.go	65.11% <0.00%> (-6.98%)	⬇️
internal/util/version/version.go	55.00% <0.00%> (-5.00%)	⬇️

Flag	Coverage Δ
integration	61.79% <61.32%> (+0.24%)	⬆️
mongodb	18.92% <7.54%> (+0.12%)	⬆️
pg	61.72% <61.32%> (+0.24%)	⬆️
unit	24.99% <34.43%> (+0.50%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

[rumyantseva]

@AlekSi @w84thesun please take a look at this quick fix. Let's not log errors if there are no errors (otherwise, the logs look quite confusing).

[w84thesun]

I think inTransaction method makes it difficult to read the code, as you need to keep it in mind when you read other methods code. Maybe introducing function like finishTx(tx pgx.Tx, err error, method string) would be enough?

[AlekSi]

#844 was merged, please resolve merge conflicts and replace a few more == with errors.Is

[w84thesun]

LGTM.