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# Gitaly code style
## Errors
### Use %v when wrapping errors
Use `%v` when wrapping errors with context.
fmt.Errorf("foo context: %v", err)
### Keep errors short
It is customary in Go to pass errors up the call stack and decorate
them. To be a good neighbor to the rest of the call stack we should keep
our errors short.
// Good
fmt.Errorf("peek diff line: %v", err)
// Too long
fmt.Errorf("ParseDiffOutput: Unexpected error while peeking: %v", err)
### Use lower case in errors
Use lower case in errors; it is OK to preserve upper case in names.
### Errors should stick to the facts
It is tempting to write errors that explain the problem that occurred.
This can be appropriate in some end-user facing situations, but it is
never appropriate for internal error messages. When your
interpretation is wrong it puts the reader on the wrong track.
Stick to the facts. Often it is enough to just describe in a few words
what we were trying to do.
### Use %q when interpolating strings
Unless it would lead to incorrect results, always use `%q` when
interpolating strings. The `%q` operator quotes strings and escapes
spaces and non-printable characters. This can save a lot of debugging
time.
## Return statements
### Don't use "naked return"
In a function with named return variables it is valid to have a plain
("naked") `return` statement, which will return the named return
variables.
In Gitaly we don't use this feature. If the function returns one or
more values, then always pass them to `return`.
## Tests
### Table-driven tests
We like table-driven tests ([Table-driven tests using subtests](https://blog.golang.org/subtests#TOC_4.), [Cheney blog post], [Golang wiki]).
- Use [subtests](https://blog.golang.org/subtests#TOC_4.) with your table-driven tests, using `t.Run`:
```
func TestTime(t *testing.T) {
testCases := []struct {
gmt string
loc string
want string
}{
{"12:31", "Europe/Zuri", "13:31"},
{"12:31", "America/New_York", "7:31"},
{"08:08", "Australia/Sydney", "18:08"},
}
for _, tc := range testCases {
t.Run(fmt.Sprintf("%s in %s", tc.gmt, tc.loc), func(t *testing.T) {
loc, err := time.LoadLocation(tc.loc)
if err != nil {
t.Fatal("could not load location")
}
gmt, _ := time.Parse("15:04", tc.gmt)
if got := gmt.In(loc).Format("15:04"); got != tc.want {
t.Errorf("got %s; want %s", got, tc.want)
}
})
}
}
```
[Cheney blog post]: https://dave.cheney.net/2013/06/09/writing-table-driven-tests-in-go
[Golang wiki]: https://github.com/golang/go/wiki/TableDrivenTests
## Stubs
Stubs should be put in the file they're expected to end up in and not in `server.go`.
So for example `BlobService::GetBlob` should end up in `internal/service/blob/get_blob.go`.
This is to guard against merge conflicts, and to make it easier to find.
To minimize diffs (and things to review in MRs) we implement the stubs as if it were
being used, even though it isn't, and should end up something like this:
```
func (s *server) GetBlob(in *pb.GetBlobRequest, stream pb.BlobService_GetBlobServer) error {
return helper.Unimplemented
}
```
instead of:
```
func (server) GetBlob(_ *pb.GetBlobRequest, _ pb.BlobService_GetBlobServer) error {
return helper.Unimplemented
}
```
## Black box and white box testing
The dominant style of testing in Gitaly is "white box" testing, meaning
test functions for package `foo` declare their own package also to be
`package foo`. This gives the test code access to package internals. Go
also provides a mechanism sometimes called "black box" testing where the
test functions are not part of the package under test: you write
`package foo_test` instead. Depending on your point of view, the lack of
access to package internals when using black-box is either a bug or a
feature.
As a team we are currently divided on which style to prefer so we are
going to allow both. In areas of the code where there is a clear
pattern, please stick with the pattern. For example, almost all our
service tests are white box.
## Prometheus metrics
Prometheus is a great tool to collect data about how our code behaves in
production. When adding new Prometheus metrics, please follow the [best
practices](https://prometheus.io/docs/practices/naming/) and be aware of
the
[gotchas](https://prometheus.io/docs/practices/instrumentation/#things-to-watch-out-for).
|
