Go update all modules - go

Question

Using this module as an example (using a specific commit so others will see
what I see):
git clone git://github.com/walles/moar
Set-Location moar
git checkout d24acdbf
I would like a way to tell Go to "update everything". Assume that the module
will work with the newest version of everything. Below are five ways I found to
do this, assume each is run on a clean clone. This results in a go.mod of 19
lines:
go get -u
This results in a go.mod of 14 lines:
go get -u
go mod tidy
This results in a go.mod of 13 lines:
go mod tidy
If I just manually delete everything in require and run go mod tidy, I get
12 lines. If I just manually delete everything in require and run go get -u, I get 11 lines. My question is, why are these methods producing different
results, and what is the "right way" to do what I am trying to do?

Answer 1

tl;dr;
this is what you want:
go get -u
go mod tidy
and to recursively update packages in any subdirectories:
go get -u ./...
The inconsistencies you are seeing is due to the inherent organic nature of software.
Using your example, commit d24acdbf of git://github.com/walles/moar most likely was checked in by the maintainer without running go mod tidy (explaining the longer 19 lines). If the maintainer had, then you would see the 13 line version you see at the end.
go get -u on it's own is more aggressive in pulling in dependencies. Also, the mere fact of updating dependencies to their latest (compatible) version, may in & of itself pull in new direct/indirect dependencies. These dependencies may grow even further if you tried this tomorrow (the latest version of some sub-dependency adds new functionality, so it needs new dependencies). So there may be a valid reason the repo maintainer fixes at a particular (non-latest) version.
go mod tidy cleans up this aggressive dependency analysis.
P.S. It's a common misconception that dependencies will shrink after go mod tidy: tracking go.sum, in some cases this file will grow after a tidy (though, not in this case)

Answer 2

Run go get -u && go mod tidy 1
More details:
go get -u (same as go get -u .) updates the package in the current directory, hence the module that provides that package, and its dependencies to the newer minor or patch releases when available. In typical projects, running this in the module root is enough, as it likely imports everything else.
go get -u ./... will expand to all packages rooted in the current directory, which effectively also updates everything (all modules that provide those packages).
Following from the above, go get -u ./foo/... will update everything that is rooted in ./foo
go get -u all updates everything including test dependencies; from Package List and Patterns
When using modules, all expands to all packages in the main module and their dependencies, including dependencies needed by tests of any of those.
go get will also add to the go.mod file the require directives for dependencies that were just updated.
go mod tidy makes sure go.mod matches the source code in the module. In your project it results in 12 lines because those are the bare minimum to match the source code.
go mod tidy will prune go.sum and go.mod by removing the unnecessary checksums and transitive dependencies (e.g. // indirect), that were added to by go get -u due to newer semver available. It may also add missing entries to go.sum.
Note that starting from Go 1.17, newly-added indirect dependencies in go.mod are arranged in a separate require block.
1: updates dependencies' newest minor/patch versions, go.mod, go.sum

Answer 3

There are some hidden dragons, here is what I recommend:
go get -u ./... walks all packages in your project. This is the command you want to use.
go get -t -u ./... walks all packages in your project and also downloads tests files of these dependencies. Probably you don’t need that.
go get -u updates in the current directory only. Useful for small single-package projects, just use the first version.
go get -u specific.com/package updates just one (or more separated by space) packages (and dependencies).
go get -u specific.com/package#version the same but to a specific version.
go get -u all updates modules from the build list from go.mod. This is useful for listing (go list -m -u all) but not too useful for updates.