cmd/godoc: add initial support for module mode

This change implements initial support for viewing Go package
documentation with godoc in module mode. There are no UI changes.

When running the godoc binary in a directory where module mode
is active (i.e., go env GOMOD reports a non-empty path), the
documentation will be shown for packages provided by modules
rather than from the GOPATH workspace.

The mode can be controlled in the same way as the go command, by
changing the GO111MODULE environment variable¹ value. For example,
'GO111MODULE=on godoc' will force godoc to run in module mode, and
'GO111MODULE=off godoc' will force godoc to run in GOPATH mode.

It is implemented by reusing the existing virtual filesystem
abstraction. The main module and all of its dependencies (in
other words, the build list²) are determined by invoking the
go list -m all command in the same directory. An attempt is
made to fill the module cache with any selected module versions
that are not already in the local module cache.

This behavior can be controlled in the same way as the go command,
by setting the GOPROXY environment variable. For example, setting
GOPROXY=off disables downloading of any modules. If any of the
modules could not be fetched, it is printed to stderr and
documentation is shown for all other available packages.

¹ https://golang.org/cmd/go/#hdr-Module_support
² https://golang.org/cmd/go/#hdr-The_main_module_and_the_build_list

Fixes golang/go#33655

Change-Id: I86f795537b65acae3771afd19d2e7cb360425467
Reviewed-on: https://go-review.googlesource.com/c/tools/+/196983
Reviewed-by: Andrew Bonventre <andybons@golang.org>
2 files changed
tree: f35402e07afca0439babc978b7bbd8becfdb7dce
  1. benchmark/
  2. blog/
  3. cmd/
  4. container/
  5. cover/
  6. go/
  7. godoc/
  8. gopls/
  9. imports/
  10. internal/
  11. playground/
  12. present/
  13. refactor/
  14. .gitattributes
  15. .gitignore
  16. AUTHORS
  17. codereview.cfg
  18. CONTRIBUTING.md
  19. CONTRIBUTORS
  20. go.mod
  21. go.sum
  22. LICENSE
  23. PATENTS
  24. README.md
README.md

Go Tools

This subrepository holds the source for various packages and tools that support the Go programming language.

Some of the tools, godoc and vet for example, are included in binary Go distributions.

Others, including the Go guru and the test coverage tool, can be fetched with go get.

Packages include a type-checker for Go and an implementation of the Static Single Assignment form (SSA) representation for Go programs.

Download/Install

The easiest way to install is to run go get -u golang.org/x/tools/.... You can also manually git clone the repository to $GOPATH/src/golang.org/x/tools.

Report Issues / Send Patches

This repository uses Gerrit for code changes. To learn how to submit changes to this repository, see https://golang.org/doc/contribute.html.

The main issue tracker for the tools repository is located at https://github.com/golang/go/issues. Prefix your issue with “x/tools/(your subdir):” in the subject line, so it is easy to find.