internal/imports: don't block completions on walks

Filesystem walks of large GOPATHs/module caches can take seconds,
especially on systems with slow filesystems like MacOS and WSL. We don't
want to block completion requests on walks finishing. At the same time,
cancelling a walk midway through results in an unusable cache, where we
don't know which parts have been scanned so far.

The best option is to run the walks in a separate goroutine. Then we can
detach and let them finish. On the other side, we need to be able to
reattach for the next completion request.

Introduce a new method on caches, ScanAndListen, which first processes
all the items in the cache, then notifies of any new items. This allows
us to reattach to an existing scan without missing anything.

The background scan introduces concurrency to the resolvers where there
wasn't any before. We can't use mutexes, because there's no way to stop
Lock() when a context expires. Use a 1-element semaphore channel to
accomplish the same effect.

Along the way: Only rescan GOPATH if the resolver has been cleared. None
of this makes sense for GOPATH without that. Fix a bug where we were
scanning the main module twice in module mode. Stop loading exports in
module tests, it slows them down a ton.

Change-Id: I978efae733ccba0c0cdc8e8fe6892bf5f15feac8
Reviewed-on: https://go-review.googlesource.com/c/tools/+/213217
Run-TryBot: Heschi Kreinick <heschi@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>
4 files changed
tree: 87f252df95da227e9dffa3c1b9542dd3fb25558a
  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. txtar/
  15. .gitattributes
  16. .gitignore
  17. AUTHORS
  18. codereview.cfg
  19. CONTRIBUTING.md
  20. CONTRIBUTORS
  21. go.mod
  22. go.sum
  23. LICENSE
  24. PATENTS
  25. 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.