cmd/go/internal/imports: make Tags and AnyTags safe for concurrent use

AnyTags turned up as a data race while running 'go test -race cmd/go'.
I'm not sure how long the race has been present.

==================
WARNING: DATA RACE
Read at 0x000001141ec0 by goroutine 8:
  cmd/go/internal/imports.AnyTags()
      /usr/local/google/home/bcmills/go/src/cmd/go/internal/imports/tags.go:45 +0x10e
  cmd/go/internal/modload.QueryPattern.func2()
      /usr/local/google/home/bcmills/go/src/cmd/go/internal/modload/query.go:539 +0x11d
  cmd/go/internal/modload.QueryPattern.func4.1()
      /usr/local/google/home/bcmills/go/src/cmd/go/internal/modload/query.go:607 +0x3db
  cmd/go/internal/modload.queryPrefixModules.func1()
      /usr/local/google/home/bcmills/go/src/cmd/go/internal/modload/query.go:677 +0xa7

Previous write at 0x000001141ec0 by goroutine 7:
  cmd/go/internal/imports.AnyTags()
      /usr/local/google/home/bcmills/go/src/cmd/go/internal/imports/tags.go:46 +0x26b
  cmd/go/internal/modload.QueryPattern.func2()
      /usr/local/google/home/bcmills/go/src/cmd/go/internal/modload/query.go:539 +0x11d
  cmd/go/internal/modload.QueryPattern.func4.1()
      /usr/local/google/home/bcmills/go/src/cmd/go/internal/modload/query.go:607 +0x3db
  cmd/go/internal/modload.queryPrefixModules.func1()
      /usr/local/google/home/bcmills/go/src/cmd/go/internal/modload/query.go:677 +0xa7

Goroutine 8 (running) created at:
  cmd/go/internal/modload.queryPrefixModules()
      /usr/local/google/home/bcmills/go/src/cmd/go/internal/modload/query.go:676 +0x284
  cmd/go/internal/modload.QueryPattern.func4()
      /usr/local/google/home/bcmills/go/src/cmd/go/internal/modload/query.go:624 +0x2e4
  cmd/go/internal/modfetch.TryProxies()
      /usr/local/google/home/bcmills/go/src/cmd/go/internal/modfetch/proxy.go:220 +0x107
  cmd/go/internal/modload.QueryPattern()
      /usr/local/google/home/bcmills/go/src/cmd/go/internal/modload/query.go:590 +0x69e
  cmd/go/internal/work.installOutsideModule()
      /usr/local/google/home/bcmills/go/src/cmd/go/internal/work/build.go:744 +0x4b0
  cmd/go/internal/work.runInstall()
      /usr/local/google/home/bcmills/go/src/cmd/go/internal/work/build.go:556 +0x217
  main.main()
      /usr/local/google/home/bcmills/go/src/cmd/go/main.go:194 +0xb94

Goroutine 7 (finished) created at:
  cmd/go/internal/modload.queryPrefixModules()
      /usr/local/google/home/bcmills/go/src/cmd/go/internal/modload/query.go:676 +0x284
  cmd/go/internal/modload.QueryPattern.func4()
      /usr/local/google/home/bcmills/go/src/cmd/go/internal/modload/query.go:624 +0x2e4
  cmd/go/internal/modfetch.TryProxies()
      /usr/local/google/home/bcmills/go/src/cmd/go/internal/modfetch/proxy.go:220 +0x107
  cmd/go/internal/modload.QueryPattern()
      /usr/local/google/home/bcmills/go/src/cmd/go/internal/modload/query.go:590 +0x69e
  cmd/go/internal/work.installOutsideModule()
      /usr/local/google/home/bcmills/go/src/cmd/go/internal/work/build.go:744 +0x4b0
  cmd/go/internal/work.runInstall()
      /usr/local/google/home/bcmills/go/src/cmd/go/internal/work/build.go:556 +0x217
  main.main()
      /usr/local/google/home/bcmills/go/src/cmd/go/main.go:194 +0xb94
==================

Change-Id: Id394978fd6ea0c30614caf8f90ee4f8e2d272843
Reviewed-on: https://go-review.googlesource.com/c/go/+/265278
Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

1 files changed, 16 insertions, 7 deletions

diff --git a/src/cmd/go/internal/imports/tags.go b/src/cmd/go/internal/imports/tags.go
index 14b4e21a02..01b448b914 100644
--- a/src/cmd/go/internal/imports/tags.go
+++ b/src/cmd/go/internal/imports/tags.go
@@ -4,17 +4,23 @@
 
 package imports
 
-import "cmd/go/internal/cfg"
+import (
+	"cmd/go/internal/cfg"
+	"sync"
+)
 
-var tags map[string]bool
+var (
+	tags     map[string]bool
+	tagsOnce sync.Once
+)
 
 // Tags returns a set of build tags that are true for the target platform.
 // It includes GOOS, GOARCH, the compiler, possibly "cgo",
 // release tags like "go1.13", and user-specified build tags.
 func Tags() map[string]bool {
-	if tags == nil {
+	tagsOnce.Do(func() {
 		tags = loadTags()
-	}
+	})
 	return tags
 }
 
@@ -36,14 +42,17 @@ func loadTags() map[string]bool {
 	return tags
 }
 
-var anyTags map[string]bool
+var (
+	anyTags     map[string]bool
+	anyTagsOnce sync.Once
+)
 
 // AnyTags returns a special set of build tags that satisfy nearly all
 // build tag expressions. Only "ignore" and malformed build tag requirements
 // are considered false.
 func AnyTags() map[string]bool {
-	if anyTags == nil {
+	anyTagsOnce.Do(func() {
 		anyTags = map[string]bool{"*": true}
-	}
+	})
 	return anyTags
 }