cmd/go: ignore build tags when 'go get' modifies build list

In module mode, 'go get' should not consider build constraints when
loading packages in order to modify the module graph. With this
change, 'go get' considers all build tags to be true except for
"ignore" and malformed build constraint expressions.

When 'go get' builds packages, it still applies build constraints for
the target platform.

Fixes #32345

Change-Id: I6dceae6f10a5185870537de730b36292271ad124
Reviewed-on: https://go-review.googlesource.com/c/go/+/179898
Reviewed-by: Bryan C. Mills <bcmills@google.com>

6 files changed, 94 insertions, 32 deletions

diff --git a/src/cmd/go/internal/imports/tags.go b/src/cmd/go/internal/imports/tags.go
index 1c22a472b8..14b4e21a02 100644
--- a/src/cmd/go/internal/imports/tags.go
+++ b/src/cmd/go/internal/imports/tags.go
@@ -8,6 +8,9 @@ import "cmd/go/internal/cfg"
 
 var tags map[string]bool
 
+// 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 {
 		tags = loadTags()
@@ -32,3 +35,15 @@ func loadTags() map[string]bool {
 	}
 	return tags
 }
+
+var anyTags map[string]bool
+
+// 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 {
+		anyTags = map[string]bool{"*": true}
+	}
+	return anyTags
+}
diff --git a/src/cmd/go/internal/modcmd/vendor.go b/src/cmd/go/internal/modcmd/vendor.go
index 44cabd5dea..75513f1d9c 100644
--- a/src/cmd/go/internal/modcmd/vendor.go
+++ b/src/cmd/go/internal/modcmd/vendor.go
@@ -166,8 +166,6 @@ func matchMetadata(dir string, info os.FileInfo) bool {
 	return false
 }
 
-var anyTagsExceptIgnore = map[string]bool{"*": true}
-
 // matchPotentialSourceFile reports whether info may be relevant to a build operation.
 func matchPotentialSourceFile(dir string, info os.FileInfo) bool {
 	if strings.HasSuffix(info.Name(), "_test.go") {
@@ -181,7 +179,7 @@ func matchPotentialSourceFile(dir string, info os.FileInfo) bool {
 		defer f.Close()
 
 		content, err := imports.ReadImports(f, false, nil)
-		if err == nil && !imports.ShouldBuild(content, anyTagsExceptIgnore) {
+		if err == nil && !imports.ShouldBuild(content, imports.AnyTags()) {
 			// The file is explicitly tagged "ignore", so it can't affect the build.
 			// Leave it out.
 			return false
diff --git a/src/cmd/go/internal/modget/get.go b/src/cmd/go/internal/modget/get.go
index e2c4c105dd..2d5eb03cd8 100644
--- a/src/cmd/go/internal/modget/get.go
+++ b/src/cmd/go/internal/modget/get.go
@@ -9,6 +9,7 @@ import (
 	"cmd/go/internal/base"
 	"cmd/go/internal/cfg"
 	"cmd/go/internal/get"
+	"cmd/go/internal/imports"
 	"cmd/go/internal/load"
 	"cmd/go/internal/modfetch"
 	"cmd/go/internal/modload"
@@ -446,9 +447,8 @@ func runGet(cmd *base.Command, args []string) {
 			// Don't load packages if pkgPatterns is empty. Both
 			// modload.ImportPathsQuiet and ModulePackages convert an empty list
 			// of patterns to []string{"."}, which is not what we want.
-			matches = modload.ImportPathsQuiet(pkgPatterns)
+			matches = modload.ImportPathsQuiet(pkgPatterns, imports.AnyTags())
 			seenPkgs = make(map[string]bool)
-			install = make([]string, 0, len(pkgPatterns))
 			for i, match := range matches {
 				arg := pkgGets[i]
 
@@ -462,7 +462,6 @@ func runGet(cmd *base.Command, args []string) {
 					continue
 				}
 
-				install = append(install, arg.path)
 				allStd := true
 				for _, pkg := range match.Pkgs {
 					if !seenPkgs[pkg] {
@@ -513,7 +512,11 @@ func runGet(cmd *base.Command, args []string) {
 		}
 		prevBuildList = buildList
 	}
-	search.WarnUnmatched(matches) // don't warn on every iteration
+	if !*getD {
+		// Only print warnings after the last iteration,
+		// and only if we aren't going to build.
+		search.WarnUnmatched(matches)
+	}
 
 	// Handle downgrades.
 	var down []module.Version
@@ -606,16 +609,17 @@ func runGet(cmd *base.Command, args []string) {
 
 	// If -d was specified, we're done after the module work.
 	// We've already downloaded modules by loading packages above.
-	// Otherwise, we need to build and install the packages matched
-	// by command line arguments.
-	// Note that 'go get -u' without any arguments results in
-	// len(install) == 1 if there's a package in the current directory.
-	// search.CleanPatterns returns "." for empty args.
-	if *getD || len(install) == 0 {
+	// Otherwise, we need to build and install the packages matched by
+	// command line arguments. This may be a different set of packages,
+	// since we only build packages for the target platform.
+	// Note that 'go get -u' without arguments is equivalent to
+	// 'go get -u .', so we'll typically build the package in the current
+	// directory.
+	if *getD || len(pkgPatterns) == 0 {
 		return
 	}
 	work.BuildInit()
-	pkgs := load.PackagesForBuild(install)
+	pkgs := load.PackagesForBuild(pkgPatterns)
 	work.InstallPackages(install, pkgs)
 }
 
diff --git a/src/cmd/go/internal/modload/load.go b/src/cmd/go/internal/modload/load.go
index 022b3f3a4f..f05975d331 100644
--- a/src/cmd/go/internal/modload/load.go
+++ b/src/cmd/go/internal/modload/load.go
@@ -51,15 +51,19 @@ var buildList []module.Version
 var loaded *loader
 
 // ImportPaths returns the set of packages matching the args (patterns),
-// adding modules to the build list as needed to satisfy new imports.
+// on the target platform. Modules may be added to the build list
+// to satisfy new imports.
 func ImportPaths(patterns []string) []*search.Match {
-	matches := ImportPathsQuiet(patterns)
+	matches := ImportPathsQuiet(patterns, imports.Tags())
 	search.WarnUnmatched(matches)
 	return matches
 }
 
-// ImportPathsQuiet is like ImportPaths but does not warn about patterns with no matches.
-func ImportPathsQuiet(patterns []string) []*search.Match {
+// ImportPathsQuiet is like ImportPaths but does not warn about patterns with
+// no matches. It also lets the caller specify a set of build tags to match
+// packages. The build tags should typically be imports.Tags() or
+// imports.AnyTags(); a nil map has no special meaning.
+func ImportPathsQuiet(patterns []string, tags map[string]bool) []*search.Match {
 	var fsDirs [][]string
 	updateMatches := func(matches []*search.Match, iterating bool) {
 		for i, m := range matches {
@@ -179,7 +183,7 @@ func ImportPathsQuiet(patterns []string) []*search.Match {
 		})
 	}
 
-	loaded = newLoader()
+	loaded = newLoader(tags)
 	loaded.load(func() []string {
 		var roots []string
 		updateMatches(matches, true)
@@ -258,12 +262,13 @@ func warnPattern(pattern string, list []string) []string {
 func ImportFromFiles(gofiles []string) {
 	InitMod()
 
-	imports, testImports, err := imports.ScanFiles(gofiles, imports.Tags())
+	tags := imports.Tags()
+	imports, testImports, err := imports.ScanFiles(gofiles, tags)
 	if err != nil {
 		base.Fatalf("go: %v", err)
 	}
 
-	loaded = newLoader()
+	loaded = newLoader(tags)
 	loaded.load(func() []string {
 		var roots []string
 		roots = append(roots, imports...)
@@ -312,7 +317,7 @@ func LoadBuildList() []module.Version {
 }
 
 func ReloadBuildList() []module.Version {
-	loaded = newLoader()
+	loaded = newLoader(imports.Tags())
 	loaded.load(func() []string { return nil })
 	return buildList
 }
@@ -338,9 +343,8 @@ func LoadVendor() []string {
 func loadAll(testAll bool) []string {
 	InitMod()
 
-	loaded = newLoader()
+	loaded = newLoader(imports.AnyTags())
 	loaded.isALL = true
-	loaded.tags = anyTags
 	loaded.testAll = testAll
 	if !testAll {
 		loaded.testRoots = true
@@ -359,15 +363,11 @@ func loadAll(testAll bool) []string {
 	return paths
 }
 
-// anyTags is a special tags map that satisfies nearly all build tag expressions.
-// Only "ignore" and malformed build tag requirements are considered false.
-var anyTags = map[string]bool{"*": true}
-
 // TargetPackages returns the list of packages in the target (top-level) module
 // matching pattern, which may be relative to the working directory, under all
 // build tag settings.
 func TargetPackages(pattern string) []string {
-	return matchPackages(pattern, anyTags, false, []module.Version{Target})
+	return matchPackages(pattern, imports.AnyTags(), false, []module.Version{Target})
 }
 
 // BuildList returns the module build list,
@@ -510,9 +510,9 @@ type loader struct {
 // LoadTests controls whether the loaders load tests of the root packages.
 var LoadTests bool
 
-func newLoader() *loader {
+func newLoader(tags map[string]bool) *loader {
 	ld := new(loader)
-	ld.tags = imports.Tags()
+	ld.tags = tags
 	ld.testRoots = LoadTests
 
 	// Inside the "std" and "cmd" modules, we prefer to use the vendor directory
diff --git a/src/cmd/go/internal/modload/query.go b/src/cmd/go/internal/modload/query.go
index 218d18373a..5e34bb5e17 100644
--- a/src/cmd/go/internal/modload/query.go
+++ b/src/cmd/go/internal/modload/query.go
@@ -12,6 +12,7 @@ import (
 	"strings"
 	"sync"
 
+	"cmd/go/internal/imports"
 	"cmd/go/internal/modfetch"
 	"cmd/go/internal/module"
 	"cmd/go/internal/search"
@@ -265,7 +266,7 @@ func QueryPattern(pattern, query string, allowed func(module.Version) bool) ([]Q
 	if i := strings.Index(pattern, "..."); i >= 0 {
 		base = pathpkg.Dir(pattern[:i+3])
 		match = func(m module.Version, root string, isLocal bool) []string {
-			return matchPackages(pattern, anyTags, false, []module.Version{m})
+			return matchPackages(pattern, imports.AnyTags(), false, []module.Version{m})
 		}
 	} else {
 		match = func(m module.Version, root string, isLocal bool) []string {
diff --git a/src/cmd/go/testdata/script/mod_get_tags.txt b/src/cmd/go/testdata/script/mod_get_tags.txt
new file mode 100644
index 0000000000..603c76983f
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_get_tags.txt
@@ -0,0 +1,44 @@
+env GO111MODULE=on
+
+[short] skip
+
+# get should add modules needed to build packages, even if those
+# dependencies are in sources excluded by build tags.
+# All build tags are considered true except "ignore".
+go mod init m
+go get -d .
+go list -m all
+stdout 'example.com/version v1.1.0'
+stdout 'rsc.io/quote v1.5.2'
+
+[short] skip
+
+# Packages that are only imported in excluded files should not be built.
+go get -x .
+stderr 'compile.* -p m '
+! stderr 'compile.* -p example.com/version '
+! stderr 'compile.* -p rsc.io/quote '
+
+-- empty.go --
+package m
+
+-- excluded.go --
+// +build windows,mips
+
+package m
+
+import _ "example.com/version"
+
+-- tools.go --
+// +build tools
+
+package tools
+
+import _ "rsc.io/quote"
+
+-- ignore.go --
+// +build ignore
+
+package ignore
+
+import _ "example.com/doesnotexist"