cmd/go/internal/modcmd: skip files with the "ignore" constraint in 'go mod vendor'

'go mod vendor' already drops test files and testdata directories, so
users should not expect the vendored module to include unnecessary
files in general.

Files tagged "ignore" are typically only used to refresh or regenerate
source files within the module to be vendored, so users of that module
do not need them.

Fixes #31088

Change-Id: I1ce9545e9b37c8e779a1826a9d494ac29d2cbfb8
Reviewed-on: https://go-review.googlesource.com/c/go/+/172978
Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/go/internal/modcmd/vendor.go

@@ -15,6 +15,7 @@ import (
 
 	"cmd/go/internal/base"
 	"cmd/go/internal/cfg"
+	"cmd/go/internal/imports"
 	"cmd/go/internal/modload"
 	"cmd/go/internal/module"
 )
@@ -100,7 +101,7 @@ func vendorPkg(vdir, pkg string) {
 	if src == "" {
 		fmt.Fprintf(os.Stderr, "internal error: no pkg for %s -> %s\n", pkg, realPath)
 	}
-	copyDir(dst, src, matchNonTest)
+	copyDir(dst, src, matchPotentialSourceFile)
 	if m := modload.PackageModule(realPath); m.Path != "" {
 		copyMetadata(m.Path, realPath, dst, src)
 	}
@@ -153,7 +154,7 @@ var metaPrefixes = []string{
 }
 
 // matchMetadata reports whether info is a metadata file.
-func matchMetadata(info os.FileInfo) bool {
+func matchMetadata(dir string, info os.FileInfo) bool {
 	name := info.Name()
 	for _, p := range metaPrefixes {
 		if strings.HasPrefix(name, p) {
@@ -163,13 +164,36 @@ func matchMetadata(info os.FileInfo) bool {
 	return false
 }
 
-// matchNonTest reports whether info is any non-test file (including non-Go files).
-func matchNonTest(info os.FileInfo) bool {
-	return !strings.HasSuffix(info.Name(), "_test.go")
+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") {
+		return false
+	}
+	if strings.HasSuffix(info.Name(), ".go") {
+		f, err := os.Open(filepath.Join(dir, info.Name()))
+		if err != nil {
+			base.Fatalf("go mod vendor: %v", err)
+		}
+		defer f.Close()
+
+		content, err := imports.ReadImports(f, false, nil)
+		if err == nil && !imports.ShouldBuild(content, anyTagsExceptIgnore) {
+			// The file is explicitly tagged "ignore", so it can't affect the build.
+			// Leave it out.
+			return false
+		}
+		return true
+	}
+
+	// We don't know anything about this file, so optimistically assume that it is
+	// needed.
+	return true
 }
 
 // copyDir copies all regular files satisfying match(info) from src to dst.
-func copyDir(dst, src string, match func(os.FileInfo) bool) {
+func copyDir(dst, src string, match func(dir string, info os.FileInfo) bool) {
 	files, err := ioutil.ReadDir(src)
 	if err != nil {
 		base.Fatalf("go mod vendor: %v", err)
@@ -178,7 +202,7 @@ func copyDir(dst, src string, match func(os.FileInfo) bool) {
 		base.Fatalf("go mod vendor: %v", err)
 	}
 	for _, file := range files {
-		if file.IsDir() || !file.Mode().IsRegular() || !match(file) {
+		if file.IsDir() || !file.Mode().IsRegular() || !match(src, file) {
 			continue
 		}
 		r, err := os.Open(filepath.Join(src, file.Name()))

src/cmd/go/testdata/script/mod_vendor.txt

@@ -38,6 +38,7 @@ stdout 'src[\\/]w'
 stderr 'src[\\/]vendor[\\/]w'
 
 ! exists vendor/x/testdata
+! exists vendor/a/foo/bar/b/ignored.go
 ! exists vendor/a/foo/bar/b/main_test.go
 
 exists vendor/a/foo/AUTHORS.txt
@@ -102,6 +103,14 @@ replace (
 
 -- a/foo/bar/b/main.go --
 package b
+-- a/foo/bar/b/ignored.go --
+// This file is intended for use with "go run"; it isn't really part of the package.
+
+// +build ignore
+
+package main
+
+func main() {}
 -- a/foo/bar/b/main_test.go --
 package b