go/build: do not record go:binary-only-package if build tags not satisfied

This is the documented (and now implemented) behavior.

Fixes #16841.

Change-Id: Ic75adc5ba18303ed9578e04284f32933f905d6a3
Reviewed-on: https://go-review.googlesource.com/31577Reviewed-by: Quentin Smith <quentin@golang.org>

src/cmd/go/go_test.go

@@ -2980,6 +2980,16 @@ func TestBinaryOnlyPackages(t *testing.T) {
 
 	tg.run("run", tg.path("src/p3/p3.go"))
 	tg.grepStdout("hello from p1", "did not see message from p1")
+
+	tg.tempFile("src/p4/p4.go", `package main`)
+	tg.tempFile("src/p4/p4not.go", `//go:binary-only-package
+
+		// +build asdf
+
+		package main
+	`)
+	tg.run("list", "-f", "{{.BinaryOnly}}", "p4")
+	tg.grepStdout("false", "did not see BinaryOnly=false for p4")
 }
 
 // Issue 16050.

src/go/build/build.go

@@ -1072,10 +1072,14 @@ func (ctxt *Context) matchFile(dir, name string, returnImports bool, allTags map
 	}
 
 	// Look for +build comments to accept or reject the file.
-	if !ctxt.shouldBuild(data, allTags, binaryOnly) && !ctxt.UseAllFiles {
+	var sawBinaryOnly bool
+	if !ctxt.shouldBuild(data, allTags, &sawBinaryOnly) && !ctxt.UseAllFiles {
 		return
 	}
 
+	if binaryOnly != nil && sawBinaryOnly {
+		*binaryOnly = true
+	}
 	match = true
 	return
 }
@@ -1119,9 +1123,8 @@ var binaryOnlyComment = []byte("//go:binary-only-package")
 //
 // marks the file as applicable only on Windows and Linux.
 //
-// If shouldBuild finds a //go:binary-only-package comment in a file that
-// should be built, it sets *binaryOnly to true. Otherwise it does
-// not change *binaryOnly.
+// If shouldBuild finds a //go:binary-only-package comment in the file,
+// it sets *binaryOnly to true. Otherwise it does not change *binaryOnly.
 //
 func (ctxt *Context) shouldBuild(content []byte, allTags map[string]bool, binaryOnly *bool) bool {
 	sawBinaryOnly := false