go/build: ignore files by extension before matching on name

Otherwise given a file like defs_nacl_amd64p32.go.~1~ we will add
"nacl" and "amd64p32" to AllTags. This was causing the
cmd/go/internal/modindex tests to fail on my system, since I had
an old editor backup file lying around.

Change-Id: Ib1c5d835e4871addae6dc78cee07c9839bb880e2
Reviewed-on: https://go-review.googlesource.com/c/go/+/446395
Reviewed-by: Bryan Mills <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
Run-TryBot: Ian Lance Taylor <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Run-TryBot: Ian Lance Taylor <[email protected]>
Auto-Submit: Ian Lance Taylor <[email protected]>

Author: ianlancetaylor

src/go/build/build.go

@@ -1423,12 +1423,12 @@ func (ctxt *Context) matchFile(dir, name string, allTags map[string]bool, binary
 	}
 	ext := name[i:]
 
-	if !ctxt.goodOSArchFile(name, allTags) && !ctxt.UseAllFiles {
+	if ext != ".go" && fileListForExt(&dummyPkg, ext) == nil {
+		// skip
 		return nil, nil
 	}
 
-	if ext != ".go" && fileListForExt(&dummyPkg, ext) == nil {
-		// skip
+	if !ctxt.goodOSArchFile(name, allTags) && !ctxt.UseAllFiles {
 		return nil, nil
 	}
 

src/go/build/build_test.go

@@ -790,3 +790,13 @@ func TestAllTags(t *testing.T) {
 		t.Errorf("GoFiles = %v, want %v", p.GoFiles, wantFiles)
 	}
 }
+
+func TestAllTagsNonSourceFile(t *testing.T) {
+	p, err := Default.ImportDir("testdata/non_source_tags", 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(p.AllTags) > 0 {
+		t.Errorf("AllTags = %v, want empty", p.AllTags)
+	}
+}

src/go/build/testdata/non_source_tags/non_source_tags.go

@@ -0,0 +1,5 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package non_source_tags

src/go/build/testdata/non_source_tags/x_arm.go.ignore

@@ -0,0 +1,5 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package non_source_tags