work around cmd/go issue relating to CompiledGoFiles

See https://golang.org/issue/28749. The improved asm test would fail:

	go parse: $WORK/imported/imported_amd64.s:1:1: expected 'package', found TEXT (and 2 more errors)

because we would incorrectly parse a non-Go file as a Go file.

Add a workaround. The original reporter's reproducer with go-ethereum
works now, as this was the last hiccup.

Fixes #555.

reverse.go

@@ -89,7 +89,7 @@ One can reverse a captured panic stack trace as follows:
 			}
 			file, err := parser.ParseFile(fset, goFile, nil, parser.SkipObjectResolution)
 			if err != nil {
-				return err
+				return fmt.Errorf("go parse: %w", err)
 			}
 			files = append(files, file)
 		}

shared.go

@@ -227,6 +227,23 @@ func appendListedPackages(packages []string, withDeps bool) error {
 			actionID := decodeHash(splitActionID(pkg.BuildID))
 			pkg.GarbleActionID = addGarbleToHash(actionID)
 		}
+
+		// Work around https://golang.org/issue/28749:
+		// cmd/go puts assembly, C, and C++ files in CompiledGoFiles.
+		//
+		// TODO: remove when upstream has fixed the bug.
+		out := pkg.CompiledGoFiles[:0]
+		for _, path := range pkg.CompiledGoFiles {
+			switch filepath.Ext(path) {
+			case "": // e.g. a generated Go file inside the build cache
+			case ".go":
+			default: // e.g. an assembly file
+				continue
+			}
+			out = append(out, path)
+		}
+		pkg.CompiledGoFiles = out
+
 		cache.ListedPackages[pkg.ImportPath] = &pkg
 	}
 

testdata/script/asm.txtar

@@ -13,6 +13,12 @@ cmp stderr main.stderr
 
 [short] stop # no need to verify this with -short
 
+# Ensure that reversing doesn't error with assembly files.
+# It should fail, as there is nothing to reverse, but without any parse error.
+stdin empty-reverse.txt
+! garble reverse .
+! stderr .
+
 garble -tiny build
 exec ./main
 cmp stderr main.stderr
@@ -101,3 +107,4 @@ TEXT ·PublicAdd(SB),$0-16
 13 36
 25 70
 7
+-- empty-reverse.txt --