cmd/cgo: add -trimpath flag allowing paths to be rewritten in outputs

cmd/cgo now has a -trimpath flag that behaves the same as the
-trimpath flag to cmd/compile. This will be used to correct paths
to cgo files that are overlaid.

The code that processes trimpath in internal/objapi has been slightly
refactored because it's currently only accessible via AbsFile, which
does some additional processing to the path names. Now an
ApplyRewrites function is exported that just applies the trimpath
rewrites.

Also remove unused srcfile argument to cmd/cgo.(*Package).godefs.

For #39958

Change-Id: I497d48d0bc2fe1f6ab2b5835cbe79f15b839ee59
Reviewed-on: https://go-review.googlesource.com/c/go/+/266358
Trust: Michael Matloob <matloob@golang.org>
Run-TryBot: Michael Matloob <matloob@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/cmd/cgo/ast.go

@@ -13,7 +13,6 @@ import (
 	"go/scanner"
 	"go/token"
 	"os"
-	"path/filepath"
 	"strings"
 )
 
@@ -44,14 +43,7 @@ func sourceLine(n ast.Node) int {
 // attached to the import "C" comment, a list of references to C.xxx,
 // a list of exported functions, and the actual AST, to be rewritten and
 // printed.
-func (f *File) ParseGo(name string, src []byte) {
-	// Create absolute path for file, so that it will be used in error
-	// messages and recorded in debug line number information.
-	// This matches the rest of the toolchain. See golang.org/issue/5122.
-	if aname, err := filepath.Abs(name); err == nil {
-		name = aname
-	}
-
+func (f *File) ParseGo(abspath string, src []byte) {
 	// Two different parses: once with comments, once without.
 	// The printer is not good enough at printing comments in the
 	// right place when we start editing the AST behind its back,
@@ -60,8 +52,8 @@ func (f *File) ParseGo(name string, src []byte) {
 	// and reprinting.
 	// In cgo mode, we ignore ast2 and just apply edits directly
 	// the text behind ast1. In godefs mode we modify and print ast2.
-	ast1 := parse(name, src, parser.ParseComments)
-	ast2 := parse(name, src, 0)
+	ast1 := parse(abspath, src, parser.ParseComments)
+	ast2 := parse(abspath, src, 0)
 
 	f.Package = ast1.Name.Name
 	f.Name = make(map[string]*Name)
@@ -88,7 +80,7 @@ func (f *File) ParseGo(name string, src []byte) {
 				cg = d.Doc
 			}
 			if cg != nil {
-				f.Preamble += fmt.Sprintf("#line %d %q\n", sourceLine(cg), name)
+				f.Preamble += fmt.Sprintf("#line %d %q\n", sourceLine(cg), abspath)
 				f.Preamble += commentText(cg) + "\n"
 				f.Preamble += "#line 1 \"cgo-generated-wrapper\"\n"
 			}

src/cmd/cgo/godefs.go

@@ -16,7 +16,7 @@ import (
 )
 
 // godefs returns the output for -godefs mode.
-func (p *Package) godefs(f *File, srcfile string) string {
+func (p *Package) godefs(f *File) string {
 	var buf bytes.Buffer
 
 	fmt.Fprintf(&buf, "// Code generated by cmd/cgo -godefs; DO NOT EDIT.\n")

src/cmd/cgo/main.go

@@ -243,6 +243,8 @@ var gccgopkgpath = flag.String("gccgopkgpath", "", "-fgo-pkgpath option used wit
 var gccgoMangler func(string) string
 var importRuntimeCgo = flag.Bool("import_runtime_cgo", true, "import runtime/cgo in generated code")
 var importSyscall = flag.Bool("import_syscall", true, "import syscall in generated code")
+var trimpath = flag.String("trimpath", "", "applies supplied rewrites or trims prefixes to recorded source file paths")
+
 var goarch, goos string
 
 func main() {
@@ -322,6 +324,13 @@ func main() {
 			input = filepath.Join(*srcDir, input)
 		}
 
+		// Create absolute path for file, so that it will be used in error
+		// messages and recorded in debug line number information.
+		// This matches the rest of the toolchain. See golang.org/issue/5122.
+		if aname, err := filepath.Abs(input); err == nil {
+			input = aname
+		}
+
 		b, err := ioutil.ReadFile(input)
 		if err != nil {
 			fatalf("%s", err)
@@ -330,6 +339,10 @@ func main() {
 			fatalf("%s", err)
 		}
 
+		// Apply trimpath to the file path. The path won't be read from after this point.
+		input, _ = objabi.ApplyRewrites(input, *trimpath)
+		goFiles[i] = input
+
 		f := new(File)
 		f.Edit = edit.NewBuffer(b)
 		f.ParseGo(input, b)
@@ -367,7 +380,7 @@ func main() {
 		p.PackagePath = f.Package
 		p.Record(f)
 		if *godefs {
-			os.Stdout.WriteString(p.godefs(f, input))
+			os.Stdout.WriteString(p.godefs(f))
 		} else {
 			p.writeOutput(f, input)
 		}

src/cmd/internal/objabi/line.go

@@ -37,27 +37,38 @@ func AbsFile(dir, file, rewrites string) string {
 		abs = filepath.Join(dir, file)
 	}
 
-	start := 0
-	for i := 0; i <= len(rewrites); i++ {
-		if i == len(rewrites) || rewrites[i] == ';' {
-			if new, ok := applyRewrite(abs, rewrites[start:i]); ok {
-				abs = new
-				goto Rewritten
-			}
-			start = i + 1
-		}
-	}
-	if hasPathPrefix(abs, GOROOT) {
+	abs, rewritten := ApplyRewrites(abs, rewrites)
+	if !rewritten && hasPathPrefix(abs, GOROOT) {
 		abs = "$GOROOT" + abs[len(GOROOT):]
 	}
 
-Rewritten:
 	if abs == "" {
 		abs = "??"
 	}
 	return abs
 }
 
+// ApplyRewrites returns the filename for file in the given directory,
+// as rewritten by the rewrites argument.
+//
+// The rewrites argument is a ;-separated list of rewrites.
+// Each rewrite is of the form "prefix" or "prefix=>replace",
+// where prefix must match a leading sequence of path elements
+// and is either removed entirely or replaced by the replacement.
+func ApplyRewrites(file, rewrites string) (string, bool) {
+	start := 0
+	for i := 0; i <= len(rewrites); i++ {
+		if i == len(rewrites) || rewrites[i] == ';' {
+			if new, ok := applyRewrite(file, rewrites[start:i]); ok {
+				return new, true
+			}
+			start = i + 1
+		}
+	}
+
+	return file, false
+}
+
 // applyRewrite applies the rewrite to the path,
 // returning the rewritten path and a boolean
 // indicating whether the rewrite applied at all.