cgo: support errno value as second return parameter

Making this work on all targets was interesting but there's now a test
in place to make sure this works on all targets that have the CGo test
enabled (which is almost all targets).

builder/picolibc.go

@@ -34,6 +34,7 @@ var libPicolibc = Library{
 			"-D__OBSOLETE_MATH_FLOAT=1", // use old math code that doesn't expect a FPU
 			"-D__OBSOLETE_MATH_DOUBLE=0",
 			"-D_WANT_IO_C99_FORMATS",
+			"-D__PICOLIBC_ERRNO_FUNCTION=__errno_location",
 			"-nostdlibinc",
 			"-isystem", newlibDir + "/libc/include",
 			"-I" + newlibDir + "/libc/tinystdio",

cgo/cgo.go

@@ -138,7 +138,8 @@ typedef unsigned long long  _Cgo_ulonglong;
 // first.
 // These functions will be modified to get a "C." prefix, so the source below
 // doesn't reflect the final AST.
-const generatedGoFilePrefix = `
+const generatedGoFilePrefixBase = `
+import "syscall"
 import "unsafe"
 
 var _ unsafe.Pointer
@@ -169,6 +170,74 @@ func __CBytes([]byte) unsafe.Pointer
 func CBytes(b []byte) unsafe.Pointer {
 	return C.__CBytes(b)
 }
+
+//go:linkname C.__get_errno_num runtime.cgo_errno
+func __get_errno_num() uintptr
+`
+
+const generatedGoFilePrefixOther = generatedGoFilePrefixBase + `
+func __get_errno() error {
+	return syscall.Errno(C.__get_errno_num())
+}
+`
+
+// Windows uses fake errno values in the syscall package.
+// See for example: https://github.com/golang/go/issues/23468
+// TinyGo uses mingw-w64 though, which does have defined errno values. Since the
+// syscall package is the standard library one we can't change it, but we can
+// map the errno values to match the values in the syscall package.
+// Source of the errno values: lib/mingw-w64/mingw-w64-headers/crt/errno.h
+const generatedGoFilePrefixWindows = generatedGoFilePrefixBase + `
+var __errno_mapping = [...]syscall.Errno{
+	1:  syscall.EPERM,
+	2:  syscall.ENOENT,
+	3:  syscall.ESRCH,
+	4:  syscall.EINTR,
+	5:  syscall.EIO,
+	6:  syscall.ENXIO,
+	7:  syscall.E2BIG,
+	8:  syscall.ENOEXEC,
+	9:  syscall.EBADF,
+	10: syscall.ECHILD,
+	11: syscall.EAGAIN,
+	12: syscall.ENOMEM,
+	13: syscall.EACCES,
+	14: syscall.EFAULT,
+	16: syscall.EBUSY,
+	17: syscall.EEXIST,
+	18: syscall.EXDEV,
+	19: syscall.ENODEV,
+	20: syscall.ENOTDIR,
+	21: syscall.EISDIR,
+	22: syscall.EINVAL,
+	23: syscall.ENFILE,
+	24: syscall.EMFILE,
+	25: syscall.ENOTTY,
+	27: syscall.EFBIG,
+	28: syscall.ENOSPC,
+	29: syscall.ESPIPE,
+	30: syscall.EROFS,
+	31: syscall.EMLINK,
+	32: syscall.EPIPE,
+	33: syscall.EDOM,
+	34: syscall.ERANGE,
+	36: syscall.EDEADLK,
+	38: syscall.ENAMETOOLONG,
+	39: syscall.ENOLCK,
+	40: syscall.ENOSYS,
+	41: syscall.ENOTEMPTY,
+	42: syscall.EILSEQ,
+}
+
+func __get_errno() error {
+	num := C.__get_errno_num()
+	if num < uintptr(len(__errno_mapping)) {
+		if mapped := __errno_mapping[num]; mapped != 0 {
+			return mapped
+		}
+	}
+	return syscall.Errno(num)
+}
 `
 
 // Process extracts `import "C"` statements from the AST, parses the comment
@@ -178,7 +247,7 @@ func CBytes(b []byte) unsafe.Pointer {
 // functions), the CFLAGS and LDFLAGS found in #cgo lines, and a map of file
 // hashes of the accessed C header files. If there is one or more error, it
 // returns these in the []error slice but still modifies the AST.
-func Process(files []*ast.File, dir, importPath string, fset *token.FileSet, cflags []string) ([]*ast.File, []string, []string, []string, map[string][]byte, []error) {
+func Process(files []*ast.File, dir, importPath string, fset *token.FileSet, cflags []string, goos string) ([]*ast.File, []string, []string, []string, map[string][]byte, []error) {
 	p := &cgoPackage{
 		packageName:     files[0].Name.Name,
 		currentDir:      dir,
@@ -210,7 +279,12 @@ func Process(files []*ast.File, dir, importPath string, fset *token.FileSet, cfl
 	// Construct a new in-memory AST for CGo declarations of this package.
 	// The first part is written as Go code that is then parsed, but more code
 	// is added later to the AST to declare functions, globals, etc.
-	goCode := "package " + files[0].Name.Name + "\n\n" + generatedGoFilePrefix
+	goCode := "package " + files[0].Name.Name + "\n\n"
+	if goos == "windows" {
+		goCode += generatedGoFilePrefixWindows
+	} else {
+		goCode += generatedGoFilePrefixOther
+	}
 	p.generated, err = parser.ParseFile(fset, dir+"/!cgo.go", goCode, parser.ParseComments)
 	if err != nil {
 		// This is always a bug in the cgo package.
@@ -225,7 +299,7 @@ func Process(files []*ast.File, dir, importPath string, fset *token.FileSet, cfl
 		switch decl := decl.(type) {
 		case *ast.FuncDecl:
 			switch decl.Name.Name {
-			case "CString", "GoString", "GoStringN", "__GoStringN", "GoBytes", "__GoBytes", "CBytes", "__CBytes":
+			case "CString", "GoString", "GoStringN", "__GoStringN", "GoBytes", "__GoBytes", "CBytes", "__CBytes", "__get_errno_num", "__get_errno", "__errno_mapping":
 				// Adjust the name to have a "C." prefix so it is correctly
 				// resolved.
 				decl.Name.Name = "C." + decl.Name.Name
@@ -1279,6 +1353,45 @@ extern __typeof(%s) %s __attribute__((alias(%#v)));
 // separate namespace (no _Cgo_ hacks like in gc).
 func (f *cgoFile) walker(cursor *astutil.Cursor, names map[string]clangCursor) bool {
 	switch node := cursor.Node().(type) {
+	case *ast.AssignStmt:
+		// An assign statement could be something like this:
+		//
+		//   val, errno := C.some_func()
+		//
+		// Check whether it looks like that, and if so, read the errno value and
+		// return it as the second return value. The call will be transformed
+		// into something like this:
+		//
+		//   val, errno := C.some_func(), C.__get_errno()
+		if len(node.Lhs) != 2 || len(node.Rhs) != 1 {
+			return true
+		}
+		rhs, ok := node.Rhs[0].(*ast.CallExpr)
+		if !ok {
+			return true
+		}
+		fun, ok := rhs.Fun.(*ast.SelectorExpr)
+		if !ok {
+			return true
+		}
+		x, ok := fun.X.(*ast.Ident)
+		if !ok {
+			return true
+		}
+		if found, ok := names[fun.Sel.Name]; ok && x.Name == "C" {
+			// Replace "C"."some_func" into "C.somefunc".
+			rhs.Fun = &ast.Ident{
+				NamePos: x.NamePos,
+				Name:    f.getASTDeclName(fun.Sel.Name, found, true),
+			}
+			// Add the errno value as the second value in the statement.
+			node.Rhs = append(node.Rhs, &ast.CallExpr{
+				Fun: &ast.Ident{
+					NamePos: node.Lhs[1].End(),
+					Name:    "C.__get_errno",
+				},
+			})
+		}
 	case *ast.CallExpr:
 		fun, ok := node.Fun.(*ast.SelectorExpr)
 		if !ok {

cgo/cgo_test.go

@@ -56,15 +56,15 @@ func TestCGo(t *testing.T) {
 			}
 
 			// Process the AST with CGo.
-			cgoFiles, _, _, _, _, cgoErrors := Process([]*ast.File{f}, "testdata", "main", fset, cflags)
+			cgoFiles, _, _, _, _, cgoErrors := Process([]*ast.File{f}, "testdata", "main", fset, cflags, "linux")
 
 			// Check the AST for type errors.
 			var typecheckErrors []error
 			config := types.Config{
 				Error: func(err error) {
 					typecheckErrors = append(typecheckErrors, err)
 				},
-				Importer: simpleImporter{},
+				Importer: newSimpleImporter(),
 				Sizes:    types.SizesFor("gccgo", "arm"),
 			}
 			_, err = config.Check("", fset, append([]*ast.File{f}, cgoFiles...), nil)
@@ -202,14 +202,33 @@ func Test_cgoPackage_isEquivalentAST(t *testing.T) {
 }
 
 // simpleImporter implements the types.Importer interface, but only allows
-// importing the unsafe package.
+// importing the syscall and unsafe packages.
 type simpleImporter struct {
+	syscallPkg *types.Package
+}
+
+func newSimpleImporter() *simpleImporter {
+	i := &simpleImporter{}
+
+	// Implement a dummy syscall package with the Errno type.
+	i.syscallPkg = types.NewPackage("syscall", "syscall")
+	obj := types.NewTypeName(token.NoPos, i.syscallPkg, "Errno", nil)
+	named := types.NewNamed(obj, nil, nil)
+	i.syscallPkg.Scope().Insert(obj)
+	named.SetUnderlying(types.Typ[types.Uintptr])
+	sig := types.NewSignatureType(nil, nil, nil, types.NewTuple(), types.NewTuple(types.NewParam(token.NoPos, i.syscallPkg, "", types.Typ[types.String])), false)
+	named.AddMethod(types.NewFunc(token.NoPos, i.syscallPkg, "Error", sig))
+	i.syscallPkg.MarkComplete()
+
+	return i
 }
 
 // Import implements the Importer interface. For testing usage only: it only
 // supports importing the unsafe package.
-func (i simpleImporter) Import(path string) (*types.Package, error) {
+func (i *simpleImporter) Import(path string) (*types.Package, error) {
 	switch path {
+	case "syscall":
+		return i.syscallPkg, nil
 	case "unsafe":
 		return types.Unsafe, nil
 	default:

cgo/testdata/basic.out.go

@@ -1,5 +1,6 @@
 package main
 
+import "syscall"
 import "unsafe"
 
 var _ unsafe.Pointer
@@ -31,6 +32,13 @@ func C.CBytes(b []byte) unsafe.Pointer {
 	return C.__CBytes(b)
 }
 
+//go:linkname C.__get_errno_num runtime.cgo_errno
+func C.__get_errno_num() uintptr
+
+func C.__get_errno() error {
+	return syscall.Errno(C.__get_errno_num())
+}
+
 type (
 	C.char      uint8
 	C.schar     int8

cgo/testdata/const.out.go

@@ -1,5 +1,6 @@
 package main
 
+import "syscall"
 import "unsafe"
 
 var _ unsafe.Pointer
@@ -31,6 +32,13 @@ func C.CBytes(b []byte) unsafe.Pointer {
 	return C.__CBytes(b)
 }
 
+//go:linkname C.__get_errno_num runtime.cgo_errno
+func C.__get_errno_num() uintptr
+
+func C.__get_errno() error {
+	return syscall.Errno(C.__get_errno_num())
+}
+
 type (
 	C.char      uint8
 	C.schar     int8

cgo/testdata/errors.out.go

@@ -24,6 +24,7 @@
 
 package main
 
+import "syscall"
 import "unsafe"
 
 var _ unsafe.Pointer
@@ -55,6 +56,13 @@ func C.CBytes(b []byte) unsafe.Pointer {
 	return C.__CBytes(b)
 }
 
+//go:linkname C.__get_errno_num runtime.cgo_errno
+func C.__get_errno_num() uintptr
+
+func C.__get_errno() error {
+	return syscall.Errno(C.__get_errno_num())
+}
+
 type (
 	C.char      uint8
 	C.schar     int8

cgo/testdata/flags.out.go

@@ -5,6 +5,7 @@
 
 package main
 
+import "syscall"
 import "unsafe"
 
 var _ unsafe.Pointer
@@ -36,6 +37,13 @@ func C.CBytes(b []byte) unsafe.Pointer {
 	return C.__CBytes(b)
 }
 
+//go:linkname C.__get_errno_num runtime.cgo_errno
+func C.__get_errno_num() uintptr
+
+func C.__get_errno() error {
+	return syscall.Errno(C.__get_errno_num())
+}
+
 type (
 	C.char      uint8
 	C.schar     int8

cgo/testdata/symbols.out.go

@@ -1,5 +1,6 @@
 package main
 
+import "syscall"
 import "unsafe"
 
 var _ unsafe.Pointer
@@ -31,6 +32,13 @@ func C.CBytes(b []byte) unsafe.Pointer {
 	return C.__CBytes(b)
 }
 
+//go:linkname C.__get_errno_num runtime.cgo_errno
+func C.__get_errno_num() uintptr
+
+func C.__get_errno() error {
+	return syscall.Errno(C.__get_errno_num())
+}
+
 type (
 	C.char      uint8
 	C.schar     int8

cgo/testdata/types.out.go

@@ -1,5 +1,6 @@
 package main
 
+import "syscall"
 import "unsafe"
 
 var _ unsafe.Pointer
@@ -31,6 +32,13 @@ func C.CBytes(b []byte) unsafe.Pointer {
 	return C.__CBytes(b)
 }
 
+//go:linkname C.__get_errno_num runtime.cgo_errno
+func C.__get_errno_num() uintptr
+
+func C.__get_errno() error {
+	return syscall.Errno(C.__get_errno_num())
+}
+
 type (
 	C.char      uint8
 	C.schar     int8

compileopts/config.go

@@ -331,6 +331,7 @@ func (c *Config) CFlags(libclang bool) []string {
 			"-isystem", filepath.Join(path, "include"),
 			"-isystem", filepath.Join(picolibcDir, "include"),
 			"-isystem", filepath.Join(picolibcDir, "tinystdio"),
+			"-D__PICOLIBC_ERRNO_FUNCTION=__errno_location",
 		)
 	case "musl":
 		root := goenv.Get("TINYGOROOT")
@@ -340,6 +341,7 @@ func (c *Config) CFlags(libclang bool) []string {
 			"-nostdlibinc",
 			"-isystem", filepath.Join(path, "include"),
 			"-isystem", filepath.Join(root, "lib", "musl", "arch", arch),
+			"-isystem", filepath.Join(root, "lib", "musl", "arch", "generic"),
 			"-isystem", filepath.Join(root, "lib", "musl", "include"),
 		)
 	case "wasi-libc":

loader/loader.go

@@ -485,7 +485,7 @@ func (p *Package) parseFiles() ([]*ast.File, error) {
 		var initialCFlags []string
 		initialCFlags = append(initialCFlags, p.program.config.CFlags(true)...)
 		initialCFlags = append(initialCFlags, "-I"+p.Dir)
-		generated, headerCode, cflags, ldflags, accessedFiles, errs := cgo.Process(files, p.program.workingDir, p.ImportPath, p.program.fset, initialCFlags)
+		generated, headerCode, cflags, ldflags, accessedFiles, errs := cgo.Process(files, p.program.workingDir, p.ImportPath, p.program.fset, initialCFlags, p.program.config.GOOS())
 		p.CFlags = append(initialCFlags, cflags...)
 		p.CGoHeaders = headerCode
 		for path, hash := range accessedFiles {

src/runtime/baremetal.go

@@ -86,3 +86,16 @@ func AdjustTimeOffset(offset int64) {
 	// TODO: do this atomically?
 	timeOffset += offset
 }
+
+// Picolibc is not configured to define its own errno value, instead it calls
+// __errno_location.
+// TODO: a global works well enough for now (same as errno on Linux with
+// -scheduler=tasks), but this should ideally be a thread-local variable stored
+// in task.Task.
+// Especially when we add multicore support for microcontrollers.
+var errno int32
+
+//export __errno_location
+func libc_errno_location() *int32 {
+	return &errno
+}