go/build: add ctxt.MatchFile

Fixes #6369.

R=dsymonds, r
CC=golang-dev
https://golang.org/cl/13708043

src/pkg/go/build/build.go

@@ -408,6 +408,14 @@ func (e *NoGoError) Error() string {
 	return "no buildable Go source files in " + e.Dir
 }
 
+func nameExt(name string) string {
+	i := strings.LastIndex(name, ".")
+	if i < 0 {
+		return ""
+	}
+	return name[i:]
+}
+
 // Import returns details about the Go package named by the import path,
 // interpreting local import paths relative to the srcDir directory.
 // If the path is a local import path naming a package that can be imported
@@ -591,58 +599,15 @@ Found:
 		if d.IsDir() {
 			continue
 		}
+
 		name := d.Name()
-		if strings.HasPrefix(name, "_") ||
+		ext := nameExt(name)
-			strings.HasPrefix(name, ".") {
-			continue
-		}
 
-		i := strings.LastIndex(name, ".")
+		match, data, filename, err := ctxt.matchFile(p.Dir, name, true, allTags)
-		if i < 0 {
-			i = len(name)
-		}
-		ext := name[i:]
-
-		if !ctxt.goodOSArchFile(name, allTags) && !ctxt.UseAllFiles {
-			if ext == ".go" {
-				p.IgnoredGoFiles = append(p.IgnoredGoFiles, name)
-			}
-			continue
-		}
-
-		switch ext {
-		case ".go", ".c", ".cc", ".cxx", ".cpp", ".s", ".h", ".hh", ".hpp", ".hxx", ".S", ".swig", ".swigcxx":
-			// tentatively okay - read to make sure
-		case ".syso":
-			// binary objects to add to package archive
-			// Likely of the form foo_windows.syso, but
-			// the name was vetted above with goodOSArchFile.
-			p.SysoFiles = append(p.SysoFiles, name)
-			continue
-		default:
-			// skip
-			continue
-		}
-
-		filename := ctxt.joinPath(p.Dir, name)
-		f, err := ctxt.openFile(filename)
 		if err != nil {
 			return p, err
 		}
-
+		if !match {
-		var data []byte
-		if strings.HasSuffix(filename, ".go") {
-			data, err = readImports(f, false)
-		} else {
-			data, err = readComments(f)
-		}
-		f.Close()
-		if err != nil {
-			return p, fmt.Errorf("read %s: %v", filename, err)
-		}
-
-		// Look for +build comments to accept or reject the file.
-		if !ctxt.shouldBuild(data, allTags) && !ctxt.UseAllFiles {
 			if ext == ".go" {
 				p.IgnoredGoFiles = append(p.IgnoredGoFiles, name)
 			}
@@ -672,6 +637,12 @@ Found:
 		case ".swigcxx":
 			p.SwigCXXFiles = append(p.SwigCXXFiles, name)
 			continue
+		case ".syso":
+			// binary objects to add to package archive
+			// Likely of the form foo_windows.syso, but
+			// the name was vetted above with goodOSArchFile.
+			p.SysoFiles = append(p.SysoFiles, name)
+			continue
 		}
 
 		pf, err := parser.ParseFile(fset, filename, data, parser.ImportsOnly|parser.ParseComments)
@@ -782,6 +753,79 @@ Found:
 	return p, pkgerr
 }
 
+// MatchFile reports whether the file with the given name in the given directory
+// matches the context and would be included in a Package created by ImportDir
+// of that directory.
+//
+// MatchFile considers the name of the file and may use ctxt.OpenFile to
+// read some or all of the file's content.
+func (ctxt *Context) MatchFile(dir, name string) (match bool, err error) {
+	match, _, _, err = ctxt.matchFile(dir, name, false, nil)
+	return
+}
+
+// matchFile determines whether the file with the given name in the given directory
+// should be included in the package being constructed.
+// It returns the data read from the file.
+// If returnImports is true and name denotes a Go program, matchFile reads
+// until the end of the imports (and returns that data) even though it only
+// considers text until the first non-comment.
+// If allTags is non-nil, matchFile records any encountered build tag
+// by setting allTags[tag] = true.
+func (ctxt *Context) matchFile(dir, name string, returnImports bool, allTags map[string]bool) (match bool, data []byte, filename string, err error) {
+	if strings.HasPrefix(name, "_") ||
+		strings.HasPrefix(name, ".") {
+		return
+	}
+
+	i := strings.LastIndex(name, ".")
+	if i < 0 {
+		i = len(name)
+	}
+	ext := name[i:]
+
+	if !ctxt.goodOSArchFile(name, allTags) && !ctxt.UseAllFiles {
+		return
+	}
+
+	switch ext {
+	case ".go", ".c", ".cc", ".cxx", ".cpp", ".s", ".h", ".hh", ".hpp", ".hxx", ".S", ".swig", ".swigcxx":
+		// tentatively okay - read to make sure
+	case ".syso":
+		// binary, no reading
+		match = true
+		return
+	default:
+		// skip
+		return
+	}
+
+	filename = ctxt.joinPath(dir, name)
+	f, err := ctxt.openFile(filename)
+	if err != nil {
+		return
+	}
+
+	if strings.HasSuffix(filename, ".go") {
+		data, err = readImports(f, false)
+	} else {
+		data, err = readComments(f)
+	}
+	f.Close()
+	if err != nil {
+		err = fmt.Errorf("read %s: %v", filename, err)
+		return
+	}
+
+	// Look for +build comments to accept or reject the file.
+	if !ctxt.shouldBuild(data, allTags) && !ctxt.UseAllFiles {
+		return
+	}
+
+	match = true
+	return
+}
+
 func cleanImports(m map[string][]token.Position) ([]string, map[string][]token.Position) {
 	all := make([]string, 0, len(m))
 	for path := range m {
@@ -1114,16 +1158,22 @@ func (ctxt *Context) goodOSArchFile(name string, allTags map[string]bool) bool {
 	}
 	n := len(l)
 	if n >= 2 && knownOS[l[n-2]] && knownArch[l[n-1]] {
-		allTags[l[n-2]] = true
+		if allTags != nil {
-		allTags[l[n-1]] = true
+			allTags[l[n-2]] = true
+			allTags[l[n-1]] = true
+		}
 		return l[n-2] == ctxt.GOOS && l[n-1] == ctxt.GOARCH
 	}
 	if n >= 1 && knownOS[l[n-1]] {
-		allTags[l[n-1]] = true
+		if allTags != nil {
+			allTags[l[n-1]] = true
+		}
 		return l[n-1] == ctxt.GOOS
 	}
 	if n >= 1 && knownArch[l[n-1]] {
-		allTags[l[n-1]] = true
+		if allTags != nil {
+			allTags[l[n-1]] = true
+		}
 		return l[n-1] == ctxt.GOARCH
 	}
 	return true

src/pkg/go/build/build_test.go

@@ -5,10 +5,12 @@
 package build
 
 import (
+	"io"
 	"os"
 	"path/filepath"
 	"reflect"
 	"runtime"
+	"strings"
 	"testing"
 )
 
@@ -142,3 +144,43 @@ func TestShouldBuild(t *testing.T) {
 		t.Errorf("shoudBuild(file3) tags = %v, want %v", m, want3)
 	}
 }
+
+type readNopCloser struct {
+	io.Reader
+}
+
+func (r readNopCloser) Close() error {
+	return nil
+}
+
+var matchFileTests = []struct {
+	name  string
+	data  string
+	match bool
+}{
+	{"foo_arm.go", "", true},
+	{"foo1_arm.go", "// +build linux\n\npackage main\n", false},
+	{"foo_darwin.go", "", false},
+	{"foo.go", "", true},
+	{"foo1.go", "// +build linux\n\npackage main\n", false},
+	{"foo.badsuffix", "", false},
+}
+
+func TestMatchFile(t *testing.T) {
+	for _, tt := range matchFileTests {
+		ctxt := Context{GOARCH: "arm", GOOS: "plan9"}
+		ctxt.OpenFile = func(path string) (r io.ReadCloser, err error) {
+			if path != "x+"+tt.name {
+				t.Fatalf("OpenFile asked for %q, expected %q", path, "x+"+tt.name)
+			}
+			return &readNopCloser{strings.NewReader(tt.data)}, nil
+		}
+		ctxt.JoinPath = func(elem ...string) string {
+			return strings.Join(elem, "+")
+		}
+		match, err := ctxt.MatchFile("x", tt.name)
+		if match != tt.match || err != nil {
+			t.Fatalf("MatchFile(%q) = %v, %v, want %v, nil", tt.name, match, err, tt.match)
+		}
+	}
+}