cmd/go: add basic support for overlays

This CL adds basic support for listing packages with overlays.
The new cmd/go/internal/fs package adds an abstraction for communicating
with the file system that will open files according to their overlaid paths,
and provides functions to override those in the build context to open
overlaid files. There is also some support for executing builds on packages
with overlays. In cmd/go/internal/work.(*Builder).build, paths are mapped
to their overlaid paths before they are given as arguments to tools.

For #39958

Change-Id: I5ec0eb9ebbca303e2f1e7dbe22ec32613bc1fd17
Reviewed-on: https://go-review.googlesource.com/c/go/+/253747
Trust: Michael Matloob <matloob@golang.org>
Trust: Jay Conrod <jayconrod@google.com>
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>

1 files changed, 2 insertions, 49 deletions

diff --git a/src/cmd/go/internal/modload/import.go b/src/cmd/go/internal/modload/import.go
index c36c8bd29b..3642de851a 100644
--- a/src/cmd/go/internal/modload/import.go
+++ b/src/cmd/go/internal/modload/import.go
@@ -17,6 +17,7 @@ import (
 	"time"
 
 	"cmd/go/internal/cfg"
+	"cmd/go/internal/fsys"
 	"cmd/go/internal/modfetch"
 	"cmd/go/internal/par"
 	"cmd/go/internal/search"
@@ -438,57 +439,9 @@ func dirInModule(path, mpath, mdir string, isLocal bool) (dir string, haveGoFile
 	// We don't care about build tags, not even "+build ignore".
 	// We're just looking for a plausible directory.
 	res := haveGoFilesCache.Do(dir, func() interface{} {
-		ok, err := isDirWithGoFiles(dir)
+		ok, err := fsys.IsDirWithGoFiles(dir)
 		return goFilesEntry{haveGoFiles: ok, err: err}
 	}).(goFilesEntry)
 
 	return dir, res.haveGoFiles, res.err
 }
-
-func isDirWithGoFiles(dir string) (bool, error) {
-	f, err := os.Open(dir)
-	if err != nil {
-		if os.IsNotExist(err) {
-			return false, nil
-		}
-		return false, err
-	}
-	defer f.Close()
-
-	names, firstErr := f.Readdirnames(-1)
-	if firstErr != nil {
-		if fi, err := f.Stat(); err == nil && !fi.IsDir() {
-			return false, nil
-		}
-
-		// Rewrite the error from ReadDirNames to include the path if not present.
-		// See https://golang.org/issue/38923.
-		var pe *os.PathError
-		if !errors.As(firstErr, &pe) {
-			firstErr = &os.PathError{Op: "readdir", Path: dir, Err: firstErr}
-		}
-	}
-
-	for _, name := range names {
-		if strings.HasSuffix(name, ".go") {
-			info, err := os.Stat(filepath.Join(dir, name))
-			if err == nil && info.Mode().IsRegular() {
-				// If any .go source file exists, the package exists regardless of
-				// errors for other source files. Leave further error reporting for
-				// later.
-				return true, nil
-			}
-			if firstErr == nil {
-				if os.IsNotExist(err) {
-					// If the file was concurrently deleted, or was a broken symlink,
-					// convert the error to an opaque error instead of one matching
-					// os.IsNotExist.
-					err = errors.New(err.Error())
-				}
-				firstErr = err
-			}
-		}
-	}
-
-	return false, firstErr
-}