Clearer error in case of Android.bp being unreadable

Bug: 64600838
Test: mkdir errtest \
      && ln -s /tmp/dontexist errtest/Android.bp \
      # and add errtest to ./Android.bp \
      && m nothing \
      # and check that the error message mentions a symlink

Change-Id: I841ec12d613f61ccc3396538062bee48c8c1ca27

context.go

@@ -19,6 +19,7 @@ import (
 	"errors"
 	"fmt"
 	"io"
+	"os"
 	"path/filepath"
 	"reflect"
 	"runtime"
@@ -793,6 +794,23 @@ func (c *Context) parseBlueprintsFile(filename string, scope *parser.Scope, root
 
 	f, err := c.fs.Open(filename)
 	if err != nil {
+		// couldn't open the file; see if we can provide a clearer error than "could not open file"
+		stats, statErr := c.fs.Lstat(filename)
+		if statErr == nil {
+			isSymlink := stats.Mode()&os.ModeSymlink != 0
+			if isSymlink {
+				err = fmt.Errorf("could not open symlink %v : %v", filename, err)
+				target, readlinkErr := os.Readlink(filename)
+				if readlinkErr == nil {
+					_, targetStatsErr := c.fs.Lstat(target)
+					if targetStatsErr != nil {
+						err = fmt.Errorf("could not open symlink %v; its target (%v) cannot be opened", filename, target)
+					}
+				}
+			} else {
+				err = fmt.Errorf("%v exists but could not be opened: %v", filename, err)
+			}
+		}
 		errsCh <- []error{err}
 		return
 	}

pathtools/fs.go

@@ -16,6 +16,7 @@ package pathtools
 
 import (
 	"bytes"
+	"errors"
 	"fmt"
 	"io"
 	"io/ioutil"
@@ -61,6 +62,7 @@ type FileSystem interface {
 	Glob(pattern string, excludes []string) (matches, dirs []string, err error)
 	glob(pattern string) (matches []string, err error)
 	IsDir(name string) (bool, error)
+	Lstat(name string) (os.FileInfo, error)
 }
 
 // osFs implements FileSystem using the local disk.
@@ -94,6 +96,10 @@ func (osFs) glob(pattern string) ([]string, error) {
 	return filepath.Glob(pattern)
 }
 
+func (osFs) Lstat(path string) (stats os.FileInfo, err error) {
+	return os.Lstat(path)
+}
+
 type mockFs struct {
 	files map[string][]byte
 	dirs  map[string]bool
@@ -150,3 +156,7 @@ func (m *mockFs) glob(pattern string) ([]string, error) {
 	}
 	return matches, nil
 }
+
+func (m *mockFs) Lstat(path string) (stats os.FileInfo, err error) {
+	return nil, errors.New("Lstat is not yet implemented in MockFs")
+}