fix(ignore, fs): allow loading ignore patterns behind symlink (fixes #10785) (#10786)

The recent change to disallow following symlinks by default conflicts
with the expected behavior of .stignore. This adds a new flag which may
be passed to OpenFile to skip default symlink-forbidding open flag.

---------

Signed-off-by: Jakob Borg <jakob@kastelo.net>

lib/fs/basicfs.go

@@ -240,16 +240,24 @@ func (f *BasicFilesystem) DirNames(name string) ([]string, error) {
 	return names, nil
 }
 
+// Open opens the file for reading, while not following symlinks
 func (f *BasicFilesystem) Open(name string) (File, error) {
 	return f.OpenFile(name, os.O_RDONLY, 0)
 }
 
+// OpenFile is the generalised file open call, using the flags to determine
+// the open semantics. We always add O_NOFOLLOW, disallowing opening files
+// by following symlinks, unless OptFollow is set.
 func (f *BasicFilesystem) OpenFile(name string, flags int, mode FileMode) (File, error) {
 	rootedName, err := f.rooted(name)
 	if err != nil {
 		return nil, err
 	}
-	flags |= alwaysOpenFlags // enforce extra bits in flags
+	if flags&OptFollow != 0 {
+		flags &^= OptFollow // unset the synthetic OptFollow bit
+	} else {
+		flags |= optNoFollow
+	}
 	fd, err := os.OpenFile(rootedName, flags, os.FileMode(mode))
 	if err != nil {
 		return nil, err
@@ -257,6 +265,8 @@ func (f *BasicFilesystem) OpenFile(name string, flags int, mode FileMode) (File,
 	return basicFile{fd, name}, err
 }
 
+// Create opens a file for writing, creating it as required. The Create will
+// fail if the destination is a symlink.
 func (f *BasicFilesystem) Create(name string) (File, error) {
 	return f.OpenFile(name, os.O_RDWR|os.O_CREATE|os.O_TRUNC, 0o666)
 }

lib/fs/basicfs_unix.go

@@ -17,7 +17,7 @@ import (
 	"syscall"
 )
 
-const alwaysOpenFlags = syscall.O_NOFOLLOW // never open symlinks as the final path component
+const optNoFollow = syscall.O_NOFOLLOW
 
 func (f *BasicFilesystem) CreateSymlink(target, name string) error {
 	name, err := f.rooted(name)

lib/fs/basicfs_windows.go

@@ -19,7 +19,7 @@ import (
 	"golang.org/x/sys/windows"
 )
 
-const alwaysOpenFlags = 0 // no extra flags
+const optNoFollow = 0 // not defined for Windows
 
 var errNotSupported = errors.New("symlinks not supported")
 

lib/fs/filesystem.go

@@ -12,6 +12,7 @@ import (
 	"fmt"
 	"io"
 	"io/fs"
+	"math/bits"
 	"os"
 	"path/filepath"
 	"strings"
@@ -173,8 +174,18 @@ const (
 	OptSync       = os.O_SYNC
 	OptTruncate   = os.O_TRUNC
 	OptWriteOnly  = os.O_WRONLY
+	OptFollow     = 1 << (bits.UintSize - 2)
 )
 
+func init() {
+	// Ensure OptFollow doesn't clash with any other flag.
+	flags := OptAppend | OptCreate | OptExclusive | OptReadOnly | OptReadWrite | OptSync | OptTruncate | OptWriteOnly
+	flags &= OptFollow
+	if flags != 0 {
+		panic("bug: flag clash")
+	}
+}
+
 // SkipDir is used as a return value from WalkFuncs to indicate that
 // the directory named in the call is to be skipped. It is not returned
 // as an error by any function.