os: document CopyFS behavior for symlinks in destination

Also clarify the permissions of created files,
and note that CopyFS will not overwrite files.

Update a few places in documentation to use 0oXXX for octal consts.

For #62484

Change-Id: I208ed2bde250304bc7fac2b93963ba57037e791e
Reviewed-on: https://go-review.googlesource.com/c/go/+/600775
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/os/dir.go

@@ -132,15 +132,17 @@ func ReadDir(name string) ([]DirEntry, error) {
 // CopyFS copies the file system fsys into the directory dir,
 // creating dir if necessary.
 //
-// Newly created directories and files have their default modes
-// where any bits from the file in fsys that are not part of the
-// standard read, write, and execute permissions will be zeroed
-// out, and standard read and write permissions are set for owner,
-// group, and others while retaining any existing execute bits from
-// the file in fsys.
+// Files are created with mode 0o666 plus any execute permissions
+// from the source, and directories are created with mode 0o777
+// (before umask).
 //
-// Symbolic links in fsys are not supported, a *PathError with Err set
-// to ErrInvalid is returned on symlink.
+// CopyFS will not overwrite existing files, and returns an error
+// if a file name in fsys already exists in the destination.
+//
+// Symbolic links in fsys are not supported. A *PathError with Err set
+// to ErrInvalid is returned when copying from a symbolic link.
+//
+// Symbolic links in dir are followed.
 //
 // Copying stops at and returns the first error encountered.
 func CopyFS(dir string, fsys fs.FS) error {

src/os/example_test.go

@@ -61,7 +61,7 @@ func ExampleFileMode() {
 		log.Fatal(err)
 	}
 
-	fmt.Printf("permissions: %#o\n", fi.Mode().Perm()) // 0400, 0777, etc.
+	fmt.Printf("permissions: %#o\n", fi.Mode().Perm()) // 0o400, 0o777, etc.
 	switch mode := fi.Mode(); {
 	case mode.IsRegular():
 		fmt.Println("regular file")

src/os/file.go

@@ -366,7 +366,7 @@ func Open(name string) (*File, error) {
 }
 
 // Create creates or truncates the named file. If the file already exists,
-// it is truncated. If the file does not exist, it is created with mode 0666
+// it is truncated. If the file does not exist, it is created with mode 0o666
 // (before umask). If successful, methods on the returned File can
 // be used for I/O; the associated file descriptor has mode O_RDWR.
 // If there is an error, it will be of type *PathError.
@@ -606,11 +606,11 @@ func UserHomeDir() (string, error) {
 // On Unix, the mode's permission bits, ModeSetuid, ModeSetgid, and
 // ModeSticky are used.
 //
-// On Windows, only the 0200 bit (owner writable) of mode is used; it
+// On Windows, only the 0o200 bit (owner writable) of mode is used; it
 // controls whether the file's read-only attribute is set or cleared.
 // The other bits are currently unused. For compatibility with Go 1.12
-// and earlier, use a non-zero mode. Use mode 0400 for a read-only
-// file and 0600 for a readable+writable file.
+// and earlier, use a non-zero mode. Use mode 0o400 for a read-only
+// file and 0o600 for a readable+writable file.
 //
 // On Plan 9, the mode's permission bits, ModeAppend, ModeExclusive,
 // and ModeTemporary are used.