[minor] umask 400 causes here-{doc,string} failure

[Stephane Chazelas <stephane.chazelas@gmail.com>]

Note: sent to bash, zsh and Schily Bourne shell dev mailing
lists (not mksh as my email provider apparently doesn't play
well with mirbsd.org's expensive greylisting, please feel free
to forward there if you don't use gmail).

That's from:
https://unix.stackexchange.com/questions/429285/cannot-create-temp-file-for-here-document-permission-denied

$ bash -c 'umask 400; cat <<< test'
bash: cannot create temp file for here-document: Permission denied
$ zsh -c 'umask 400; cat <<< test'
zsh:1: can't create temp file for here document: permission denied
$ bosh -c 'umask 400; cat << EOF
test
EOF'
bosh: /tmp/sh193220: cannot open
$ mksh -c 'umask 400; cat <<< test'
mksh: can't open temporary file /tmp/sh933f2z.tmp: Permission denied

Those shells use temporary files to store the content of the
here-documents as the Bourne shell initially did, and open them
in read-only mode to make it cat's stdin.

When umask contains the 0400 bit, the file is created without
read permission to the user, hence the error upon that second
open().

(note that bosh also leaves the temp file behind in that
case).

I can think of several ways to address it:

1- do nothing and blame the user as the user explicitly asked
for files to be unreadable (but then again, it's not obvious
to the user that heredocs imply a temp file)

2- do like AT&T ksh/tcsh (or yash for big heredocs that don't
fit in the pipe buffer) and open the file only once for both
writing the content and making it the command's stdin (with a
lseek() to beginning in between). That means the fd ends up
being writable though I can't see it being a huge problem. (Yash
actually gives the file 000 permissions here regardless of the
umask with open("/tmp/yash-ECCFE6268", O_RDWR|O_CREAT|O_EXCL,
0), but see below about =(...) emulation)

3. do like dash/yash/rc/es and use a pipe instead of a temp
file. That means having to fork a process to feed the data (or
like yash fall back to a temp file for big heredocs). That also
means the fd is no longer seekable

The change could break some scripts for bash, as on Linux (where
/dev/fd/n behaves differently from other *nices), we see some
doing:

cmd1 /dev/fd/3 3<<< "$(cmd2)"

to emulate zsh's cmd1 =(cmd2) (command substitution using a temp
file). (A 0400 umask also makes a =(...) file unreadable, but
definitely here it's the user's problem).

4. Reset the umask temporarily to 077 before creating the temp
file (and block trapped signals until it's restored).


2 would have my preference.

-- 
Stephane