[PATCH v2 3/3] input: preadfd: read standard input byte-wise

[наб <nabijaczleweli@nabijaczleweli.xyz>]

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POSIX Issue 7, XCU, sh, STDIN says:
  When the shell is using standard input and it invokes a command that
  also uses standard input, the shell shall ensure that the standard
  input file pointer points directly after the command it has read when
  the command begins execution. It shall not read ahead in such a manner
  that any characters intended to be read by the invoked command are
  consumed by the shell (whether interpreted by the shell or not) or
  that characters that are not read by the invoked command are not seen
  by the shell.

I.e.
  sh <<EOF
  id
  cat
  good!
  EOF
must execute id, then execute cat, then the cat must copy "good!"
to the standard output stream, and similarly
  sh <<"EOF"
  id
  read Q
  good!
  echo Q$Q
  EOF
must execute id, then read "good!" into Q, then echo "Qgood!".

Heretofor the output was as such:
  uid=1000(nabijaczleweli) gid=100(users) groups=100(users)
  ./dash: 3: good!: not found
and as such (with -x):
  + id
  uid=1000(nabijaczleweli) gid=100(users) groups=100(users)
  + read Q
  + good!
  sh: 3: good!: not found
  + echo Q
  Q
and a strace confirms:
  read(0, "id\ncat\ngood!\n", 8192)       = 13
  read(0, "id\nread Q\ngood!\necho Q$Q\n", 8192) = 25

Reading the standard input byte-by-byte is the obvious solution to this
issue, Just Works, and is how all of shells do it on non-seekable input
(we could, theoretically, read regular files block-wise,
then seek within them after parsing, but the complexity out-weighs
the rarity of running sh < program; we could also do whole-line reads
on teletypes in icanon mode, but, again, the gain here is miniscule
for an interactive session, and the mode can change at any time, so...).
Naturally, we keep reading block-wise from not-standard-input.

With this patch, we observe the correct
  uid=1000(nabijaczleweli) gid=100(users) groups=100(users)
  good!
and
  + id
  uid=1000(nabijaczleweli) gid=100(users) groups=100(users)
  + read Q
  + echo Qgood!
  Qgood!

Fixes: https://bugs.debian.org/862907
---
 src/input.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/input.c b/src/input.c
index ec075f5..6b6113e 100644
--- a/src/input.c
+++ b/src/input.c
@@ -195,7 +195,7 @@ retry:
 
 	} else
 #endif
-		nr = read(parsefile->fd, buf, IBUFSIZ - 1);
+		nr = read(parsefile->fd, buf, parsefile->fd == 0 ? 1 : IBUFSIZ - 1);
 
 
 	if (nr < 0) {
-- 
2.30.2

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