echo: always process escapes when POSIXLY_CORRECT is set

* src/echo.c (main): Always enable backslash processing if
POSIXLY_CORRECT is set.
* tests/misc/echo.sh: Add (the first) test for the echo command.
* tests/local.mk: Reference the new test.
* tests/misc/printf.sh: Update a stale comment.
* doc/coreutils.texi (echo invocation). Mention that POSIXLY_CORRECT
now always enables backslash processing.
* NEWS: Mention the change in behavior.
Fixes https://bugs.gnu.org/32703
Issue identified by Eric Blake.

NEWS

@@ -9,6 +9,12 @@ GNU coreutils NEWS                                    -*- outline -*-
   [This bug was present in "the beginning".]
 
   df no longer corrupts displayed multibyte characters on macOS.
+  [bug introduced with coreutils-8.18]
+
+** Changes in behavior
+
+  echo now always processes backslash escapes when the POSIXLY_CORRECT
+  environment variable is set.
 
   When possible 'ln A B' now merely links A to B and reports an error
   if this fails, instead of statting A and B before linking.  This

doc/coreutils.texi

@@ -12608,7 +12608,7 @@ If the @env{POSIXLY_CORRECT} environment variable is set, then when
 @command{echo}'s first argument is not @option{-n} it outputs
 option-like arguments instead of treating them as options.  For
 example, @code{echo -ne hello} outputs @samp{-ne hello} instead of
-plain @samp{hello}.
+plain @samp{hello}.  Also backslash escapes are always enabled.
 
 POSIX does not require support for any options, and says
 that the behavior of @command{echo} is implementation-defined if any

src/echo.c

@@ -108,8 +108,9 @@ int
 main (int argc, char **argv)
 {
   bool display_return = true;
+  bool posixly_correct = getenv ("POSIXLY_CORRECT");
   bool allow_options =
-    (! getenv ("POSIXLY_CORRECT")
+    (! posixly_correct
      || (! DEFAULT_ECHO_TO_XPG && 1 < argc && STREQ (argv[1], "-n")));
 
   /* System V machines already have a /bin/sh with a v9 behavior.
@@ -189,7 +190,7 @@ main (int argc, char **argv)
 
 just_echo:
 
-  if (do_v9)
+  if (do_v9 || posixly_correct)
     {
       while (argc > 0)
         {

tests/local.mk

@@ -237,6 +237,7 @@ all_tests =					\
   tests/fmt/base.pl				\
   tests/fmt/long-line.sh			\
   tests/fmt/goal-option.sh			\
+  tests/misc/echo.sh				\
   tests/misc/env.sh				\
   tests/misc/ptx.pl				\
   tests/misc/test.pl				\

tests/misc/echo.sh

@@ -0,0 +1,66 @@
+#!/bin/sh
+# basic tests for echo
+
+# Copyright (C) 2018 Free Software Foundation, Inc.
+
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+prog='env echo'
+
+. "${srcdir=.}/tests/init.sh"; path_prepend_ ./src
+print_ver_ echo
+
+
+# Verify the methods of specifying "Escape":
+# Note 4 octal digits are allowed (unlike printf which uses up to 3)
+printf '%s\n' . . . . . | tr . '\033' > exp
+$prog -n -e '\x1b\n\e\n\33\n\033\n\0033\n' > out || fail=1
+compare exp out || fail=1
+
+# Incomplete hex escapes are output as is (unlike printf)
+printf '\\x\n' > exp
+$prog -n -e '\x\n' > out || fail=1
+compare exp out || fail=1
+
+# Always output -- (unlike printf)
+$prog -- 'foo' > out || fail=1
+$prog -n -e -- 'foo\n' >> out || fail=1
+cat <<\EOF > exp
+-- foo
+-- foo
+EOF
+compare exp out || fail=1
+
+# Ensure \c stops processing
+$prog -e 'foo\n\cbar' > out || fail=1
+printf 'foo\n' > exp
+compare exp out || fail=1
+
+# With POSIXLY_CORRECT:
+#   only -n as the first (separate) option enables option processing
+#   -E is ignored
+#   escapes are processed by default
+POSIXLY_CORRECT=1 $prog -n -E 'foo\n' > out || fail=1
+POSIXLY_CORRECT=1 $prog -nE 'foo' >> out || fail=1
+POSIXLY_CORRECT=1 $prog -E -n 'foo' >> out || fail=1
+POSIXLY_CORRECT=1 $prog --version >> out || fail=1
+cat <<\EOF > exp
+foo
+-nE foo
+-E -n foo
+--version
+EOF
+compare exp out || fail=1
+
+Exit $fail

tests/misc/printf.sh

@@ -60,9 +60,8 @@ $prog '5 % +d\n' 234  >> out || fail=1
 # coreutils-5.0.1, it would print six bytes: "6 \41\n".
 $prog '6 \41\n' | tr '\41' '!' >> out
 
-# Note that as of coreutils-5.0.1, printf with a format of '\0002x'
-# prints a NUL byte followed by the digit '2' and an 'x'.
-# By contrast bash's printf outputs the same thing as $(printf '\2x') does.
+# Note that as of coreutils-5.0.1, printf with a format of '\0002y'
+# prints a NUL byte followed by the digit '2' and a 'y'.
 $prog '7 \2y \02y \002y \0002y\n' |tr '\0\2' '*=' >> out
 
 $prog '8 %b %b %b %b\n' '\1y' '\01y' '\001y' '\0001y'|tr '\1' = >> out