bash incorrectly handles 'echo' + SIGPIPE under some situations on Linux

[cks]

Configuration Information [Automatically generated, do not change]:
Machine: x86_64
OS: linux-gnu
Compiler: gcc
Compilation CFLAGS:  -DPROGRAM='bash' -DCONF_HOSTTYPE='x86_64' 
-DCONF_OSTYPE='linux-gnu' -DCONF_MACHTYPE='x86_64-pc-linux-gnu' 
-DCONF_VENDOR='pc' -DLOCALEDIR='/usr/share/locale' -DPACKAGE='bash' -DSHELL 
-DHAVE_CONFIG_H   -I.  -I../. -I.././include -I.././lib  -D_FORTIFY_SOURCE=2 -g 
-O2 -fstack-protector --param=ssp-buffer-size=4 -Wformat 
-Werror=format-security -Wall
uname output: Linux apps0 3.13.0-65-generic #106-Ubuntu SMP Fri Oct 2 22:08:27 
UTC 2015 x86_64 x86_64 x86_64 GNU/Linux
Machine Type: x86_64-pc-linux-gnu

Bash Version: 4.3
Patch Level: 11
Release Status: release

Description:

If Bash writes to stdout with the built in echo and gets a SIGPIPE,
it can incorrectly buffer and then repeat this would-have-been output
in at least $(...) things invoked in a cleanup function and in fact
in some other contexts as well. To see this in action, create the
following script as /tmp/repro:

        #!/bin/bash
        function cleanup() {
                r1=$(/bin/echo one)
                r2=$(/bin/echo two)
                echo $r1 '!' $r2 1>&2
                #echo $r1 '!' $r2 >>/tmp/logout
        }
        trap cleanup EXIT
        sleep 1
        echo final

Run it as '/tmp/repro | false'. The output produced is the clearly
incorrect:
        $ /tmp/repro | false
        final
        one final ! two final

If you switch the 'echo' to the commented out version, the bare 'final'
disappears but the other two remain.

Although I am filing this bug report from an Ubuntu 14.04 LTS machine,
this issue also reproduces with the Fedora 22 and Fedora 23 versions
of 'version 4.3.42(1)-release' and probably on other versions as well.

For your potential reference, the original issue that uncovered this
problem is covered here:
        https://github.com/golang/go/issues/13789