mapfile -n seeks ahead an extra line

[Dan Douglas]

From: Dan Douglas
To: bug-bash@gnu.org
Subject: mapfile -n seeks ahead an extra line

Configuration Information [Automatically generated, do not change]:
Machine: x86_64
OS: linux-gnu
Compiler: x86_64-pc-linux-gnu-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./include -I. -I./include -I./lib  
-DDEFAULT_PATH_VALUE='/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin'
 -DSTANDARD_UTILS_PATH='/bin:/usr/bin:/sbin:/usr/sbin' 
-DSYS_BASHRC='/etc/bash/bashrc' -DSYS_BASH_LOGOUT='/etc/bash/bash_logout' 
-DNON_INTERACTIVE_LOGIN_SHELLS -DSSH_SOURCE_BASHRC -march=native -Ofast -mmmx 
-pipe -floop-interchange -floop-strip-mine -floop-block -ggdb
uname output: Linux smorgbox 3.4.2-pf+ #62 SMP PREEMPT Mon Jun 11 16:24:00 CDT 
2012 x86_64 Intel(R) Core(TM) i7 CPU 920 @ 2.67GHz GenuineIntel GNU/Linux
Machine Type: x86_64-pc-linux-gnu

Bash Version: 4.2
Patch Level: 29
Release Status: release

Description:
    mapfile -n eats a line too many, discarding the last.

Repeat-By:
    $ printf '%s\n' {a..f} | { mapfile -tn 2 a; mapfile -t b; declare -p a b; }
    declare -a a='([0]="a" [1]="b")'
    declare -a b='([0]="d" [1]="e" [2]="f")'

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