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BASH 4.4 mapfile/readarray/read builtins mis-behaving with pipe

Windows 10 Pro 10.0.17134 N/A Build 17134 patched through 15 July 2018
Cygwin 2.10.0(0.325/5/3)
Bash 4.4.12(3)

Cygwin man pages show:

  mapfile [-d delim] [-n count] [-O origin] [-s count] [-t] [-u fd]  [-C  callback]  [-c quantum] [array]   readarray  [-d delim] [-n count] [-O origin] [-s count] [-t] [-u fd] [-C callback] [-c        quantum] [array]     Read lines from the standard input into the indexed array variable  array,  or ...

 read [-ers] [-a aname] [-d delim] [-i text] [-n nchars] [-N nchars]  [-p  prompt]  [-t timeout] [-u fd] [name ...]
     One  line  is read from the standard input, or ...

So the expectation is that all bash stdin re-directions such as pipes and left chevs should work.  For mapfile/readarry, the output array variable MAPFILE is described as being created if not array_var argument is given to mapfile.

A) Searched cygwin email list with terms 'mapfile', 'readarray' and 'read builtin'.  No obvious hits of recent vintage in summary result list.

B) Confirm, not using mapfile/readarry/read, that bash stdin redirs work as expected.

echo multi-line_arg | /bin/cat

/bin/cat < some_unix_fmt_file

/bin/cat <<END

all work as expected.

C) Test mapfile/readarray/read in various stdin redir situations. Basic harnesses are :

mapfile/readarry HARNESS:
stdin or fd 0 feed to mapfile
: "${MAPFILE[@]:?MAPFILE null or unset}"
if get past the ${:?} test then dump MAPFILE with a simple /for /loop as: for ent in "${MAPFILE[@]}" ; do echo ="${ent}"= ; done

stdin feed to 'read foo'
: "${foo:?foo null or unset}"
if get past the ${:?} test then dump variable foo with echo ="${foo}"=

echo multi-line_arg | mapfile
cat multi-line_unix_fmt_file | mapfile
Fail with MAPFILE null or unset
If 'for' loop forced, "${MAPFILE[@]}" expands to zero words.

echo multi-line_arg | read foo
cat multi-line_unix_fmt_file | read foo
Fail with variable foo null or unset

CASES: left chev re-directions
mapfile < some_unix_fmt_file
mapfile <<END multi-line_here_doc
read < some_unix_fmt_file
read <<END multi-line_here_doc
Work as expected

CASES: mapfile or read in a sub-shell
echo multi-line_arg | ( mapfile ; stuff )
echo multi-line_arg | ( read foo; stuff )
Work as expected.  However, since mapfile/read result vars are not accessible in parent shell context, can force a notable re-structuring of a script to use sub-shells.

For bash builtins mapfile/readarray and read, stdin pipe re-direction directly to the builtin fails as if the builtin read()/fread() sees a silent pre-mature EOF on the pipe.  In the sub-shell tests, the pipe re-direction happens for the sub-shell as a whole before commands internal to the sub-shell get processed for individual re-dirs, so no re-dir processing directly for the builtin.

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