shell script error management in busybox ash

["Roberto A. Foglietta" <roberto.foglietta@gmail.com>]

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Dear all,

 Denys Vlasenko, the maintainer of BusyBox, asked me to organise a
task to synchronise busybox ash and dash in such a way the future
developments could benefit from each other's contributions. In
particular this need raised after these patches has been applied:

ash: add bash-like ERR trap and set -E
https://git.busybox.net/busybox/commit/?id=e0bf3df0205d5ccef52df67b1760b8b54f15ec6e

ash: fix LINENO in functions
https://git.busybox.net/busybox/commit/?id=d6c9cbc0727cc3875672ae280ec129514a9d8594

ash: LINENO starts from 0 in -c SCRIPT mode
https://git.busybox.net/busybox/commit/?id=64aa86b720641cb50be9636e6c20d4dbbea6fed0

ash: introduce bash-like $FUNCNAME
https://git.busybox.net/busybox/commit/?id=704c596563a5b4da4349b272d0c9c71aacea34a7

 These patches derive from a larger patch that I developed to improve
the error management/debug in busybox ash. In brief these patches add
the following functionalities:

 - trap ERR and set -E added
 - LINENO became a global variable
 - FUNCNAME global variable added

 Developing these patches we realised that the use of EXEXIT and EXEND
exceptions complicates the code and do not add any value. Moreover,
the code that these exceptions trigger (everywhere the setjmp()
returns true) restores some values and frees some memory but the traps
need the information before the restore, not the restored values. For
these reason I developed three patches here in attachment:

 1. get rid of EXEXIT in exitcmd() and fix a bug about return value
 2. get rid of EXEND at the end of evaltree()
 3. remove the EXEXIT and EXEND altogether

 The description of these patches are simply arbitrary and temporary.
I hope we will agree on these details once we are going to port them
to dash.

 Applying these patches the output printed by trap ERR and trap EXIT
is what was expected to be demonstrating that to avoid these
exceptions the code became straightforward. We are here to check your
interest in porting these patches to dash and your will to apply.

 Our focus is related to the patches listed above but in the long run
it is not limited to them but also includes the following patches.
These patches have been applied in the meantime that others made
changes to ash.c as well. The full list of patches and their order is
listed here:

https://git.busybox.net/busybox/log/

 Other patches that have been applied that might be a convenience to
port in dash are the following:

ash: eval: Check nflag in evaltree instead of cmdloop
https://git.busybox.net/busybox/commit/?id=41beb53787ec798a27f336c4758cb5ebd8f0c75a

ash: eval: Do not cache value of eflag in evaltree
https://git.busybox.net/busybox/commit/?id=f415e21a7dce1d4f4b760fddfaba85c551681e11

ash: parser: Fix handling of empty aliases
https://git.busybox.net/busybox/commit/?id=30af5938afad076e12b8ece123cab0b8bc92a596

ash: parser: Save and restore heredoclist in expandstr
https://git.busybox.net/busybox/commit/?id=1c06ddd8bbbd6906e5bf00ec93e04d5090718be9

ash: use pgetc_eatbnl() in more places, take 3
https://git.busybox.net/busybox/commit/?id=c54025612711a6b1997efd99206b9fbcaa5a29cf

ash: parser: Fix alias expansion after heredoc or newlines
https://git.busybox.net/busybox/commit/?id=8c68ae8416c8b54222eb3cd1d4908a570147e134

ash: parser: Get rid of PEOA
https://git.busybox.net/busybox/commit/?id=48cb983b136fb74c61db594a30e18bdc42b7264c

ash: eval: Prevent recursive PS4 expansion
https://git.busybox.net/busybox/commit/?id=eb607777697f4c5eb2dfd86e5837a8c379f65979

ash: fix ignoreeof option
https://git.busybox.net/busybox/commit/?id=0beee209778870888c3a80a9ae57e74888bc8e7b

ash: stopped jobs should only prevent exit from interactive shell
https://git.busybox.net/busybox/commit/?id=50239a665c88f5a95ce41146804500f5da90b19e

ash: let ignoreeof only affect interactive shells
https://git.busybox.net/busybox/commit/?id=5726df5f94f973eaa097d9853ceff2bd6b748d97

 I wish to receive your feedback and I hope you are interested in the porting.

 Best regards,
--
Roberto A. Foglietta
+39.349.33.30.697

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