PR/48875 CVS commit: src/bin/sh

["Robert Elz" <kre%netbsd.org@localhost>]

The following reply was made to PR bin/48875; it has been noted by GNATS.

From: "Robert Elz" <kre%netbsd.org@localhost>
To: gnats-bugs%gnats.NetBSD.org@localhost
Cc: 
Subject: PR/48875 CVS commit: src/bin/sh
Date: Sun, 19 Aug 2018 23:50:27 +0000

 Module Name:	src
 Committed By:	kre
 Date:		Sun Aug 19 23:50:27 UTC 2018
 
 Modified Files:
 	src/bin/sh: error.h eval.c input.c input.h main.c parser.c trap.c
 	    trap.h
 
 Log Message:
 PR bin/48875 (is related, and ameliorated, but not exactly "fixed")
 
 Import a whole set of tree evaluation enhancements from FreeBSD.
 
 With these, before forking, the shell predicts (often) when all it will
 have to do after forking (in the parent) is wait for the child and then
 exit with the status from the child, and in such a case simply does not
 fork, but rather allows the child to take over the parent's role.
 
 This turns out to handle the particular test case from PR bin/48875 in
 such a way that it works as hoped, rather than as it did (the delay there
 was caused by an extra copy of the shell hanging around waiting for the
 background child to complete ... and keeping the command substitution
 stdout open, so the "real" parent had to wait in case more output appeared).
 
 As part of doing this, redirection processing for compound commands gets
 moved out of evalsubshell() and into a new evalredir(), which allows us
 to properly handle errors occurring while performing those redirects,
 and not mishandle (as in simply forget) fd's which had been moved out
 of the way temporarily.
 
 evaltree() has its degree of recursion reduced by making it loop to
 handle the subsequent operation: that is instead of (for any binop
 like ';' '&&' (etc)) where it used to
 	evaltree(node->left);
 	evaltree(node->right);
 	return;
 it now does (kind of)
 	next = node;
 	while ((node = next) != NULL) {
 		next = NULL;
 
 		if (node is a binary op) {
 			evaltree(node->left);
 			if appropriate /* if && test for success, etc */
 				next = node->right;
 			continue;
 		}
 		/* similar for loops, etc */
 	}
 which can be a good saving, as while the left side (now) tends to be
 (usually) a simple (or simpleish) command, the right side can be many
 commands (in a command sequence like a; b; c; d; ...  the node at the
 top of the tree will now have "a" as its left node, and the tree for
 b; c; d; ... as its right node - until now everything was evaluated
 recursively so it made no difference, and the tree was constructed
 the other way).
 
 if/while/... statements are done similarly, recurse to evaluate the
 condition, then if the (or one of the) body parts is to be evaluated,
 set next to that, and loop (previously it recursed).
 
 There is more to do in this area (particularly in the way that case
 statements are processed - we can avoid recursion there as well) but
 that can wait for another day.
 
 While doing all of this we keep much better track of when the shell is
 just going to exit once the current tree is evaluated (with a new
 predicate at_eof() to tell us that we have, for sure, reached the end
 of the input stream, that is, this shell will, for certain, not be reading
 more command input) and use that info to avoid unneeded forks.   For that
 we also need another new predicate (have_traps()) to determine of there
 are any caught traps which might occur - if there are, we need to remain
 to (potentially) handle them, so these optimisations will not occur (to
 make the issue in PR 48875 appear again, run the same code, but with a
 trap set to execute some code when a signal (or EXIT) occurs - note that
 the trap must be set in the appropriate level of sub-shell to have this
 effect, any caught traps are cleared in a subshell whenever one is created).
 
 There is still work to be done to handle traps properly, whatever
 weirdness they do (some of which is related to some of this.)
 
 These changes do not need man page updates, but 48875 does - an update
 to sh.1 will be forthcoming once it is decided what it should say...
 
 Once again, all the heavy lifting for this set of changes comes directly
 (with thanks) from the FreeBSD shell.
 
 XXX pullup-8 (but not very soon)
 
 
 To generate a diff of this commit:
 cvs rdiff -u -r1.20 -r1.21 src/bin/sh/error.h src/bin/sh/input.h
 cvs rdiff -u -r1.158 -r1.159 src/bin/sh/eval.c
 cvs rdiff -u -r1.62 -r1.63 src/bin/sh/input.c
 cvs rdiff -u -r1.74 -r1.75 src/bin/sh/main.c
 cvs rdiff -u -r1.149 -r1.150 src/bin/sh/parser.c
 cvs rdiff -u -r1.44 -r1.45 src/bin/sh/trap.c
 cvs rdiff -u -r1.22 -r1.23 src/bin/sh/trap.h
 
 Please note that diffs are not public domain; they are subject to the
 copyright notices on the relevant files.