Re: [PATCH 29/29] t/test-lib: teach --chain-lint to detect broken &&-chains in subshells

Eric Sunshine [off-list ref] writes:

The --chain-lint option detects broken &&-chains by forcing the test to
exit early (as the very first step) with a sentinel value. If that
sentinel is the test's overall exit code, then the &&-chain is intact;
if not, then the chain is broken. Unfortunately, this detection does not
extend to &&-chains within subshells even when the subshell itself is
properly linked into the outer &&-chain.

Address this shortcoming by eliminating the subshell during the
"linting" phase and incorporating its body directly into the surrounding
&&-chain. To keep this transformation cheap, no attempt is made at
properly parsing shell code. Instead, the manipulations are purely
textual. For example:

    statement1 &&
    (
        statement2 &&
        statement3
    ) &&
    statement4

is transformed to:

    statement1 &&
        statement2 &&
        statement3 &&
    statement4

so, with --chain-lint, we would transform this

	mkdir -p a/b/c &&
	(
		cd a/b/c
		rm -fr ../../*
	) &&
	statement 4

into this sequence

	(exit $sentinel) &&
	mkdir -p a/b/c &&
		cd a/b/c
		rm -fr ../../* &&
	statement 4

and then rely on the non-zero exit to cancel all the remainder?

We didn't create nor cd to the t/trash$num/a/b/c thanks to the &&
chain, and end up running rm -fr ../../* from inside t/trash$num?

Hmmmmm....