Make COMPOSER_BIN_DIR env or _composer_bin_dir global available to binaries

[Seldaek]

Fixes #10389 - or at least adds a utility for packages with problems to fix it by doing something like this:

if [[ -z "$COMPOSER_BIN_DIR" ]]; then
  DIR="$( cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd )"
else
  DIR="$COMPOSER_BIN_DIR"
fi

Instead of simply DIR="$( cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd )" to find the Composer bin dir.

The problem is before Composer 2.2, when a binary was symlinked into bin-dir, it got executed as vendor/bin/foo which ran the script really stored in vendor/foo/bar/bin/foo, but $BASH_SOURCE[0] was set to vendor/bin/foo still, so that allowed people to discover the bin dir. Composer 2.2 always creates proxies instead of symlinking, so now the proxy runs vendor/foo/bar/bin/foo directly and so the resolved dir is vendor/foo/bar/bin instead of bin-dir, which breaks some assumptions.

These scripts were kinda broken anyway as they relied on symlinks which was just one way binaries could be installed, and so they were working on some envs but not all. The current 2.2 model is simpler as it's always working the same, but it will require some adjustments.

[Seldaek]

@johnstevenson do you see any problem with this?

[johnstevenson]

No, except that COMPOSER_BIN_DIR there will include a trailing \, if that matters.

I'm curious why this %~dp0 stuff is needed, when you know the location anyway - or will it move?

[Seldaek]

I have no idea why it's there anymore :) I saw the trailing slash, but i am assuming this will safely be ignored by most usages.

[Seldaek]

It's been in since the very beginning https://github.com/composer/composer/pull/140/files#diff-254824db37e271a109993b63f65d2072eaeeaf7b89996c8ccc0f937cb81ed2d0R173-R195 but perhaps it's not needed at all. Perhaps it was needed to create an absolute path to help with cygwin and such crappy envs. Anyway unless we have a good reason to get rid of it I'd probably leave it in.

[johnstevenson]

I think it was just copied from the way Composer-Setup.exe does it. Whatever, it is certainly not a crucial enough issue to be worrying about at the moment, so I'll shut up!

[johnstevenson]

Actually, in case of spaces in directory names it should probably be:
"SET COMPOSER_BIN_DIR=\"%~dp0\"\r\n".

[Seldaek]

You sure? Because the one above BIN_TARGET does not have it, it only uses quotes when using the variable, not setting it.

[johnstevenson]

Sorry, you're correct. I had assumed that because BIN_TARGET was being escaped, this would also need it. But it would appear that even though SET is a command, it uses its full command-line and ignores any whitespace.

Note entirely sure why BIN_TARGET is escaped and then has any surrounding quotes removed, but I'm not going to worry about that.