build: give BlocksRuntime hidden visibility

[compnerd]

Since BlocksRuntime is currently built statically, we should ensure that it does
not leak symbols into the target library or executable it is being linked into.
This will require consumers to explicitly link against the BlocksRuntime.

[compnerd]

CC: @kevints @MadCoder

[MadCoder]

I don't think this works, there are symbols that need to be unique for all users of the blocks runtime, and if Swift has such a runtime and dispatch, they must agree on the isa's of blocks.

I don't feel I'm competent review this, you need Swift runtime people to look into that.

[kevints]

This seems like the wrong approach - Block_retain and Block_release are globals, and the Block_retain and Block_release that your program see need to be the same ones that libdispatch see.

Please see my comments in
swiftlang/swift#19640 (comment)
and
swiftlang/swift#19640 (comment) for a detailed explanation of why.

[kevints]

This won't work as it only sets the default visibility (-fvisibility=hidden), source files can still override it with __attribute__(visibility("default")) (which the BLOCK_EXPORT macro defines).

In fact this is already set on line 20: 1eee0f9#diff-af3b638bc2a3e6c650974192a53c7291R20

Since your ultimate goal is one BlocksRuntime per program, have you considered removing STATIC from the add_library(BlocksRuntime) command above? I think that would result in a BlocksRuntime that respects BUILD_SHARED_LIBS. You should also make sure to install it, e.g.

install(TARGETS BlocksRuntime DESTINATION "${INSTALL_TARGET_LIBDIR}")

[compnerd]

Yeah, there is already a PR for that (and this was meant as a stop gap until we get that merged. Since we discussed that we are going to actively pursue that first, this is no longer relevant.