[swift-corelibs-dev] libdispatch import order / nondeterminism

[Drew Crawford]

I have built the Feb 8th snapshot into /usr/local on Linux 64.  I have enabled libdispatch, and it's installed to /usr/local as well.  Additionally, I cherry-picked the PR https://github.com/apple/swift/pull/1212 <https://github.com/apple/swift/pull/1212>.  It compiles, installs, everything's fine.

My /usr/local/include/dispatch directory has

# ls /usr/local/include/dispatch/
Dispatch.swiftdoc     base.h   data.h	   group.h	    io.h	object.h  queue.h      source.h
Dispatch.swiftmodule  block.h  dispatch.h  introspection.h  module.map	once.h	  semaphore.h  time.h

Now I attempt to use Dispatch via

swift -Xcc -fblocks -L/usr/local/lib -I /usr/local/include/dispatch -lBlocksRuntime -L/usr/lib/x86_64-linux-gnu

This is because:

-Xcc -fblocks to pick up blocks support
-I /usr/local/include/dispatch to pick up /usr/local/include/dispatch/Dispatch.swiftmodule (as well as module.modulemap etc.)
-lBlocksRuntime -L/usr/lib/x86_64-linux-gnu to pick up blocks runtime

Okay.  So now I import Dispatch:

Welcome to Swift version 3.0-dev (LLVM a7663bb722, Clang 4ca3c7fa28, Swift 319a912638). Type :help for assistance.
  1> import Dispatch

What I find is about half the time, this works just fine.  The other half the time, I hit an import error:

> <module-includes>:1:10: note: in file included from <module-includes>:1:
> #include "/usr/local/include/dispatch/dispatch.h"
>          ^
> /usr/local/include/dispatch/dispatch.h:34:10: note: in file included from /usr/local/include/dispatch/dispatch.h:34:
> #include <fcntl.h>
>          ^
> /usr/include/fcntl.h:67:11: note: in file included from /usr/include/fcntl.h:67:
> # include <time.h>
>           ^
> /usr/local/include/dispatch/time.h:25:2: error: "Please #include <dispatch/dispatch.h> instead of this file directly."
> #error "Please #include <dispatch/dispatch.h> instead of this file directly."
>  ^

What seems to be happening here is that we interpret /usr/include/fcntl.h's #include <time.h> to be /usr/local/include/dispatch/time.h instead of /usr/include/time.h.

This behavior is puzzling to me.

I don't understand why the behavior is nondeterministic.  Shouldn't an import failure be deterministic behavior?  Or are we just trying imports in random order?  That seems like a bad implementation.
I don't understand how to make sure fcntl's `time.h` goes to the right time.h without also losing Dispatch.swiftmodule and module.modulemap in the exchange.

Thanks in advance.

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