[Libguestfs] nbdkit / mingw support

[Richard W.M. Jones]

Eric:

Yifan Gu has posted a few patches for mingw support.  My comments
below.

https://github.com/gyf304/nbdkit/commit/a37c4ca6546dfc4e96e305af97b62e5a9d6174ca

* I think the SHARED_LDFLAGS idea is good.  I pushed a slightly
  different take on the idea here:
  https://github.com/libguestfs/nbdkit/commit/1d634009ab8e43592065ec469df6312400525cc8
  It's slightly different from what Yifan posted above, because I
  replaced -module -avoid-version -shared with $(SHARED_LDFLAGS),
  adding -no-undefined additionally on mingw.

* Change to .gitignore is obviously fine, but I kept the file sorted:
  https://github.com/libguestfs/nbdkit/commit/002da4f4ca279a798fea1bf8749558d643d7c186

* Defining -D_spawnv=spawnv seems OK, but should be a separate patch.
  I didn't push anything.

* Unclear why AM_CONDITIONAL(MINGW) is needed.  It doesn't seem to be
  used anywhere unless I'm missing something.  However if conditional
  compilation for mingw is needed for something, then this would be fine.

* Unclear why it's necessary to rename nbdkit-common.h -> nbdkit-compat.h ?
  While this isn't API, it seems we should not rename it unless there's
  a good reason.

* Functions are indirected through struct nbdkit_functions
  _nbdkit_functions which I guess is necessary because of PE linking
  (we've had similar problems with PE linking restrictions in other
  programs).  However I wasn't very clear how this actually works, and
  we cannot break existing Linux binaries, so this needs more thought
  and must be conditional on Windows only.

https://github.com/gyf304/nbdkit/commit/4f238c3e52a70b855dbb5f3eb3084d357b7c8071

* This one should be folded into the eventual nbdkit_functions patch.

https://github.com/gyf304/nbdkit/commit/20d888fbb8ccf33f0975a070df0d8e3d929b71f8

* Is there no way to get the socket peer on Windows?

Rich.

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