Fix CI hangs on win32

After much debugging, it turns out that OpenBlas is sometimes hanging
on exit due to bad assumptions in its thread management. The upstream
discussion is at OpenMathLib/OpenBLAS#2350, but this
should fix our frequent win32 CI failures in the meantime.

deps/blas.mk

@@ -99,7 +99,12 @@ $(BUILDDIR)/$(OPENBLAS_SRC_DIR)/openblas-skylakexdgemm.patch-applied: $(BUILDDIR
 		patch -p1 -f < $(SRCDIR)/patches/openblas-skylakexdgemm.patch
 	echo 1 > $@
 
-$(BUILDDIR)/$(OPENBLAS_SRC_DIR)/build-configured: $(BUILDDIR)/$(OPENBLAS_SRC_DIR)/openblas-skylakexdgemm.patch-applied
+$(BUILDDIR)/$(OPENBLAS_SRC_DIR)/openblas-winexit.patch-applied: $(BUILDDIR)/$(OPENBLAS_SRC_DIR)/source-extracted
+	cd $(BUILDDIR)/$(OPENBLAS_SRC_DIR) && \
+		patch -p1 -f < $(SRCDIR)/patches/openblas-winexit.patch
+	echo 1 > $@
+
+$(BUILDDIR)/$(OPENBLAS_SRC_DIR)/build-configured: $(BUILDDIR)/$(OPENBLAS_SRC_DIR)/openblas-skylakexdgemm.patch-applied $(BUILDDIR)/$(OPENBLAS_SRC_DIR)/openblas-winexit.patch-applied
 	echo 1 > $@
 
 $(BUILDDIR)/$(OPENBLAS_SRC_DIR)/build-compiled: $(BUILDDIR)/$(OPENBLAS_SRC_DIR)/build-configured

deps/patches/openblas-winexit.patch

@@ -0,0 +1,27 @@
+commit 7f4a96a353c06c2fdecb0b2464800bcf8fab6fd7
+Author: Keno Fischer <[email protected]>
+Date:   Sun Dec 29 15:08:13 2019 -0500
+
+    win32: Don't run cleanup if we're about to exit anyway
+
+    If the process is about to exit, there's no point trying to do
+    a bunch of work to clean up resources. The kernel will release
+    them much more efficiently when the process exits at the end
+    of this function.
+
+diff --git a/exports/dllinit.c b/exports/dllinit.c
+index 02ff092e..56aafa95 100644
+--- a/exports/dllinit.c
++++ b/exports/dllinit.c
+@@ -48,7 +48,10 @@ BOOL APIENTRY DllMain(HINSTANCE hInst, DWORD reason, LPVOID reserved) {
+   }
+
+   if (reason == DLL_PROCESS_DETACH) {
+-    gotoblas_quit();
++    // If the process is about to exit, don't bother releasing any resources
++    // The kernel is much better at bulk releasing then.
++    if (!reserved)
++      gotoblas_quit();
+   }
+
+   return TRUE;