[coll] Improve event loop. (#10199)

- Add a test for blocking calls.
- Do not require the queue to be empty after waking up; this frees up the thread to answer blocking calls.
- Handle EOF in read.
- Improve the error message in the result. Allow concatenation of multiple results.

R-package/src/Makevars.in

@@ -99,6 +99,7 @@ OBJECTS= \
     $(PKGROOT)/src/context.o \
     $(PKGROOT)/src/logging.o \
     $(PKGROOT)/src/global_config.o \
+    $(PKGROOT)/src/collective/result.o \
     $(PKGROOT)/src/collective/allgather.o \
     $(PKGROOT)/src/collective/allreduce.o \
     $(PKGROOT)/src/collective/broadcast.o \

R-package/src/Makevars.win

@@ -99,6 +99,7 @@ OBJECTS= \
     $(PKGROOT)/src/context.o \
     $(PKGROOT)/src/logging.o \
     $(PKGROOT)/src/global_config.o \
+    $(PKGROOT)/src/collective/result.o \
     $(PKGROOT)/src/collective/allgather.o \
     $(PKGROOT)/src/collective/allreduce.o \
     $(PKGROOT)/src/collective/broadcast.o \