[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.

tests/cpp/collective/test_loop.cc

@@ -1,5 +1,5 @@
 /**
- * Copyright 2023, XGBoost Contributors
+ * Copyright 2023-2024, XGBoost Contributors
  */
 #include <gtest/gtest.h>                // for ASSERT_TRUE, ASSERT_EQ
 #include <xgboost/collective/socket.h>  // for TCPSocket, Connect, SocketFinalize, SocketStartup
@@ -28,18 +28,25 @@ class LoopTest : public ::testing::Test {
 
     auto domain = SockDomain::kV4;
     pair_.first = TCPSocket::Create(domain);
-    auto port = pair_.first.BindHost();
-    pair_.first.Listen();
+    in_port_t port{0};
+    auto rc = Success() << [&] {
+      port = pair_.first.BindHost();
+      return Success();
+    } << [&] {
+      pair_.first.Listen();
+      return Success();
+    };
+    SafeColl(rc);
 
     auto const& addr = SockAddrV4::Loopback().Addr();
-    auto rc = Connect(StringView{addr}, port, 1, timeout, &pair_.second);
-    ASSERT_TRUE(rc.OK());
+    rc = Connect(StringView{addr}, port, 1, timeout, &pair_.second);
+    SafeColl(rc);
     rc = pair_.second.NonBlocking(true);
-    ASSERT_TRUE(rc.OK());
+    SafeColl(rc);
 
     pair_.first = pair_.first.Accept();
     rc = pair_.first.NonBlocking(true);
-    ASSERT_TRUE(rc.OK());
+    SafeColl(rc);
 
     loop_ = std::shared_ptr<Loop>{new Loop{timeout}};
   }
@@ -74,8 +81,26 @@ TEST_F(LoopTest, Op) {
   loop_->Submit(rop);
 
   auto rc = loop_->Block();
-  ASSERT_TRUE(rc.OK()) << rc.Report();
+  SafeColl(rc);
 
   ASSERT_EQ(rbuf[0], wbuf[0]);
 }
+
+TEST_F(LoopTest, Block) {
+  // We need to ensure that a blocking call doesn't go unanswered.
+  auto op = Loop::Op::Sleep(2);
+
+  common::Timer t;
+  t.Start();
+  loop_->Submit(op);
+  t.Stop();
+  // submit is non-blocking
+  ASSERT_LT(t.ElapsedSeconds(), 1);
+
+  t.Start();
+  auto rc = loop_->Block();
+  t.Stop();
+  SafeColl(rc);
+  ASSERT_GE(t.ElapsedSeconds(), 1);
+}
 }  // namespace xgboost::collective

tests/cpp/collective/test_result.cc

@@ -0,0 +1,31 @@
+/**
+ *  Copyright 2024, XGBoost Contributors
+ */
+#include <gtest/gtest.h>
+#include <xgboost/collective/result.h>
+
+namespace xgboost::collective {
+TEST(Result, Concat) {
+  auto rc0 = Fail("foo");
+  auto rc1 = Fail("bar");
+  auto rc = std::move(rc0) + std::move(rc1);
+  ASSERT_NE(rc.Report().find("foo"), std::string::npos);
+  ASSERT_NE(rc.Report().find("bar"), std::string::npos);
+
+  auto rc2 = Fail("Another", std::move(rc));
+  auto assert_that = [](Result const& rc) {
+    ASSERT_NE(rc.Report().find("Another"), std::string::npos);
+    ASSERT_NE(rc.Report().find("foo"), std::string::npos);
+    ASSERT_NE(rc.Report().find("bar"), std::string::npos);
+  };
+  assert_that(rc2);
+
+  auto empty = Success();
+  auto rc3 = std::move(empty) + std::move(rc2);
+  assert_that(rc3);
+
+  empty = Success();
+  auto rc4 = std::move(rc3) + std::move(empty);
+  assert_that(rc4);
+}
+}  // namespace xgboost::collective