commit 115e9b3df1058f1a6d2c1096a9389caa3d16e098
author Patrick Williams <patrick@stwcx.xyz> Fri Sep 23 16:20:59 2022 -0500
committer Patrick Williams <patrick@stwcx.xyz> Fri Oct 14 06:35:51 2022 -0500
tree cd7b09a6ea03da570416619653acadd680af1ba5
parent 10483c94e48fa040004d8dca8a10c9d80c8180e4

async: context: allow many exceptions

Allow many spawned tasks to throw exceptions and queue them up,
re-throwing them one-by-one from `context::run` calls.  Similarly
enhance `context::scope` so that many `context::empty` completions
can complete with error.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I8744e0a3b8b0663ebd5ca26db0f5943688afe62a

src/async/context.cpp

diff --git a/src/async/context.cpp b/src/async/context.cpp
index b16cf3e..6d50301 100644
--- a/src/async/context.cpp
+++ b/src/async/context.cpp
@@ -142,10 +142,10 @@
     wait_for_wait_process_stopped();
 
     // Wait for all the internal tasks to complete.
-    std::this_thread::sync_wait(internal_tasks.empty() |
-                                execution::upon_error([&](auto&& e) {
-                                    pending_exception = std::move(e);
-                                }));
+    std::this_thread::sync_wait(
+        internal_tasks.empty() | execution::upon_error([&](auto&& e) {
+            pending_exceptions.emplace_back(std::move(e));
+        }));
 
     // Finish up the loop and join the thread.
     // (There shouldn't be anything going on by this point anyhow.)
@@ -177,7 +177,7 @@
 
         if (e)
         {
-            pending_exception = std::move(e);
+            pending_exceptions.emplace_back(std::move(e));
         }
     }
 
@@ -225,7 +225,7 @@
     // we get an exception.
     auto keep_running = [this]() {
         std::lock_guard l{lock};
-        return !final_stop.stop_requested() && !pending_exception;
+        return !final_stop.stop_requested() && pending_exceptions.empty();
     };
 
     // If we are suppose to keep running, start the run loop.
@@ -284,9 +284,11 @@
 {
     {
         std::lock_guard l{lock};
-        if (pending_exception)
+        if (!pending_exceptions.empty())
         {
-            std::rethrow_exception(std::exchange(pending_exception, {}));
+            auto e = pending_exceptions.front();
+            pending_exceptions.pop_front();
+            std::rethrow_exception(std::move(e));
         }
     }
 }
@@ -338,7 +340,7 @@
         ctx.caller_wait.wait(lock, [&] {
             return (ctx.pending != nullptr) || (ctx.staged != nullptr) ||
                    ctx.final_stop.stop_requested() ||
-                   (ctx.pending_exception != nullptr);
+                   !ctx.pending_exceptions.empty();
         });
 
         // Save the waiter as pending.

src/async/scope.cpp

diff --git a/src/async/scope.cpp b/src/async/scope.cpp
index a990e41..d9eb8a7 100644
--- a/src/async/scope.cpp
+++ b/src/async/scope.cpp
@@ -28,36 +28,27 @@
         std::lock_guard l{lock};
         --pending_count; // decrement count.
 
-        if (e && pending_exception)
+        if (e)
         {
-            // Received a second exception without delivering the first
-            // to a pending completion.  Terminate using the first one.
-            try
-            {
-                std::rethrow_exception(std::exchange(pending_exception, {}));
-            }
-            catch (...)
-            {
-                std::terminate();
-            }
+            pending_exceptions.emplace_back(std::exchange(e, {}));
         }
 
         // If the scope is complete, get the pending completion, if it exists.
-        if (e || (pending_count == 0))
+        if (!pending_exceptions.empty() || (pending_count == 0))
         {
             p = std::exchange(pending, nullptr);
         }
-
-        // If we have an exception but no pending completion, save it away.
-        if (e && !p)
-        {
-            pending_exception = std::move(e);
-        }
     }
 
     if (p)
     {
-        p->complete(std::move(e));
+        std::exception_ptr fwd_exception = {};
+        if (!pending_exceptions.empty())
+        {
+            fwd_exception = pending_exceptions.front();
+            pending_exceptions.pop_front();
+        }
+        p->complete(std::move(fwd_exception));
     }
 }
 
@@ -79,13 +70,14 @@
         {
             done = false;
         }
-        else if (s.pending_count == 0)
+        else if (!s.pending_exceptions.empty())
         {
+            e = s.pending_exceptions.front();
+            s.pending_exceptions.pop_front();
             done = true;
         }
-        else if (s.pending_exception)
+        else if (s.pending_count == 0)
         {
-            e = std::exchange(s.pending_exception, {});
             done = true;
         }
         else