From d5402b8527a87a887b516d5cdf630acb54ecbcb5 Mon Sep 17 00:00:00 2001
From: John Ericson <John.Ericson@Obsidian.Systems>
Date: Mon, 29 Sep 2025 16:35:59 -0400
Subject: [PATCH 1/3] Encapsulate `curlFileTransfer::State:quit`

It is allowed to read it, and to set it to `false`, but not to set it
to `true`.
---
 src/libstore/filetransfer.cc | 23 +++++++++++++++++------
 1 file changed, 17 insertions(+), 6 deletions(-)

diff --git a/src/libstore/filetransfer.cc b/src/libstore/filetransfer.cc
index a162df1ad..72153dfdd 100644
--- a/src/libstore/filetransfer.cc
+++ b/src/libstore/filetransfer.cc
@@ -594,10 +594,21 @@ struct curlFileTransfer : public FileTransfer
             }
         };
 
-        bool quit = false;
         std::
             priority_queue<std::shared_ptr<TransferItem>, std::vector<std::shared_ptr<TransferItem>>, EmbargoComparator>
                 incoming;
+    private:
+        bool quitting = false;
+    public:
+        void quit()
+        {
+            quitting = true;
+        }
+
+        bool isQuitting()
+        {
+            return quitting;
+        }
     };
 
     Sync<State> state_;
@@ -649,7 +660,7 @@ struct curlFileTransfer : public FileTransfer
         /* Signal the worker thread to exit. */
         {
             auto state(state_.lock());
-            state->quit = true;
+            state->quit();
         }
 #ifndef _WIN32 // TODO need graceful async exit support on Windows?
         writeFull(wakeupPipe.writeSide.get(), " ", false);
@@ -750,7 +761,7 @@ struct curlFileTransfer : public FileTransfer
                         break;
                     }
                 }
-                quit = state->quit;
+                quit = state->isQuitting();
             }
 
             for (auto & item : incoming) {
@@ -778,7 +789,7 @@ struct curlFileTransfer : public FileTransfer
             auto state(state_.lock());
             while (!state->incoming.empty())
                 state->incoming.pop();
-            state->quit = true;
+            state->quit();
         }
     }
 
@@ -789,7 +800,7 @@ struct curlFileTransfer : public FileTransfer
 
         {
             auto state(state_.lock());
-            if (state->quit)
+            if (state->isQuitting())
                 throw nix::Error("cannot enqueue download request because the download thread is shutting down");
             state->incoming.push(item);
         }
@@ -845,7 +856,7 @@ ref<FileTransfer> getFileTransfer()
 {
     static ref<curlFileTransfer> fileTransfer = makeCurlFileTransfer();
 
-    if (fileTransfer->state_.lock()->quit)
+    if (fileTransfer->state_.lock()->isQuitting())
         fileTransfer = makeCurlFileTransfer();
 
     return fileTransfer;

From 1f65b08d947d9ab7eb397eebe49609963e003641 Mon Sep 17 00:00:00 2001
From: John Ericson <John.Ericson@Obsidian.Systems>
Date: Mon, 29 Sep 2025 16:37:12 -0400
Subject: [PATCH 2/3] `curlFileTransfer::State:quit` emptys the queue

Whoever first calls `quit` now empties the queue, instead of waiting for
the worker thread to do it.

(Note that in the unwinding case, the worker thread is still the first
to call `quit`, though.)
---
 src/libstore/filetransfer.cc | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/src/libstore/filetransfer.cc b/src/libstore/filetransfer.cc
index 72153dfdd..f8f5b48e0 100644
--- a/src/libstore/filetransfer.cc
+++ b/src/libstore/filetransfer.cc
@@ -603,6 +603,9 @@ struct curlFileTransfer : public FileTransfer
         void quit()
         {
             quitting = true;
+            /* We wil not be processing any more incomming requests */
+            while (!incoming.empty())
+                incoming.pop();
         }
 
         bool isQuitting()
@@ -787,8 +790,6 @@ struct curlFileTransfer : public FileTransfer
 
         {
             auto state(state_.lock());
-            while (!state->incoming.empty())
-                state->incoming.pop();
             state->quit();
         }
     }

From 86fb5b24a9cb528d87cb02efb89483353a4b6c44 Mon Sep 17 00:00:00 2001
From: John Ericson <John.Ericson@Obsidian.Systems>
Date: Mon, 29 Sep 2025 16:43:45 -0400
Subject: [PATCH 3/3] `curlFileTransfer::workerThreadEntry` Only call `quit` if
 we need to.

---
 src/libstore/filetransfer.cc | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/src/libstore/filetransfer.cc b/src/libstore/filetransfer.cc
index f8f5b48e0..59fc75ed0 100644
--- a/src/libstore/filetransfer.cc
+++ b/src/libstore/filetransfer.cc
@@ -781,14 +781,18 @@ struct curlFileTransfer : public FileTransfer
 
     void workerThreadEntry()
     {
+        // Unwinding or because someone called `quit`.
+        bool normalExit = true;
         try {
             workerThreadMain();
         } catch (nix::Interrupted & e) {
+            normalExit = false;
         } catch (std::exception & e) {
             printError("unexpected error in download thread: %s", e.what());
+            normalExit = false;
         }
 
-        {
+        if (!normalExit) {
             auto state(state_.lock());
             state->quit();
         }