From 43d6a79863d5e28b6a2cd98ecb22fe4ff491057e Mon Sep 17 00:00:00 2001
From: eldritch horrors <pennae@lix.systems>
Date: Wed, 23 Jul 2025 15:12:03 +0200
Subject: [PATCH] libstore: work around capnp fd passing bug

capnp does not handle fd passing correctly in all circumstances. we hit
such cirumstances when passing large closures path lists to build-hook.
since capnp seems to ignore fds passed in non-final segments of any rpc
message we just ensure that the capability including the log fd will be
small enough to not be fragmented on the receiving side of the channel.

cf https://github.com/capnproto/capnproto/issues/2359

Change-Id: Id22309264936b3a57bcc68a0753c3bfb3c9a43d2
---
 lix/legacy/build-remote.cc             | 15 +++++++++++----
 lix/libstore/build/derivation-goal.cc  |  2 +-
 lix/libstore/build/hook-instance.capnp |  4 ++--
 3 files changed, 14 insertions(+), 7 deletions(-)

diff --git a/lix/legacy/build-remote.cc b/lix/legacy/build-remote.cc
index 48f8eed2b..c90e3f77e 100644
--- a/lix/legacy/build-remote.cc
+++ b/lix/legacy/build-remote.cc
@@ -213,11 +213,18 @@ struct AcceptedBuild final : rpc::build_remote::HookInstance::AcceptedBuild::Ser
     ref<Store> store;
     StorePath drvPath;
     BuilderConnection builder;
+    rpc::build_remote::HookInstance::BuildLogger::Client buildLogger;
 
-    AcceptedBuild(ref<Store> store, StorePath drvPath, BuilderConnection builder)
+    AcceptedBuild(
+        ref<Store> store,
+        StorePath drvPath,
+        BuilderConnection builder,
+        rpc::build_remote::HookInstance::BuildLogger::Client buildLogger
+    )
         : store(store)
         , drvPath(drvPath)
         , builder(std::move(builder))
+        , buildLogger(std::move(buildLogger))
     {
     }
 
@@ -382,6 +389,7 @@ kj::Promise<void> Instance::build(BuildContext context)
         auto drvPath = from(context.getParams().getDrvPath(), *store);
         auto requiredFeatures =
             rpc::to<std::set<std::string>>(context.getParams().getRequiredFeatures());
+        auto buildLogger = context.getParams().getBuildLogger();
 
         auto result = TRY_AWAIT(connectToBuilder(
             store, drvPath, machines, maxBuildJobs, amWilling, neededSystem, requiredFeatures
@@ -403,7 +411,7 @@ kj::Promise<void> Instance::build(BuildContext context)
 
         auto ac = context.getResults().initResult().initGood().initAccept();
         RPC_FILL(ac, setMachineName, builder->storeUri);
-        ac.setMachine(kj::heap<AcceptedBuild>(store, drvPath, std::move(*builder)));
+        ac.setMachine(kj::heap<AcceptedBuild>(store, drvPath, std::move(*builder), buildLogger));
     } catch (...) {
         RPC_FILL(context.getResults(), initResult, std::current_exception());
     }
@@ -412,8 +420,7 @@ kj::Promise<void> Instance::build(BuildContext context)
 kj::Promise<void> AcceptedBuild::run(RunContext context)
 {
     try {
-        auto builderLogger = context.getParams().getBuildLogger();
-        const int logFD = (co_await builderLogger.getFd()).orDefault(-1);
+        const int logFD = (co_await buildLogger.getFd()).orDefault(-1);
         if (logFD < 0) {
             throw Error("build-hook needs a logFD from the builder to build");
         }
diff --git a/lix/libstore/build/derivation-goal.cc b/lix/libstore/build/derivation-goal.cc
index b93f851d3..46da79d6e 100644
--- a/lix/libstore/build/derivation-goal.cc
+++ b/lix/libstore/build/derivation-goal.cc
@@ -1085,6 +1085,7 @@ try {
     RPC_FILL(buildReq, setNeededSystem, drv->platform);
     RPC_FILL(buildReq, initDrvPath, drvPath, worker.store);
     RPC_FILL(buildReq, initRequiredFeatures, parsedDrv->getRequiredSystemFeatures());
+    buildReq.setBuildLogger(kj::heap<BuildHookLogger>(std::move(logPipe.writeSide)));
     auto buildRespPromise = buildReq.send();
     auto buildResp = TRY_AWAIT_RPC(buildRespPromise);
 
@@ -1112,7 +1113,6 @@ try {
     /* Tell the hook all the inputs that have to be copied to the
        remote system. */
     RPC_FILL(runReq, initInputs, inputPaths, worker.store);
-    runReq.setBuildLogger(kj::heap<BuildHookLogger>(std::move(logPipe.writeSide)));
 
     /* Tell the hooks the missing outputs that have to be copied back
        from the remote system. */
diff --git a/lix/libstore/build/hook-instance.capnp b/lix/libstore/build/hook-instance.capnp
index 81c8e962a..05840e8d5 100644
--- a/lix/libstore/build/hook-instance.capnp
+++ b/lix/libstore/build/hook-instance.capnp
@@ -16,7 +16,6 @@ interface HookInstance {
     run @0 (
       inputs :List(StoreTypes.StorePath), # actual a set
       wantedOutputs :List(Data), # actually StringSet
-      buildLogger :BuildLogger,
     ) -> (result :Types.ResultV);
   }
 
@@ -36,6 +35,7 @@ interface HookInstance {
     amWilling :Bool,
     neededSystem :Data,
     drvPath :StoreTypes.StorePath,
-    requiredFeatures :List(Data)
+    requiredFeatures :List(Data),
+    buildLogger :BuildLogger,
   ) -> (result :Types.Result(BuildResponse));
 }