libstore: downsize hook pipes

don't keep fds open we're not using. currently this does not cause any
problems, but it does increase the size of our fd table needlessly and
in the future, when we have proper async processing, having builderOut
open in the daemon once the hook has been fully started is problematic

Change-Id: I6e7fb773b280b042873103638d3e04272ca1e4fc

src/libstore/build/derivation-goal.cc

@@ -822,8 +822,8 @@ int DerivationGoal::getChildStatus()
 
 void DerivationGoal::closeReadPipes()
 {
-    hook->builderOut.readSide.reset();
+    hook->builderOut.reset();
-    hook->fromHook.readSide.reset();
+    hook->fromHook.reset();
     builderOutFD = nullptr;
 }
 
@@ -1135,7 +1135,7 @@ HookReply DerivationGoal::tryBuildHook(bool inBuildSlot)
         while (true) {
             auto s = [&]() {
                 try {
-                    return readLine(worker.hook.instance->fromHook.readSide.get());
+                    return readLine(worker.hook.instance->fromHook.get());
                 } catch (Error & e) {
                     e.addTrace({}, "while reading the response from the build hook");
                     throw;
@@ -1171,7 +1171,7 @@ HookReply DerivationGoal::tryBuildHook(bool inBuildSlot)
         if (e.errNo == EPIPE) {
             printError(
                 "build hook died unexpectedly: %s",
-                chomp(drainFD(worker.hook.instance->fromHook.readSide.get())));
+                chomp(drainFD(worker.hook.instance->fromHook.get())));
             worker.hook.instance.reset();
             return rpDecline;
         } else
@@ -1181,7 +1181,7 @@ HookReply DerivationGoal::tryBuildHook(bool inBuildSlot)
     hook = std::move(worker.hook.instance);
 
     try {
-        machineName = readLine(hook->fromHook.readSide.get());
+        machineName = readLine(hook->fromHook.get());
     } catch (Error & e) {
         e.addTrace({}, "while reading the machine name from the build hook");
         throw;
@@ -1204,15 +1204,15 @@ HookReply DerivationGoal::tryBuildHook(bool inBuildSlot)
     }
 
     hook->sink = FdSink();
-    hook->toHook.writeSide.reset();
+    hook->toHook.reset();
 
     /* Create the log file and pipe. */
     Path logFile = openLogFile();
 
     std::set<int> fds;
-    fds.insert(hook->fromHook.readSide.get());
+    fds.insert(hook->fromHook.get());
-    fds.insert(hook->builderOut.readSide.get());
+    fds.insert(hook->builderOut.get());
-    builderOutFD = &hook->builderOut.readSide;
+    builderOutFD = &hook->builderOut;
     worker.childStarted(shared_from_this(), fds, false);
 
     return rpAccept;
@@ -1309,7 +1309,7 @@ Goal::WorkResult DerivationGoal::handleChildOutput(int fd, std::string_view data
         return StillAlive{};
     }
 
-    if (hook && fd == hook->fromHook.readSide.get()) {
+    if (hook && fd == hook->fromHook.get()) {
         for (auto c : data)
             if (c == '\n') {
                 auto json = parseJSONMessage(currentHookLine);

src/libstore/build/hook-instance.cc

@@ -26,18 +26,21 @@ HookInstance::HookInstance()
     args.push_back(std::to_string(verbosity));
 
     /* Create a pipe to get the output of the child. */
-    fromHook.create();
+    Pipe fromHook_;
+    fromHook_.create();
 
     /* Create the communication pipes. */
-    toHook.create();
+    Pipe toHook_;
+    toHook_.create();
 
     /* Create a pipe to get the output of the builder. */
-    builderOut.create();
+    Pipe builderOut_;
+    builderOut_.create();
 
     /* Fork the hook. */
     pid = startProcess([&]() {
 
-        if (dup2(fromHook.writeSide.get(), STDERR_FILENO) == -1)
+        if (dup2(fromHook_.writeSide.get(), STDERR_FILENO) == -1)
             throw SysError("cannot pipe standard error into log file");
 
         commonChildInit();
@@ -45,16 +48,16 @@ HookInstance::HookInstance()
         if (chdir("/") == -1) throw SysError("changing into /");
 
         /* Dup the communication pipes. */
-        if (dup2(toHook.readSide.get(), STDIN_FILENO) == -1)
+        if (dup2(toHook_.readSide.get(), STDIN_FILENO) == -1)
             throw SysError("dupping to-hook read side");
 
         /* Use fd 4 for the builder's stdout/stderr. */
-        if (dup2(builderOut.writeSide.get(), 4) == -1)
+        if (dup2(builderOut_.writeSide.get(), 4) == -1)
             throw SysError("dupping builder's stdout/stderr");
 
         /* Hack: pass the read side of that fd to allow build-remote
            to read SSH error messages. */
-        if (dup2(builderOut.readSide.get(), 5) == -1)
+        if (dup2(builderOut_.readSide.get(), 5) == -1)
             throw SysError("dupping builder's stdout/stderr");
 
         execv(buildHook.c_str(), stringsToCharPtrs(args).data());
@@ -63,10 +66,11 @@ HookInstance::HookInstance()
     });
 
     pid.setSeparatePG(true);
-    fromHook.writeSide.reset();
+    fromHook = std::move(fromHook_.readSide);
-    toHook.readSide.reset();
+    toHook = std::move(toHook_.writeSide);
+    builderOut = std::move(builderOut_.readSide);
 
-    sink = FdSink(toHook.writeSide.get());
+    sink = FdSink(toHook.get());
     std::map<std::string, Config::SettingInfo> settings;
     globalConfig.getSettings(settings);
     for (auto & setting : settings)
@@ -78,7 +82,7 @@ HookInstance::HookInstance()
 HookInstance::~HookInstance()
 {
     try {
-        toHook.writeSide.reset();
+        toHook.reset();
         if (pid) pid.kill();
     } catch (...) {
         ignoreException();

src/libstore/build/hook-instance.hh

@@ -10,19 +10,19 @@ namespace nix {
 struct HookInstance
 {
     /**
-     * Pipes for talking to the build hook.
+     * Pipe for talking to the build hook.
      */
-    Pipe toHook;
+    AutoCloseFD toHook;
 
     /**
      * Pipe for the hook's standard output/error.
      */
-    Pipe fromHook;
+    AutoCloseFD fromHook;
 
     /**
      * Pipe for the builder's standard output/error.
      */
-    Pipe builderOut;
+    AutoCloseFD builderOut;
 
     /**
      * The process ID of the hook.