PathSubstitutionGoal: Clean up pipe

If there were many top-level goals (which are not destroyed until the
very end), commands like

  $ nix copy --to 'ssh://localhost?remote-store=/tmp/nix' \
    /run/current-system --no-check-sigs --substitute-on-destination

could fail with "Too many open files". So now we do some explicit
cleanup from amDone(). It would be cleaner to separate goals from
their temporary internal state, but that would be a bigger refactor.

Backport 8a29052cb2

src/libstore/build.cc

@@ -179,6 +179,8 @@ public:
 
     virtual string key() = 0;
 
+    virtual void cleanup() { }
+
 protected:
 
     virtual void amDone(ExitCode result);
@@ -424,6 +426,8 @@ void Goal::amDone(ExitCode result)
     }
     waiters.clear();
     worker.removeGoal(shared_from_this());
+
+    cleanup();
 }
 
 
@@ -3895,6 +3899,8 @@ public:
     void handleChildOutput(int fd, const string & data) override;
     void handleEOF(int fd) override;
 
+    void cleanup() override;
+
     Path getStorePath() { return storePath; }
 
     void amDone(ExitCode result) override
@@ -3918,15 +3924,7 @@ SubstitutionGoal::SubstitutionGoal(const Path & storePath, Worker & worker, Repa
 
 SubstitutionGoal::~SubstitutionGoal()
 {
-    try {
+    cleanup();
-        if (thr.joinable()) {
-            // FIXME: signal worker thread to quit.
-            thr.join();
-            worker.childTerminated(this);
-        }
-    } catch (...) {
-        ignoreException();
-    }
 }
 
 
@@ -3961,6 +3959,8 @@ void SubstitutionGoal::tryNext()
 {
     trace("trying next substituter");
 
+    cleanup();
+
     if (subs.size() == 0) {
         /* None left.  Terminate this goal and let someone else deal
            with it. */
@@ -4088,7 +4088,7 @@ void SubstitutionGoal::tryToRun()
     thr = std::thread([this]() {
         try {
             /* Wake up the worker loop when we're done. */
-            Finally updateStats([this]() { outPipe.writeSide = -1; });
+            Finally updateStats([this]() { outPipe.writeSide.close(); });
 
             Activity act(*logger, actSubstitute, Logger::Fields{storePath, sub->getUri()});
             PushActivity pact(act.id);
@@ -4172,6 +4172,20 @@ void SubstitutionGoal::handleEOF(int fd)
     if (fd == outPipe.readSide.get()) worker.wakeUp(shared_from_this());
 }
 
+void SubstitutionGoal::cleanup()
+{
+    try {
+        if (thr.joinable()) {
+            // FIXME: signal worker thread to quit.
+            thr.join();
+            worker.childTerminated(this);
+        }
+
+        outPipe.close();
+    } catch (...) {
+        ignoreException();
+    }
+}
 
 //////////////////////////////////////////////////////////////////////
 

src/libutil/util.cc

@@ -753,6 +753,7 @@ void AutoCloseFD::close()
         if (::close(fd) == -1)
             /* This should never happen. */
             throw SysError(format("closing file descriptor %1%") % fd);
+        fd = -1;
     }
 }
 
@@ -770,6 +771,12 @@ int AutoCloseFD::release()
     return oldFD;
 }
 
+void Pipe::close()
+{
+    readSide.close();
+    writeSide.close();
+}
+
 
 void Pipe::create()
 {
@@ -1080,7 +1087,7 @@ void runProgram2(const RunOptions & options)
         throw SysError("executing '%1%'", options.program);
     }, processOptions);
 
-    out.writeSide = -1;
+    out.writeSide.close();
 
     std::thread writerThread;
 
@@ -1093,7 +1100,7 @@ void runProgram2(const RunOptions & options)
 
 
     if (source) {
-        in.readSide = -1;
+        in.readSide.close();
         writerThread = std::thread([&]() {
             try {
                 std::vector<unsigned char> buf(8 * 1024);
@@ -1110,7 +1117,7 @@ void runProgram2(const RunOptions & options)
             } catch (...) {
                 promise.set_exception(std::current_exception());
             }
-            in.writeSide = -1;
+            in.writeSide.close();
         });
     }
 

src/libutil/util.hh

@@ -190,7 +190,6 @@ public:
 class AutoCloseFD
 {
     int fd;
-    void close();
 public:
     AutoCloseFD();
     AutoCloseFD(int fd);
@@ -202,6 +201,7 @@ public:
     int get() const;
     explicit operator bool() const;
     int release();
+    void close();
 };
 
 
@@ -210,6 +210,7 @@ class Pipe
 public:
     AutoCloseFD readSide, writeSide;
     void create();
+    void close();
 };