* No longer block while waiting for a lock on a store path. Instead

poll for it (i.e. if we can't acquire the lock, then let the main
  select() loop wait for at most a few seconds and then try again).
  This improves parallelism: if two nix-store processes are both
  trying to build a path at the same time, the second one shouldn't
  block; it should first see if it can build other goals.  Also, it
  prevents the deadlocks that have been occuring in Hydra lately,
  where a process waits for a lock held by another process that's
  waiting for a lock held by the first.

  The downside is that polling isn't really elegant, but POSIX doesn't
  provide a way to wait for locks in a select() loop.  The only
  solution would be to spawn a thread for each lock to do a blocking
  fcntl() and then signal the main thread, but that would require
  pthreads.

src/libstore/build.cc

@@ -8,6 +8,7 @@
 #include <map>
 #include <iostream>
 #include <sstream>
+#include <algorithm>
 #include <boost/shared_ptr.hpp>
 #include <boost/weak_ptr.hpp>
 #include <boost/enable_shared_from_this.hpp>
@@ -200,6 +201,12 @@ private:
     /* Goals waiting for busy paths to be unlocked. */
     WeakGoals waitingForAnyGoal;
     
+    /* Goals sleeping for a few seconds (polling a lock). */
+    WeakGoals waitingForAWhile;
+
+    /* Last time the goals in `waitingForAWhile' where woken up. */
+    time_t lastWokenUp;
+    
 public:
 
     LocalStore & store;
@@ -246,6 +253,12 @@ public:
        wait for some resource that some other goal is holding. */
     void waitForAnyGoal(GoalPtr goal);
     
+    /* Wait for a few seconds and then retry this goal.  Used when
+       waiting for a lock held by another process.  This kind of
+       polling is inefficient, but POSIX doesn't really provide a way
+       to wait for multiple locks in the main select() loop. */
+    void waitForAWhile(GoalPtr goal);
+    
     /* Loop until the specified top-level goals have finished. */
     void run(const Goals & topGoals);
 
@@ -952,10 +965,14 @@ void DerivationGoal::tryToBuild()
         }
     
     /* Obtain locks on all output paths.  The locks are automatically
-       released when we exit this function or Nix crashes. */
-    /* !!! nonblock */
-    outputLocks.lockPaths(outputPaths(drv.outputs),
-        (format("waiting for lock on %1%") % showPaths(outputPaths(drv.outputs))).str());
+       released when we exit this function or Nix crashes.  If we
+       can't acquire the lock, then continue; hopefully some other
+       goal can start a build, and if not, the main loop will sleep a
+       few seconds and then retry this goal. */
+    if (!outputLocks.lockPaths(outputPaths(drv.outputs), "", false)) {
+        worker.waitForAWhile(shared_from_this());
+        return;
+    }
 
     /* Now check again whether the outputs are valid.  This is because
        another process may have started building in parallel.  After
@@ -2205,8 +2222,10 @@ void SubstitutionGoal::tryToRun()
     
     /* Acquire a lock on the output path. */
     outputLock = boost::shared_ptr<PathLocks>(new PathLocks);
-    outputLock->lockPaths(singleton<PathSet>(storePath),
-        (format("waiting for lock on `%1%'") % storePath).str());
+    if (!outputLock->lockPaths(singleton<PathSet>(storePath), "", false)) {
+        worker.waitForAWhile(shared_from_this());
+        return;
+    }
 
     /* Check again whether the path is invalid. */
     if (worker.store.isValidPath(storePath)) {
@@ -2372,6 +2391,7 @@ Worker::Worker(LocalStore & store)
     if (working) abort();
     working = true;
     nrChildren = 0;
+    lastWokenUp = 0;
 }
 
 
@@ -2440,9 +2460,7 @@ void Worker::removeGoal(GoalPtr goal)
     }
 
     /* Wake up goals waiting for any goal to finish. */
-    for (WeakGoals::iterator i = waitingForAnyGoal.begin();
-         i != waitingForAnyGoal.end(); ++i)
-    {
+    foreach (WeakGoals::iterator, i, waitingForAnyGoal) {
         GoalPtr goal = i->lock();
         if (goal) wakeUp(goal);
     }
@@ -2539,6 +2557,13 @@ void Worker::waitForAnyGoal(GoalPtr goal)
 }
 
 
+void Worker::waitForAWhile(GoalPtr goal)
+{
+    debug("wait for a while");
+    waitingForAWhile.insert(goal);
+}
+
+
 void Worker::run(const Goals & _topGoals)
 {
     for (Goals::iterator i = _topGoals.begin();
@@ -2566,10 +2591,9 @@ void Worker::run(const Goals & _topGoals)
         if (topGoals.empty()) break;
 
         /* Wait for input. */
-        if (!children.empty())
+        if (!children.empty() || !waitingForAWhile.empty())
             waitForInput();
         else
-            /* !!! not when we're polling */
             assert(!awake.empty());
     }
 
@@ -2592,22 +2616,36 @@ void Worker::waitForInput()
        the logger pipe of a build, we assume that the builder has
        terminated. */
 
+    bool useTimeout = false;
+    struct timeval timeout;
+    timeout.tv_usec = 0;
+    time_t before = time(0);
+        
     /* If we're monitoring for silence on stdout/stderr, sleep until
        the first deadline for any child. */
-    struct timeval timeout;
     if (maxSilentTime != 0) {
         time_t oldest = 0;
         foreach (Children::iterator, i, children) {
             oldest = oldest == 0 || i->second.lastOutput < oldest
                 ? i->second.lastOutput : oldest;
         }
-        time_t now = time(0);
-        timeout.tv_sec = (time_t) (oldest + maxSilentTime) <= now ? 0 :
-            oldest + maxSilentTime - now;
-        timeout.tv_usec = 0;
+        useTimeout = true;
+        timeout.tv_sec = std::max((time_t) 0, oldest + maxSilentTime - before);
         printMsg(lvlVomit, format("sleeping %1% seconds") % timeout.tv_sec);
     }
 
+    /* If we are polling goals that are waiting for a lock, then wake
+       up after a few seconds at most. */
+    int wakeUpInterval = 3;
+        
+    if (!waitingForAWhile.empty()) {
+        useTimeout = true;
+        if (lastWokenUp == 0 && children.empty())
+            printMsg(lvlError, "waiting for locks...");
+        if (lastWokenUp == 0 || lastWokenUp > before) lastWokenUp = before;
+        timeout.tv_sec = std::max((time_t) 0, lastWokenUp + wakeUpInterval - before);
+    } else lastWokenUp = 0;
+
     /* Use select() to wait for the input side of any logger pipe to
        become `available'.  Note that `available' (i.e., non-blocking)
        includes EOF. */
@@ -2621,12 +2659,12 @@ void Worker::waitForInput()
         }
     }
 
-    if (select(fdMax, &fds, 0, 0, maxSilentTime != 0 ? &timeout : 0) == -1) {
+    if (select(fdMax, &fds, 0, 0, useTimeout ? &timeout : 0) == -1) {
         if (errno == EINTR) return;
         throw SysError("waiting for input");
     }
 
-    time_t now = time(0);
+    time_t after = time(0);
 
     /* Process all available file descriptors. */
 
@@ -2662,13 +2700,13 @@ void Worker::waitForInput()
                         % goal->getName() % rd);
                     string data((char *) buffer, rd);
                     goal->handleChildOutput(*k, data);
-                    j->second.lastOutput = now;
+                    j->second.lastOutput = after;
                 }
             }
         }
 
         if (maxSilentTime != 0 &&
-            now - j->second.lastOutput >= (time_t) maxSilentTime)
+            after - j->second.lastOutput >= (time_t) maxSilentTime)
         {
             printMsg(lvlError,
                 format("%1% timed out after %2% seconds of silence")
@@ -2676,6 +2714,15 @@ void Worker::waitForInput()
             goal->cancel();
         }
     }
+
+    if (!waitingForAWhile.empty() && lastWokenUp + wakeUpInterval >= after) {
+        lastWokenUp = after;
+        foreach (WeakGoals::iterator, i, waitingForAWhile) {
+            GoalPtr goal = i->lock();
+            if (goal) wakeUp(goal);
+        }
+        waitingForAWhile.clear();
+    }
 }
 
 

src/libstore/globals.cc

@@ -24,7 +24,7 @@ Verbosity buildVerbosity = lvlInfo;
 unsigned int maxBuildJobs = 1;
 bool readOnlyMode = false;
 string thisSystem = "unset";
-unsigned int maxSilentTime = 0;
+time_t maxSilentTime = 0;
 Paths substituters;
 bool useBuildHook = true;
 bool printBuildTrace = false;

src/libstore/globals.hh

@@ -65,7 +65,7 @@ extern string thisSystem;
 /* The maximum time in seconds that a builer can go without producing
    any output on stdout/stderr before it is killed.  0 means
    infinity. */
-extern unsigned int maxSilentTime;
+extern time_t maxSilentTime;
 
 /* The substituters.  There are programs that can somehow realise a
    store path without building, e.g., by downloading it or copying it

src/libstore/pathlocks.cc

@@ -141,9 +141,9 @@ PathLocks::PathLocks(const PathSet & paths, const string & waitMsg)
 }
 
 
-void PathLocks::lockPaths(const PathSet & _paths, const string & waitMsg)
+bool PathLocks::lockPaths(const PathSet & _paths,
+    const string & waitMsg, bool wait)
 {
-    /* May be called only once! */
     assert(fds.empty());
     
     /* Note that `fds' is built incrementally so that the destructor
@@ -174,8 +174,15 @@ void PathLocks::lockPaths(const PathSet & _paths, const string & waitMsg)
 
             /* Acquire an exclusive lock. */
             if (!lockFile(fd, ltWrite, false)) {
-                if (waitMsg != "") printMsg(lvlError, waitMsg);
-                lockFile(fd, ltWrite, true);
+                if (wait) {
+                    if (waitMsg != "") printMsg(lvlError, waitMsg);
+                    lockFile(fd, ltWrite, true);
+                } else {
+                    /* Failed to lock this path; release all other
+                       locks. */
+                    unlock();
+                    return false;
+                }
             }
 
             debug(format("lock acquired on `%1%'") % lockPath);
@@ -199,6 +206,8 @@ void PathLocks::lockPaths(const PathSet & _paths, const string & waitMsg)
         fds.push_back(FDPair(fd.borrow(), lockPath));
         lockedPaths.insert(lockPath);
     }
+
+    return true;
 }
 
 

src/libstore/pathlocks.hh

@@ -33,8 +33,9 @@ public:
     PathLocks();
     PathLocks(const PathSet & paths,
         const string & waitMsg = "");
-    void lockPaths(const PathSet & _paths,
-        const string & waitMsg = "");
+    bool lockPaths(const PathSet & _paths,
+        const string & waitMsg = "",
+        bool wait = true);
     ~PathLocks();
     void unlock();
     void setDeletion(bool deletePaths);