* Start of concurrent garbage collection. Processes write temporary

roots to a per-process temporary file in /nix/var/nix/temproots
  while holding a write lock on that file.  The garbage collector
  acquires read locks on all those files, thus blocking further
  progress in other Nix processes, and reads the sets of temporary
  roots.

Makefile.am

@@ -24,6 +24,7 @@ init-state:
 	$(INSTALL) $(INIT_FLAGS) -d $(DESTDIR)$(localstatedir)/log/nix
 	$(INSTALL) $(INIT_FLAGS) -d $(DESTDIR)$(localstatedir)/nix/profiles
 	$(INSTALL) $(INIT_FLAGS) -d $(DESTDIR)$(localstatedir)/nix/gcroots
+	$(INSTALL) $(INIT_FLAGS) -d $(DESTDIR)$(localstatedir)/nix/temproots
 	$(INSTALL) $(INIT_FLAGS) $(GROUP_WRITABLE) -d $(DESTDIR)$(localstatedir)/nix/gcroots/tmp
 	$(INSTALL) $(INIT_FLAGS) $(GROUP_WRITABLE) -d $(DESTDIR)$(localstatedir)/nix/gcroots/channels
 	rm -f $(DESTDIR)$(localstatedir)/nix/gcroots/profiles

scripts/nix-collect-garbage.in

@@ -9,6 +9,7 @@ my $storeDir = "@storedir@";
 my %alive;
 
 my $gcOper = "--delete";
+my $extraArgs = "";
 
 my @roots = ();
 
@@ -19,6 +20,9 @@ for (my $i = 0; $i < scalar @ARGV; $i++) {
     if ($arg eq "--delete" || $arg eq "--print-live" || $arg eq "--print-dead") {
         $gcOper = $arg;
     }
+    elsif ($arg =~ /^-v+$/) {
+        $extraArgs = "$extraArgs $arg";
+    }
     else { die "unknown argument `$arg'" };
 }
 
@@ -66,7 +70,7 @@ findRoots 1, $rootsDir;
 
 
 # Run the collector with the roots we found.
-my $pid = open2(">&1", \*WRITE, "@bindir@/nix-store --gc $gcOper")
+my $pid = open2(">&1", \*WRITE, "@bindir@/nix-store --gc $gcOper $extraArgs")
     or die "cannot run `nix-store --gc'";
 
 foreach my $root (@roots) {

src/libstore/build.cc

@@ -13,6 +13,7 @@
 #include "references.hh"
 #include "pathlocks.hh"
 #include "globals.hh"
+#include "gc.hh"
 
 
 /* !!! TODO derivationFromPath shouldn't be used here */
@@ -59,7 +60,6 @@ protected:
     /* Whether amDone() has been called. */
     bool done;
 
-    
     Goal(Worker & worker) : worker(worker)
     {
         done = false;
@@ -442,6 +442,10 @@ void DerivationGoal::haveStoreExpr()
     /* Get the derivation. */
     drv = derivationFromPath(drvPath);
 
+    for (DerivationOutputs::iterator i = drv.outputs.begin();
+         i != drv.outputs.end(); ++i)
+        addTempRoot(i->second.path);
+
     /* Check what outputs paths are not already valid. */
     PathSet invalidOutputs = checkPathValidity(false);
 
@@ -1308,6 +1312,8 @@ void SubstitutionGoal::init()
 {
     trace("init");
 
+    addTempRoot(storePath);
+    
     /* If the path already exists we're done. */
     if (isValidPath(storePath)) {
         amDone();

src/libstore/gc.cc

@@ -1,20 +1,146 @@
 #include "globals.hh"
 #include "gc.hh"
 #include "build.hh"
+#include "pathlocks.hh"
+
+#include <boost/shared_ptr.hpp>
 
 #include <sys/types.h>
 #include <sys/stat.h>
+#include <errno.h>
+#include <fcntl.h>
 #include <unistd.h>
 
 
+static string tempRootsDir = "temproots";
+
+/* The file to which we write our temporary roots. */
+Path fnTempRoots;
+static AutoCloseFD fdTempRoots;
+
+
+void addTempRoot(const Path & path)
+{
+    /* Create the temporary roots file for this process. */
+    if (fdTempRoots == -1) {
+
+        while (1) {
+            fnTempRoots = (format("%1%/%2%/%3%")
+                % nixStateDir % tempRootsDir % getpid()).str();
+        
+            fdTempRoots = open(fnTempRoots.c_str(), O_RDWR | O_CREAT | O_TRUNC, 0600);
+            if (fdTempRoots == -1)
+                throw SysError(format("opening temporary roots file `%1%'") % fnTempRoots);
+
+            debug(format("acquiring read lock on `%1%'") % fnTempRoots);
+            lockFile(fdTempRoots, ltRead, true);
+
+            /* Check whether the garbage collector didn't get in our
+               way. */
+            struct stat st;
+            if (fstat(fdTempRoots, &st) == -1)
+                throw SysError(format("statting `%1%'") % fnTempRoots);
+            if (st.st_size == 0) break;
+            
+            /* The garbage collector deleted this file before we could
+               get a lock.  (It won't delete the file after we get a
+               lock.)  Try again. */
+        }
+
+    }
+
+    /* Upgrade the lock to a write lock.  This will cause us to block
+       if the garbage collector is holding our lock. */
+    debug(format("acquiring write lock on `%1%'") % fnTempRoots);
+    lockFile(fdTempRoots, ltWrite, true);
+
+    string s = path + '\0';
+    writeFull(fdTempRoots, (const unsigned char *) s.c_str(), s.size());
+
+    /* Downgrade to a read lock. */
+    debug(format("downgrading to read lock on `%1%'") % fnTempRoots);
+    lockFile(fdTempRoots, ltRead, true);
+}
+
+
+typedef shared_ptr<AutoCloseFD> FDPtr;
+typedef list<FDPtr> FDs;
+
+
+static void readTempRoots(PathSet & tempRoots, FDs & fds)
+{
+    /* Read the `temproots' directory for per-process temporary root
+       files. */
+    Strings tempRootFiles = readDirectory(
+        (format("%1%/%2%") % nixStateDir % tempRootsDir).str());
+
+    for (Strings::iterator i = tempRootFiles.begin();
+         i != tempRootFiles.end(); ++i)
+    {
+        Path path = (format("%1%/%2%/%3%") % nixStateDir % tempRootsDir % *i).str();
+
+        debug(format("reading temporary root file `%1%'") % path);
+            
+        FDPtr fd(new AutoCloseFD(open(path.c_str(), O_RDWR, 0666)));
+        if (*fd == -1) {
+            /* It's okay if the file has disappeared. */
+            if (errno == ENOENT) continue;
+            throw SysError(format("opening temporary roots file `%1%'") % path);
+        }
+
+        /* Try to acquire a write lock without blocking.  This can
+           only succeed if the owning process has died.  In that case
+           we don't care about its temporary roots. */
+        if (lockFile(*fd, ltWrite, false)) {
+            printMsg(lvlError, format("removing stale temporary roots file `%1%'")
+                % path);
+            /* !!! write token, unlink */
+            continue;
+        }
+
+        /* Acquire a read lock.  This will prevent the owning process
+           from upgrading to a write lock, therefore it will block in
+           addTempRoot(). */
+        debug(format("waiting for read lock on `%1%'") % path);
+        lockFile(*fd, ltRead, true);
+
+        /* Read the entire file. */
+        struct stat st;
+        if (fstat(*fd, &st) == -1)
+            throw SysError(format("statting `%1%'") % path);
+        unsigned char buf[st.st_size]; /* !!! stack space */
+        readFull(*fd, buf, st.st_size);
+        debug(format("FILE SIZE %1%") % st.st_size);
+
+        /* Extract the roots. */
+        string contents((char *) buf, st.st_size);
+        unsigned int pos = 0, end;
+
+        while ((end = contents.find((char) 0, pos)) != string::npos) {
+            Path root(contents, pos, end - pos);
+            debug(format("got temporary root `%1%'") % root);
+            assertStorePath(root);
+            tempRoots.insert(root);
+            pos = end + 1;
+        }
+
+        fds.push_back(fd); /* keep open */
+    }
+}
+
+
 void collectGarbage(const PathSet & roots, GCAction action,
     PathSet & result)
 {
     result.clear();
     
-    /* !!! TODO: Acquire an exclusive lock on the gcroots directory.
-       This prevents the set of live paths from increasing after this
-       point. */
+    /* !!! TODO: Acquire the global GC root.  This prevents
+       a) New roots from being added.
+       b) Processes from creating new temporary root files. */
+
+    /* !!! Restrict read permission on the GC root.  Otherwise any
+       process that can open the file for reading can DoS the
+       collector. */
     
     /* Determine the live paths which is just the closure of the
        roots under the `references' relation. */
@@ -27,6 +153,16 @@ void collectGarbage(const PathSet & roots, GCAction action,
         return;
     }
 
+    /* Read the temporary roots.  This acquires read locks on all
+       per-process temporary root files.  So after this point no paths
+       can be added to the set of temporary roots. */
+    PathSet tempRoots;
+    FDs fds;
+    readTempRoots(tempRoots, fds);
+
+    for (FDs::iterator i = fds.begin(); i != fds.end(); ++i)
+        debug(format("FD %1%") % (int) **i);
+    
     /* !!! TODO: Try to acquire (without blocking) exclusive locks on
        the files in the `pending' directory.  Delete all files for
        which we managed to acquire such a lock (since if we could get
@@ -50,6 +186,11 @@ void collectGarbage(const PathSet & roots, GCAction action,
             continue;
         }
 
+        if (tempRoots.find(path) != tempRoots.end()) {
+            debug(format("temporary root `%1%'") % path);
+            continue;
+        }
+
         debug(format("dead path `%1%'") % path);
         result.insert(path);
 
@@ -57,100 +198,10 @@ void collectGarbage(const PathSet & roots, GCAction action,
             printMsg(lvlInfo, format("deleting `%1%'") % path);
             deleteFromStore(path);
         }
-        
+
+        /* Only delete lock files if the path is belongs to doesn't
+           exist and isn't a temporary root and we can acquire an
+           exclusive lock on it. */
+        /* !!! */
     }
 }
-
-
-
-#if 0
-void followLivePaths(Path nePath, PathSet & live)
-{
-    /* Just to be sure, canonicalise the path.  It is important to do
-       this here and in findDeadPath() to ensure that a live path is
-       not mistaken for a dead path due to some non-canonical
-       representation. */
-    nePath = canonPath(nePath);
-    
-    if (live.find(nePath) != live.end()) return;
-    live.insert(nePath);
-
-    startNest(nest, lvlDebug, format("following `%1%'") % nePath);
-    assertStorePath(nePath);
-
-    if (isValidPath(nePath)) {
-
-        /* !!! should make sure that no substitutes are used */
-        StoreExpr ne = storeExprFromPath(nePath);
-
-        /* !!! painfully similar to requisitesWorker() */
-        if (ne.type == StoreExpr::neClosure)
-            for (ClosureElems::iterator i = ne.closure.elems.begin();
-                 i != ne.closure.elems.end(); ++i)
-            {
-                Path p = canonPath(i->first);
-                if (live.find(p) == live.end()) {
-                    debug(format("found live `%1%'") % p);
-                    assertStorePath(p);
-                    live.insert(p);
-                }
-            }
-    
-        else if (ne.type == StoreExpr::neDerivation)
-            for (PathSet::iterator i = ne.derivation.inputs.begin();
-                 i != ne.derivation.inputs.end(); ++i)
-                followLivePaths(*i, live);
-
-        else abort();
-        
-    }
-
-    Path nfPath;
-    if (querySuccessor(nePath, nfPath))
-        followLivePaths(nfPath, live);
-}
-
-
-PathSet findLivePaths(const Paths & roots)
-{
-    PathSet live;
-
-    startNest(nest, lvlDebug, "finding live paths");
-
-    for (Paths::const_iterator i = roots.begin(); i != roots.end(); ++i)
-        followLivePaths(*i, live);
-
-    return live;
-}
-
-
-PathSet findDeadPaths(const PathSet & live, time_t minAge)
-{
-    PathSet dead;
-
-    startNest(nest, lvlDebug, "finding dead paths");
-
-    time_t now = time(0);
-
-    Strings storeNames = readDirectory(nixStore);
-
-    for (Strings::iterator i = storeNames.begin(); i != storeNames.end(); ++i) {
-        Path p = canonPath(nixStore + "/" + *i);
-
-        if (minAge > 0) {
-            struct stat st;
-            if (lstat(p.c_str(), &st) != 0)
-                throw SysError(format("obtaining information about `%1%'") % p);
-            if (st.st_atime + minAge >= now) continue;
-        }
-        
-        if (live.find(p) == live.end()) {
-            debug(format("dead path `%1%'") % p);
-            dead.insert(p);
-        } else
-            debug(format("live path `%1%'") % p);
-    }
-    
-    return dead;
-}
-#endif

src/libstore/gc.hh

@@ -3,6 +3,7 @@
 
 #include "util.hh"
 
+
 /* Garbage collector operation. */
 typedef enum { gcReturnLive, gcReturnDead, gcDeleteDead } GCAction;
 
@@ -14,4 +15,9 @@ typedef enum { gcReturnLive, gcReturnDead, gcDeleteDead } GCAction;
 void collectGarbage(const PathSet & roots, GCAction action,
     PathSet & result);
 
+/* Register a temporary GC root.  This root will automatically
+   disappear when this process exits. */
+void addTempRoot(const Path & path);
+
+
 #endif /* !__GC_H */

src/libstore/store.cc

@@ -12,6 +12,7 @@
 #include "db.hh"
 #include "archive.hh"
 #include "pathlocks.hh"
+#include "gc.hh"
 
 
 /* Nix database. */
@@ -468,6 +469,8 @@ Path addToStore(const Path & _srcPath)
     string baseName = baseNameOf(srcPath);
     Path dstPath = makeStorePath("source", h, baseName);
 
+    addTempRoot(dstPath);
+
     if (!readOnlyMode && !isValidPath(dstPath)) { 
 
         /* The first check above is an optimisation to prevent
@@ -512,6 +515,8 @@ Path addTextToStore(const string & suffix, const string & s,
 
     Path dstPath = makeStorePath("text", hash, suffix);
     
+    addTempRoot(dstPath);
+
     if (!readOnlyMode && !isValidPath(dstPath)) {
 
         PathSet lockPaths;

src/libutil/util.cc

@@ -234,8 +234,8 @@ Path createTempDir()
 
 void writeStringToFile(const Path & path, const string & s)
 {
-    AutoCloseFD fd = open(path.c_str(),
-        O_CREAT | O_EXCL | O_WRONLY, 0666);
+    AutoCloseFD fd(open(path.c_str(),
+        O_CREAT | O_EXCL | O_WRONLY, 0666));
     if (fd == -1)
         throw SysError(format("creating file `%1%'") % path);
     writeFull(fd, (unsigned char *) s.c_str(), s.size());
@@ -375,6 +375,12 @@ AutoCloseFD::AutoCloseFD(int fd)
 }
 
 
+AutoCloseFD::AutoCloseFD(const AutoCloseFD & fd)
+{
+    abort();
+}
+
+
 AutoCloseFD::~AutoCloseFD()
 {
     try {
@@ -392,7 +398,7 @@ void AutoCloseFD::operator =(int fd)
 }
 
 
-AutoCloseFD::operator int()
+AutoCloseFD::operator int() const
 {
     return fd;
 }
@@ -401,6 +407,7 @@ AutoCloseFD::operator int()
 void AutoCloseFD::close()
 {
     if (fd != -1) {
+        debug(format("closing fd %1%") % fd);
         if (::close(fd) == -1)
             /* This should never happen. */
             throw SysError("closing file descriptor");

src/libutil/util.hh

@@ -179,9 +179,10 @@ class AutoCloseFD
 public:
     AutoCloseFD();
     AutoCloseFD(int fd);
+    AutoCloseFD(const AutoCloseFD & fd);
     ~AutoCloseFD();
     void operator =(int fd);
-    operator int();
+    operator int() const;
     void close();
     bool isOpen();
     int borrow();

tests/Makefile.am

@@ -36,9 +36,10 @@ nix-pull.sh: dependencies.nix
 gc.sh: dependencies.nix
 gc-concurrent.sh: gc-concurrent.nix
 
-TESTS = init.sh hash.sh lang.sh simple.sh dependencies.sh locking.sh parallel.sh \
-  build-hook.sh substitutes.sh substitutes2.sh fallback.sh nix-push.sh gc.sh \
-  gc-concurrent.sh verify.sh nix-pull.sh
+#TESTS = init.sh hash.sh lang.sh simple.sh dependencies.sh locking.sh parallel.sh \
+#  build-hook.sh substitutes.sh substitutes2.sh fallback.sh nix-push.sh gc.sh \
+#  gc-concurrent.sh verify.sh nix-pull.sh
+TESTS = init.sh gc-concurrent.sh 
 
 XFAIL_TESTS =
 

tests/gc-concurrent.sh

@@ -1,6 +1,8 @@
 storeExpr=$($TOP/src/nix-instantiate/nix-instantiate gc-concurrent.nix)
 outPath=$($TOP/src/nix-store/nix-store -q $storeExpr)
 
+ls -l test-tmp/state/temproots
+
 
 # Start a build in the background.
 $TOP/src/nix-store/nix-store -rvv "$storeExpr" &
@@ -8,7 +10,7 @@ pid=$!
 
 # Run the garbage collector while the build is running.
 sleep 2
-$NIX_BIN_DIR/nix-collect-garbage
+$NIX_BIN_DIR/nix-collect-garbage -vvvvv
 
 # Wait for the build to finish.
 echo waiting for pid $pid to finish...

tests/init.sh

@@ -26,6 +26,7 @@ ln -s $TOP/scripts/readmanifest.pm $NIX_BIN_DIR/nix/
 mkdir -p "$NIX_LOCALSTATE_DIR"/nix/manifests
 mkdir -p "$NIX_LOCALSTATE_DIR"/nix/gcroots
 mkdir -p "$NIX_LOCALSTATE_DIR"/log/nix
+mkdir -p "$NIX_LOCALSTATE_DIR"/temproots
 
 mkdir $NIX_DATA_DIR/nix
 cp -prd $TOP/corepkgs $NIX_DATA_DIR/nix/