* Fix and simplify the garbage collector (it's still not concurrent,

though).  In particular it's now much easier to register a GC root.
  Just place a symlink to whatever store path it is that you want to
  keep in /nix/var/nix/gcroots.

src/libstore/build.cc

@@ -458,7 +458,7 @@ void DerivationGoal::haveStoreExpr()
          i != invalidOutputs.end(); ++i)
         /* Don't bother creating a substitution goal if there are no
            substitutes. */
-        if (querySubstitutes(*i).size() > 0)
+        if (querySubstitutes(noTxn, *i).size() > 0)
             addWaitee(worker.makeSubstitutionGoal(*i));
 
     if (waitees.empty()) /* to prevent hang (no wake-up event) */
@@ -1315,7 +1315,7 @@ void SubstitutionGoal::init()
     }
 
     /* Read the substitutes. */
-    subs = querySubstitutes(storePath);
+    subs = querySubstitutes(noTxn, storePath);
 
     /* To maintain the closure invairant, we first have to realise the
        paths referenced by this one. */

src/libstore/gc.cc

@@ -1,12 +1,68 @@
 #include "globals.hh"
 #include "gc.hh"
-
+#include "build.hh"
 
 #include <sys/types.h>
 #include <sys/stat.h>
 #include <unistd.h>
 
 
+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. */
+    
+    /* Determine the live paths which is just the closure of the
+       roots under the `references' relation. */
+    PathSet livePaths;
+    for (PathSet::const_iterator i = roots.begin(); i != roots.end(); ++i)
+        computeFSClosure(canonPath(*i), livePaths);
+
+    if (action == gcReturnLive) {
+        result = livePaths;
+        return;
+    }
+
+    /* !!! 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
+       such a lock, that means that the process that owned the file
+       has died). */
+
+    /* !!! TODO: Acquire shared locks on all files in the pending
+       directories.  This prevents the set of pending paths from
+       increasing while we are garbage-collecting.  Read the set of
+       pending paths from those files. */
+
+    /* Read the Nix store directory to find all currently existing
+       paths. */
+    Strings storeNames = readDirectory(nixStore);
+
+    for (Strings::iterator i = storeNames.begin(); i != storeNames.end(); ++i) {
+        Path path = canonPath(nixStore + "/" + *i);
+
+        if (livePaths.find(path) != livePaths.end()) {
+            debug(format("live path `%1%'") % path);
+            continue;
+        }
+
+        debug(format("dead path `%1%'") % path);
+        result.insert(path);
+
+        if (action == gcDeleteDead) {
+            printMsg(lvlInfo, format("deleting `%1%'") % path);
+            deleteFromStore(path);
+        }
+        
+    }
+}
+
+
+
 #if 0
 void followLivePaths(Path nePath, PathSet & live)
 {

src/libstore/gc.hh

@@ -3,24 +3,15 @@
 
 #include "util.hh"
 
+/* Garbage collector operation. */
+typedef enum { gcReturnLive, gcReturnDead, gcDeleteDead } GCAction;
 
-/* Determine the set of "live" store paths, given a set of root store
-   expressions.  The live store paths are those that are reachable
-   from the roots.  The roots are reachable by definition.  Any path
-   mentioned in a reachable store expression is also reachable.  If a
-   derivation store expression is reachable, then its successor (if it
-   exists) if also reachable.  It is not an error for store
-   expressions not to exist (since this can happen on derivation store
-   expressions, for instance, due to the substitute mechanism), but
-   successor links are followed even for non-existant derivations. */
-PathSet findLivePaths(const Paths & roots);
-
-/* Given a set of "live" store paths, determine the set of "dead"
-   store paths (which are simply all store paths that are not in the
-   live set).  The value `minAge' specifies the minimum age in seconds
-   for an unreachable file to be considered dead (0 meaning that any
-   unreachable file is dead). */
-PathSet findDeadPaths(const PathSet & live, time_t minAge);
-
+/* If `action' is set to `soReturnLive', return the set of paths
+   reachable from (i.e. in the closure of) the specified roots.  If
+   `action' is `soReturnDead', return the set of paths not reachable
+   from the roots.  If `action' is `soDeleteDead', actually delete the
+   latter set. */
+void collectGarbage(const PathSet & roots, GCAction action,
+    PathSet & result);
 
 #endif /* !__GC_H */

src/libstore/store.cc

@@ -363,9 +363,9 @@ void registerSubstitute(const Transaction & txn,
 }
 
 
-Substitutes querySubstitutes(const Path & srcPath)
+Substitutes querySubstitutes(const Transaction & txn, const Path & srcPath)
 {
-    return readSubstitutes(noTxn, srcPath);
+    return readSubstitutes(txn, srcPath);
 }
 
 
@@ -411,6 +411,13 @@ static void invalidatePath(const Path & path, Transaction & txn)
     debug(format("unregistering path `%1%'") % path);
 
     nixDB.delPair(txn, dbValidPaths, path);
+
+    /* Clear the `references' entry for this path, as well as the
+       inverse `referers' entries; but only if there are no
+       substitutes for this path.  This maintains the cleanup
+       invariant. */
+    if (querySubstitutes(txn, path).size() == 0)
+        setReferences(txn, path, PathSet());
 }
 
 

src/libstore/store.hh

@@ -45,7 +45,7 @@ void registerSubstitute(const Transaction & txn,
     const Path & srcPath, const Substitute & sub);
 
 /* Return the substitutes for the given path. */
-Substitutes querySubstitutes(const Path & srcPath);
+Substitutes querySubstitutes(const Transaction & txn, const Path & srcPath);
 
 /* Deregister all substitutes. */
 void clearSubstitutes();