* nix-store, nix-instantiate: added an option `--add-root' to

immediately add the result as a permanent GC root.  This is the only
  way to prevent a race with the garbage collector.  For instance, the
  old style

    ln -s $(nix-store -r $(nix-instantiate foo.nix)) \
      /nix/var/nix/gcroots/result

  has two time windows in which the garbage collector can interfere
  (by GC'ing the derivation and the output, respectively).  On the
  other hand,

    nix-store --add-root /nix/var/nix/gcroots/result -r \
      $(nix-instantiate --add-root /nix/var/nix/gcroots/drv \
        foo.nix)

  is safe.

* nix-build: use `--add-root' to prevent GC races.

src/libstore/gc.cc

@@ -39,6 +39,48 @@ static int openGCLock(LockType lockType)
 }
 
 
+static void createDirs(const Path & path)
+{
+    if (path == "") return;
+    createDirs(dirOf(path));
+    if (!pathExists(path))
+        if (mkdir(path.c_str(), 0777) == -1)
+            throw SysError(format("creating directory `%1%'") % path);
+}
+
+
+Path addPermRoot(const Path & _storePath, const Path & _gcRoot)
+{
+    Path storePath(canonPath(_storePath));
+    Path gcRoot(canonPath(_gcRoot));
+
+    Path rootsDir = canonPath((format("%1%/%2%") % nixStateDir % "gcroots").str());
+    
+    if (string(gcRoot, 0, rootsDir.size() + 1) != rootsDir + "/")
+        throw Error(format(
+            "path `%1%' is not a valid garbage collector root; "
+            "it's not in the `%1%' directory")
+            % gcRoot % rootsDir);
+
+    /* Grab the global GC root.  This prevents the set of permanent
+       roots from increasing while a GC is in progress. */
+    AutoCloseFD fdGCLock = openGCLock(ltRead);
+
+    /* Create directories up to `gcRoot'. */
+    createDirs(dirOf(gcRoot));
+
+    /* Remove the old symlink. */
+    unlink(gcRoot.c_str());
+
+    /* And create the new own. */
+    if (symlink(storePath.c_str(), gcRoot.c_str()) == -1)
+        throw SysError(format("symlinking `%1%' to `%2%'")
+            % gcRoot % storePath);
+
+    return gcRoot;
+}
+
+
 static string tempRootsDir = "temproots";
 
 /* The file to which we write our temporary roots. */
@@ -210,6 +252,9 @@ void collectGarbage(const PathSet & roots, GCAction action,
        b) Processes from creating new temporary root files. */
     AutoCloseFD fdGCLock = openGCLock(ltWrite);
 
+    /* !!! Find the roots here, after we've grabbed the GC lock, since
+       the set of permanent roots cannot increase now. */
+
     /* Determine the live paths which is just the closure of the
        roots under the `references' relation. */
     PathSet livePaths;
@@ -264,6 +309,7 @@ void collectGarbage(const PathSet & roots, GCAction action,
        will not work anymore because we get cycles. */
     storePaths = topoSort(storePaths2);
 
+    /* Try to delete store paths in the topologically sorted order. */
     for (Paths::iterator i = storePaths.begin(); i != storePaths.end(); ++i) {
 
         debug(format("considering deletion of `%1%'") % *i);

src/libstore/gc.hh

@@ -26,5 +26,8 @@ void addTempRoot(const Path & path);
    as a (permanent) root. */
 void removeTempRoots();
 
+/* Register a permanent GC root. */
+Path addPermRoot(const Path & storePath, const Path & gcRoot);
+
 
 #endif /* !__GC_H */

src/libstore/store.cc

@@ -168,7 +168,7 @@ void copyPath(const Path & src, const Path & dst)
 }
 
 
-static bool isInStore(const Path & path)
+bool isStorePath(const Path & path)
 {
     return path[0] == '/'
         && path.compare(0, nixStore.size(), nixStore) == 0
@@ -180,7 +180,7 @@ static bool isInStore(const Path & path)
 
 void assertStorePath(const Path & path)
 {
-    if (!isInStore(path))
+    if (!isStorePath(path))
         throw Error(format("path `%1%' is not in the Nix store") % path);
 }
 
@@ -579,7 +579,7 @@ void verifyStore()
         if (!pathExists(path)) {
             printMsg(lvlError, format("path `%1%' disappeared") % path);
             invalidatePath(path, txn);
-        } else if (!isInStore(path)) {
+        } else if (!isStorePath(path)) {
             printMsg(lvlError, format("path `%1%' is not in the Nix store") % path);
             invalidatePath(path, txn);
         } else

src/libstore/store.hh

@@ -62,6 +62,8 @@ void registerValidPath(const Transaction & txn,
 /* Throw an exception if `path' is not directly in the Nix store. */
 void assertStorePath(const Path & path);
 
+bool isStorePath(const Path & path);
+
 /* "Fix", or canonicalise, the meta-data of the files in a store path
    after it has been built.  In particular:
    - the last modification date on each file is set to 0 (i.e.,