* The determination of the root set should be made by the privileged

process, so forward the operation.
* Spam the user about GC misconfigurations (NIX-71).
* findRoots: skip all roots that are unreadable - the warnings with
  which we spam the user should be enough.

src/libstore/gc.cc

@@ -91,12 +91,6 @@ void LocalStore::addIndirectRoot(const Path & path)
 }
 
 
-typedef std::map<Path, Path> Roots;
-
-
-static void findRoots(Roots & roots, bool ignoreUnreadable);
-
-
 Path addPermRoot(const Path & _storePath, const Path & _gcRoot,
     bool indirect, bool allowOutsideRootsDir)
 {
@@ -122,16 +116,16 @@ Path addPermRoot(const Path & _storePath, const Path & _gcRoot,
             
         createSymlink(gcRoot, storePath, false);
 
+    }
+
     /* Check that the root can be found by the garbage collector. */
-        Roots roots;
-        findRoots(roots, true);
+    Roots roots = store->findRoots();
     if (roots.find(gcRoot) == roots.end())
         printMsg(lvlError, 
             format(
                 "warning: the garbage collector does not find `%1%' as a root; "
                 "therefore, `%2%' might be removed by the garbage collector")
             % gcRoot % storePath);
-    }
         
     /* Grab the global GC root, causing us to block while a GC is in
        progress.  This prevents the set of permanent roots from
@@ -308,8 +302,10 @@ static void readTempRoots(PathSet & tempRoots, FDs & fds)
 
 
 static void findRoots(const Path & path, bool recurseSymlinks,
-    bool ignoreUnreadable, Roots & roots)
+    bool deleteStale, Roots & roots)
 {
+    try {
+        
         struct stat st;
         if (lstat(path.c_str(), &st) == -1)
             throw SysError(format("statting `%1%'") % path);
@@ -319,7 +315,7 @@ static void findRoots(const Path & path, bool recurseSymlinks,
         if (S_ISDIR(st.st_mode)) {
             Strings names = readDirectory(path);
             for (Strings::iterator i = names.begin(); i != names.end(); ++i)
-            findRoots(path + "/" + *i, recurseSymlinks, ignoreUnreadable, roots);
+                findRoots(path + "/" + *i, recurseSymlinks, deleteStale, roots);
         }
 
         else if (S_ISLNK(st.st_mode)) {
@@ -337,29 +333,42 @@ static void findRoots(const Path & path, bool recurseSymlinks,
             }
 
             else if (recurseSymlinks) {
-            struct stat st2;
-            if (lstat(target.c_str(), &st2) == 0)
-                findRoots(target, false, ignoreUnreadable, roots);
-            else if (ignoreUnreadable && errno == EACCES)
-                /* ignore */ ;
-            else if (errno == ENOENT || errno == ENOTDIR) {
+                if (pathExists(target))
+                    findRoots(target, false, deleteStale, roots);
+                else if (deleteStale) {
                     printMsg(lvlInfo, format("removing stale link from `%1%' to `%2%'") % path % target);
-                /* Note that we only delete when recursing, i.e., when
-                   we are still in the `gcroots' tree.  We never
-                   delete stuff outside that tree. */
+                    /* Note that we only delete when recursing, i.e.,
+                       when we are still in the `gcroots' tree.  We
+                       never delete stuff outside that tree. */
                     unlink(path.c_str());
                 }
+            }
+        }
+
+    }
+
+    catch (SysError & e) {
+        /* We only ignore permanent failures. */
+        if (e.errNo == EACCES || e.errNo == ENOENT || e.errNo == ENOTDIR)
+            printMsg(lvlInfo, format("cannot read potential root `%1%'") % path);
         else
-                throw SysError(format("statting `%1%'") % target);
-        }
+            throw;
     }
 }
 
 
-static void findRoots(Roots & roots, bool ignoreUnreadable)
+static Roots findRoots(bool deleteStale)
 {
+    Roots roots;
     Path rootsDir = canonPath((format("%1%/%2%") % nixStateDir % gcRootsDir).str());
-    findRoots(rootsDir, true, ignoreUnreadable, roots);
+    findRoots(rootsDir, true, deleteStale, roots);
+    return roots;
+}
+
+
+Roots LocalStore::findRoots()
+{
+    return nix::findRoots(false);
 }
 
 
@@ -437,8 +446,7 @@ void collectGarbage(GCAction action, const PathSet & pathsToDelete,
 
     /* Find the roots.  Since we've grabbed the GC lock, the set of
        permanent roots cannot increase now. */
-    Roots rootMap;
-    findRoots(rootMap, false);
+    Roots rootMap = ignoreLiveness ? Roots() : findRoots(true);
 
     PathSet roots;
     for (Roots::iterator i = rootMap.begin(); i != rootMap.end(); ++i)

src/libstore/local-store.hh

@@ -64,6 +64,8 @@ public:
     void addIndirectRoot(const Path & path);
     
     void syncWithGC();
+
+    Roots findRoots();
 };
 
 

src/libstore/remote-store.cc

@@ -18,6 +18,23 @@
 namespace nix {
 
 
+Path readStorePath(Source & from)
+{
+    Path path = readString(from);
+    assertStorePath(path);
+    return path;
+}
+
+
+PathSet readStorePaths(Source & from)
+{
+    PathSet paths = readStringSet(from);
+    for (PathSet::iterator i = paths.begin(); i != paths.end(); ++i)
+        assertStorePath(*i);
+    return paths;
+}
+
+
 RemoteStore::RemoteStore()
 {
     string remoteMode = getEnv("NIX_REMOTE");
@@ -179,7 +196,7 @@ void RemoteStore::queryReferences(const Path & path,
     writeInt(wopQueryReferences, to);
     writeString(path, to);
     processStderr();
-    PathSet references2 = readStringSet(from);
+    PathSet references2 = readStorePaths(from);
     references.insert(references2.begin(), references2.end());
 }
 
@@ -190,7 +207,7 @@ void RemoteStore::queryReferrers(const Path & path,
     writeInt(wopQueryReferrers, to);
     writeString(path, to);
     processStderr();
-    PathSet referrers2 = readStringSet(from);
+    PathSet referrers2 = readStorePaths(from);
     referrers.insert(referrers2.begin(), referrers2.end());
 }
 
@@ -207,7 +224,7 @@ Path RemoteStore::addToStore(const Path & _srcPath, bool fixed,
     writeString(hashAlgo, to);
     dumpPath(srcPath, to);
     processStderr();
-    Path path = readString(from);
+    Path path = readStorePath(from);
     return path;
 }
 
@@ -221,7 +238,7 @@ Path RemoteStore::addTextToStore(const string & suffix, const string & s,
     writeStringSet(references, to);
     
     processStderr();
-    Path path = readString(from);
+    Path path = readStorePath(from);
     return path;
 }
 
@@ -270,6 +287,21 @@ void RemoteStore::syncWithGC()
 }
 
 
+Roots RemoteStore::findRoots()
+{
+    writeInt(wopFindRoots, to);
+    processStderr();
+    unsigned int count = readInt(from);
+    Roots result;
+    while (count--) {
+        Path link = readString(from);
+        Path target = readStorePath(from);
+        result[link] = target;
+    }
+    return result;
+}
+
+
 void RemoteStore::processStderr()
 {
     unsigned int msg;

src/libstore/remote-store.hh

@@ -53,6 +53,8 @@ public:
     
     void syncWithGC();
     
+    Roots findRoots();
+    
 private:
     AutoCloseFD fdSocket;
     FdSink to;

src/libstore/store-api.hh

@@ -2,6 +2,7 @@
 #define __STOREAPI_H
 
 #include <string>
+#include <map>
 
 #include <boost/shared_ptr.hpp>
 
@@ -33,6 +34,9 @@ struct Substitute
 typedef list<Substitute> Substitutes;
 
 
+typedef std::map<Path, Path> Roots;
+
+
 class StoreAPI 
 {
 public:
@@ -118,6 +122,11 @@ public:
        In either case the permanent root is seen by the collector. */
     virtual void syncWithGC() = 0;
 
+    /* Find the roots of the garbage collector.  Each root is a pair
+       (link, storepath) where `link' is the path of the symlink
+       outside of the Nix store that point to `storePath'.  */
+    virtual Roots findRoots() = 0;
+
 };
 
 

src/libstore/worker-protocol.hh

@@ -2,6 +2,9 @@
 #define __WORKER_PROTOCOL_H
 
 
+namespace nix {
+
+
 #define WORKER_MAGIC_1 0x6e697864
 #define WORKER_MAGIC_2 0x6478696e
 
@@ -21,6 +24,7 @@ typedef enum {
     wopAddTempRoot,
     wopAddIndirectRoot,
     wopSyncWithGC,
+    wopFindRoots,
 } WorkerOp;
 
 
@@ -34,4 +38,11 @@ typedef enum {
 #define DEFAULT_SOCKET_PATH "/daemon.socket"
 
 
+Path readStorePath(Source & from);
+PathSet readStorePaths(Source & from);
+
+    
+}
+
+
 #endif /* !__WORKER_PROTOCOL_H */

src/libutil/types.hh

@@ -34,6 +34,7 @@ public:
 class SysError : public Error
 {
 public:
+    int errNo;
     SysError(const format & f);
 };
 

src/libutil/util.cc

@@ -35,6 +35,7 @@ Error & Error::addPrefix(const format & f)
 
 SysError::SysError(const format & f)
     : Error(format("%1%: %2%") % f.str() % strerror(errno))
+    , errNo(errno)
 {
 }
 

src/nix-worker/main.cc

@@ -23,23 +23,6 @@ using namespace nix;
 #endif
 
 
-static Path readStorePath(Source & from)
-{
-    Path path = readString(from);
-    assertStorePath(path);
-    return path;
-}
-
-
-static PathSet readStorePaths(Source & from)
-{
-    PathSet paths = readStringSet(from);
-    for (PathSet::iterator i = paths.begin(); i != paths.end(); ++i)
-        assertStorePath(*i);
-    return paths;
-}
-
-
 static FdSource from(STDIN_FILENO);
 static FdSink to(STDOUT_FILENO);
 
@@ -286,6 +269,18 @@ static void performOp(Source & from, Sink & to, unsigned int op)
         break;
     }
 
+    case wopFindRoots: {
+        startWork();
+        Roots roots = store->findRoots();
+        stopWork();
+        writeInt(roots.size(), to);
+        for (Roots::iterator i = roots.begin(); i != roots.end(); ++i) {
+            writeString(i->first, to);
+            writeString(i->second, to);
+        }
+        break;
+    }
+
     default:
         throw Error(format("invalid operation %1%") % op);
     }