* Garbage collector: added an option `--use-atime' to delete paths in

order of ascending last access time.  This is useful in conjunction
  with --max-freed or --max-links to prefer deleting non-recently used
  garbage, which is good (especially in the build farm) since garbage
  may become live again.

  The code could easily be modified to accept other criteria for
  ordering garbage by changing the comparison operator used by the
  priority queue in collectGarbage().

src/libstore/gc.cc

@@ -5,6 +5,9 @@
 
 #include <boost/shared_ptr.hpp>
 
+#include <functional>
+#include <queue>
+
 #include <sys/types.h>
 #include <sys/stat.h>
 #include <errno.h>
@@ -439,29 +442,38 @@ Paths topoSortPaths(const PathSet & paths)
 }
 
 
+static time_t lastFileAccessTime(const Path & path)
+{
+    struct stat st;
+    if (lstat(path.c_str(), &st) == -1)
+        throw SysError(format("statting `%1%'") % path);
+
+    if (S_ISDIR(st.st_mode)) {
+        time_t last = 0;
+	Strings names = readDirectory(path);
+	for (Strings::iterator i = names.begin(); i != names.end(); ++i) {
+            time_t t = lastFileAccessTime(path + "/" + *i);
+            if (t > last) last = t;
+        }
+        return last;
+    }
+
+    else if (S_ISLNK(st.st_mode)) return 0;
+
+    else return st.st_atime;
+}
+
+
 struct GCLimitReached { };
 
 
 void LocalStore::tryToDelete(const GCOptions & options, GCResults & results, 
-    const PathSet & livePaths, const PathSet & tempRootsClosed, PathSet & done, 
+    PathSet & done, const Path & path)
-    const Path & path)
 {
     if (done.find(path) != done.end()) return;
     done.insert(path);
 
-    debug(format("considering deletion of `%1%'") % path);
+    startNest(nest, lvlDebug, format("looking at `%1%'") % path);
-        
-    if (livePaths.find(path) != livePaths.end()) {
-        if (options.action == GCOptions::gcDeleteSpecific)
-            throw Error(format("cannot delete path `%1%' since it is still alive") % path);
-        debug(format("live path `%1%'") % path);
-        return;
-    }
-
-    if (tempRootsClosed.find(path) != tempRootsClosed.end()) {
-        debug(format("temporary root `%1%'") % path);
-        return;
-    }
         
     /* Delete all the referrers first.  They must be garbage too,
        since if they were in the closure of some live path, then this
@@ -469,15 +481,15 @@ void LocalStore::tryToDelete(const GCOptions & options, GCResults & results,
        deleteFromStore() below still makes sure that the referrer set
        has become empty, just in case. */
     PathSet referrers;
-    if (store->isValidPath(path))
+    if (isValidPath(path))
         queryReferrers(path, referrers);
     foreach (PathSet::iterator, i, referrers)
-        if (*i != path)
+        if (*i != path) tryToDelete(options, results, done, *i);
-            tryToDelete(options, results, livePaths, tempRootsClosed, done, *i);
 
-    debug(format("dead path `%1%'") % path);
     results.paths.insert(path);
 
+    if (!pathExists(path)) return;
+                
     /* If just returning the set of dead paths, we also return the
        space that would be freed if we deleted them. */
     if (options.action == GCOptions::gcReturnDead) {
@@ -505,8 +517,6 @@ void LocalStore::tryToDelete(const GCOptions & options, GCResults & results,
     }
 #endif
 
-    if (!pathExists(path)) return;
-                
     printMsg(lvlInfo, format("deleting `%1%'") % path);
             
     /* Okay, it's safe to delete. */
@@ -538,6 +548,35 @@ void LocalStore::tryToDelete(const GCOptions & options, GCResults & results,
 }
 
 
+struct CachingAtimeComparator : public std::binary_function<Path, Path, bool> 
+{
+    std::map<Path, time_t> cache;
+
+    time_t lookup(const Path & p)
+    {
+        std::map<Path, time_t>::iterator i = cache.find(p);
+        if (i != cache.end()) return i->second;
+        debug(format("computing atime of `%1%'") % p);
+        cache[p] = lastFileAccessTime(p);
+        assert(cache.find(p) != cache.end());
+        return cache[p];
+    }
+        
+    bool operator () (const Path & p1, const Path & p2)
+    {
+        return lookup(p2) < lookup(p1);
+    }
+};
+
+
+string showTime(const string & format, time_t t)
+{
+    char s[128];
+    strftime(s, sizeof s, format.c_str(), gmtime(&t));
+    return string(s);
+}
+
+
 void LocalStore::collectGarbage(const GCOptions & options, GCResults & results)
 {
     bool gcKeepOutputs =
@@ -587,8 +626,8 @@ void LocalStore::collectGarbage(const GCOptions & options, GCResults & results)
             /* Note that the deriver need not be valid (e.g., if we
                previously ran the collector with `gcKeepDerivations'
                turned off). */
-            Path deriver = store->queryDeriver(*i);
+            Path deriver = queryDeriver(*i);
-            if (deriver != "" && store->isValidPath(deriver))
+            if (deriver != "" && isValidPath(deriver))
                 computeFSClosure(deriver, livePaths);
         }
     }
@@ -608,7 +647,7 @@ void LocalStore::collectGarbage(const GCOptions & options, GCResults & results)
 		if (gcLevel >= gcKeepOutputsThreshold)    
 		    for (DerivationOutputs::iterator j = drv.outputs.begin();
                          j != drv.outputs.end(); ++j)
-			if (store->isValidPath(j->second.path))
+			if (isValidPath(j->second.path))
 			    computeFSClosure(j->second.path, livePaths);
             }
     }
@@ -633,7 +672,7 @@ void LocalStore::collectGarbage(const GCOptions & options, GCResults & results)
        means that it has already been closed). */
     PathSet tempRootsClosed;
     for (PathSet::iterator i = tempRoots.begin(); i != tempRoots.end(); ++i)
-        if (store->isValidPath(*i))
+        if (isValidPath(*i))
             computeFSClosure(*i, tempRootsClosed);
         else
             tempRootsClosed.insert(*i);
@@ -644,29 +683,110 @@ void LocalStore::collectGarbage(const GCOptions & options, GCResults & results)
        can be deleted. */
     
     /* Read the Nix store directory to find all currently existing
-       paths. */
+       paths and filter out all live paths. */
     printMsg(lvlError, format("reading the Nix store..."));
     PathSet storePaths;
+    
     if (options.action != GCOptions::gcDeleteSpecific) {
         Paths entries = readDirectory(nixStore);
-        for (Paths::iterator i = entries.begin(); i != entries.end(); ++i)
+        foreach (Paths::iterator, i, entries) {
-            storePaths.insert(canonPath(nixStore + "/" + *i));
+            Path path = canonPath(nixStore + "/" + *i);
-    } else {
+            if (livePaths.find(path) == livePaths.end() &&
+                tempRootsClosed.find(path) == tempRootsClosed.end())
+                storePaths.insert(path);
+        }
+    }
+
+    else {
         foreach (PathSet::iterator, i, options.pathsToDelete) {
             assertStorePath(*i);
             storePaths.insert(*i);
+            if (livePaths.find(*i) != livePaths.end())
+                throw Error(format("cannot delete path `%1%' since it is still alive") % *i);
+            if (tempRootsClosed.find(*i) != tempRootsClosed.end())
+                throw Error(format("cannot delete path `%1%' since it is temporarily in use") % *i);
         }
     }
 
-    /* Try to delete store paths in the topologically sorted order. */
+    if (options.action == GCOptions::gcReturnDead) {
-    printMsg(lvlError, options.action == GCOptions::gcReturnDead
+        results.paths.insert(storePaths.begin(), storePaths.end());
-        ? format("looking for garbage...")
+        return;
-        : format("deleting garbage..."));
+    }
+
+    /* Delete all dead store paths (or until one of the stop
+       conditions is reached). */
 
     PathSet done;
     try {
+
+        if (!options.useAtime) {
+            /* Delete the paths, respecting the partial ordering
+               determined by the references graph. */
+            printMsg(lvlError, format("deleting garbage..."));
             foreach (PathSet::iterator, i, storePaths)
-            tryToDelete(options, results, livePaths, tempRootsClosed, done, *i);
+                tryToDelete(options, results, done, *i);
+        }
+
+        else {
+
+            /* Delete in order of ascending last access time, still
+               maintaining the partial ordering of the reference
+               graph.  Note that we can't use a topological sort for
+               this because that takes time O(V+E), and in this case
+               E=O(V^2) (i.e. the graph is dense because of the edges
+               due to the atime ordering).  So instead we put all
+               deletable paths in a priority queue (ordered by atime),
+               and after deleting a path, add additional paths that
+               have become deletable to the priority queue. */
+
+            CachingAtimeComparator atimeComp;
+
+            /* Create a priority queue that orders paths by ascending
+               atime.  This is why C++ needs type inferencing... */
+            std::priority_queue<Path, vector<Path>, binary_function_ref_adapter<CachingAtimeComparator> > prioQueue =
+                std::priority_queue<Path, vector<Path>, binary_function_ref_adapter<CachingAtimeComparator> >(binary_function_ref_adapter<CachingAtimeComparator>(&atimeComp));
+
+           /* Initially put the paths that are invalid or have no
+              referrers into the priority queue. */
+            printMsg(lvlError, format("finding deletable paths..."));
+            foreach (PathSet::iterator, i, storePaths) {
+                /* We can safely delete a path if it's invalid or
+                   it has no referrers.  Note that all the invalid
+                   paths will be deleted in the first round. */
+                if (isValidPath(*i)) {
+                    if (queryReferrersNoSelf(*i).empty()) prioQueue.push(*i);
+                } else prioQueue.push(*i);
+            }
+
+            debug(format("%1% initially deletable paths") % prioQueue.size());
+
+            /* Now delete everything in the order of the priority
+               queue until nothing is left. */
+            while (!prioQueue.empty()) {
+                Path path = prioQueue.top(); prioQueue.pop();
+                printMsg(lvlTalkative, format("atime %1%: %2%") % showTime("%F %H:%M:%S", atimeComp.cache[path]) % path);
+
+                PathSet references;
+                if (isValidPath(path)) references = queryReferencesNoSelf(path);
+
+                tryToDelete(options, results, done, path);
+
+                /* For each reference of the current path, see if the
+                   reference has now become deletable (i.e. is in the
+                   set of dead paths and has no referrers left).  If
+                   so add it to the priority queue. */
+                foreach (PathSet::iterator, i, references) {
+                    if (storePaths.find(*i) != storePaths.end() &&
+                        queryReferrersNoSelf(*i).empty())
+                    {
+                        debug(format("path `%1%' has become deletable") % *i);
+                        prioQueue.push(*i);
+                    }
+                }
+            }
+            
+        }
+        
     } catch (GCLimitReached & e) {
     }
 }

src/libstore/local-store.hh

@@ -168,8 +168,7 @@ private:
     void upgradeStore12();
 
     void tryToDelete(const GCOptions & options, GCResults & results,
-        const PathSet & livePaths, const PathSet & tempRootsClosed, PathSet & done, 
+        PathSet & done, const Path & path);
-        const Path & path);
 
     void startSubstituter(const Path & substituter,
         RunningSubstituter & runningSubstituter);

src/libstore/store-api.cc

@@ -14,6 +14,7 @@ GCOptions::GCOptions()
     ignoreLiveness = false;
     maxFreed = ULLONG_MAX;
     maxLinks = 0;
+    useAtime = false;
 }
 
 

src/libstore/store-api.hh

@@ -63,6 +63,18 @@ struct GCOptions
        has dropped below `maxLinks'. */
     unsigned int maxLinks;
 
+    /* Delete paths in order of ascending last access time.  I.e.,
+       prefer deleting unrecently used paths.  Useful in conjunction
+       with `maxFreed' and `maxLinks' (or manual interruption).  The
+       access time of a path is defined as the highest atime of any
+       non-directory, non-symlink file under that path.  Directories
+       and symlinks are ignored because their atimes are frequently
+       mass-updated, e.g. by `locate'.  Note that optimiseStore()
+       somewhat reduces the usefulness of this option: it hard-links
+       regular files and symlink together, giving them a "shared"
+       atime. */
+    bool useAtime;
+
     GCOptions();
 };
 
@@ -116,11 +128,29 @@ public:
     virtual void queryReferences(const Path & path,
         PathSet & references) = 0;
 
+    /* Like queryReferences, but with self-references filtered out. */
+    PathSet queryReferencesNoSelf(const Path & path)
+    {
+        PathSet res;
+        queryReferences(path, res);
+        res.erase(path);
+        return res;
+    }
+
     /* Queries the set of incoming FS references for a store path.
        The result is not cleared. */
     virtual void queryReferrers(const Path & path,
         PathSet & referrers) = 0;
 
+    /* Like queryReferrers, but with self-references filtered out. */
+    PathSet queryReferrersNoSelf(const Path & path)
+    {
+        PathSet res;
+        queryReferrers(path, res);
+        res.erase(path);
+        return res;
+    }
+
     /* Query the deriver of a store path.  Return the empty string if
        no deriver has been set. */
     virtual Path queryDeriver(const Path & path) = 0;

src/libutil/util.hh

@@ -303,6 +303,29 @@ bool hasSuffix(const string & s, const string & suffix);
 void ignoreException();
 
 
+/* STL functions such as sort() pass a binary function object around
+   by value, so it gets cloned a lot.  This is bad if the function
+   object has state or is simply large.  This adapter wraps the
+   function object to simulate passing by reference. */
+template<class F>
+struct binary_function_ref_adapter
+{
+    F * p;
+
+    binary_function_ref_adapter(F * _p)
+    {
+        p = _p;
+    }
+    
+    typename F::result_type operator () (
+        const typename F::first_argument_type & x,
+        const typename F::second_argument_type & y)
+    {
+        return (*p)(x, y);
+    }
+};
+
+
 }
 
 

src/nix-store/nix-store.cc

@@ -501,17 +501,14 @@ static string showBytes(unsigned long long bytes, unsigned long long blocks)
 
 struct PrintFreed 
 {
-    bool show, dryRun;
+    bool show;
     const GCResults & results;
-    PrintFreed(bool show, bool dryRun, const GCResults & results)
+    PrintFreed(bool show, const GCResults & results)
-        : show(show), dryRun(dryRun), results(results) { }
+        : show(show), results(results) { }
     ~PrintFreed() 
     {
         if (show)
-            cout << format(
+            cout << format("%1% freed\n")
-                (dryRun
-                    ? "%1% would be freed\n"
-                    : "%1% freed\n"))
                 % showBytes(results.bytesFreed, results.blocksFreed);
     }
 };
@@ -525,19 +522,17 @@ static void opGC(Strings opFlags, Strings opArgs)
     GCResults results;
     
     /* Do what? */
-    for (Strings::iterator i = opFlags.begin();
+    foreach (Strings::iterator, i, opFlags)
-         i != opFlags.end(); ++i)
         if (*i == "--print-roots") options.action = GCOptions::gcReturnRoots;
         else if (*i == "--print-live") options.action = GCOptions::gcReturnLive;
         else if (*i == "--print-dead") options.action = GCOptions::gcReturnDead;
         else if (*i == "--delete") options.action = GCOptions::gcDeleteDead;
         else if (*i == "--max-freed") options.maxFreed = getIntArg(*i, i, opFlags.end());
         else if (*i == "--max-links") options.maxLinks = getIntArg(*i, i, opFlags.end());
+        else if (*i == "--use-atime") options.useAtime = true;
         else throw UsageError(format("bad sub-operation `%1%' in GC") % *i);
 
-    PrintFreed freed(
+    PrintFreed freed(options.action == GCOptions::gcDeleteDead, results);
-        options.action == GCOptions::gcDeleteDead || options.action == GCOptions::gcReturnDead,
-        options.action == GCOptions::gcReturnDead, results);
     store->collectGarbage(options, results);
 
     if (options.action != GCOptions::gcDeleteDead)
@@ -554,17 +549,15 @@ static void opDelete(Strings opFlags, Strings opArgs)
     GCOptions options;
     options.action = GCOptions::gcDeleteSpecific;
     
-    for (Strings::iterator i = opFlags.begin();
+    foreach (Strings::iterator, i, opFlags)
-         i != opFlags.end(); ++i)
         if (*i == "--ignore-liveness") options.ignoreLiveness = true;
         else throw UsageError(format("unknown flag `%1%'") % *i);
 
-    for (Strings::iterator i = opArgs.begin();
+    foreach (Strings::iterator, i, opArgs)
-         i != opArgs.end(); ++i)
         options.pathsToDelete.insert(followLinksToStorePath(*i));
     
     GCResults results;
-    PrintFreed freed(true, false, results);
+    PrintFreed freed(true, results);
     store->collectGarbage(options, results);
 }