From b33da599c5c1b06a32a3eeac58f95481d10f821d Mon Sep 17 00:00:00 2001
From: Eelco Dolstra <e.dolstra@tudelft.nl>
Date: Thu, 22 Dec 2011 15:55:53 +0000
Subject: [PATCH] * In the garbage collector, delete invalid paths before
 deleting   unreachable paths.  This matters when using --max-freed etc.:  
 unreachable paths could become reachable again, so it's nicer to   keep them
 if there is "real" garbage to be deleted.  Also, don't use   readDirectory()
 but read the Nix store and delete invalid paths in   parallel.  This reduces
 GC latency on very large Nix stores.

---
 src/libstore/gc.cc  | 46 ++++++++++++++++++++++++++++++++++-----------
 src/libutil/util.cc | 10 +++++++++-
 src/libutil/util.hh |  1 +
 3 files changed, 45 insertions(+), 12 deletions(-)

diff --git a/src/libstore/gc.cc b/src/libstore/gc.cc
index a99bb1a81..20f194e6e 100644
--- a/src/libstore/gc.cc
+++ b/src/libstore/gc.cc
@@ -617,27 +617,51 @@ void LocalStore::collectGarbage(const GCOptions & options, GCResults & results)
         
     } else {
         
-        printMsg(lvlError, format("reading the Nix store..."));
-        Paths entries = readDirectory(nixStore);
-
-        /* Randomise the order in which we delete entries to make the
-           collector less biased towards deleting paths that come
-           alphabetically first (e.g. /nix/store/000...).  This
-           matters when using --max-freed etc. */
-        vector<Path> entries_(entries.begin(), entries.end());
-        random_shuffle(entries_.begin(), entries_.end());
-
         if (shouldDelete(state.options.action))
             printMsg(lvlError, format("deleting garbage..."));
         else
             printMsg(lvlError, format("determining live/dead paths..."));
     
         try {
+
+            AutoCloseDir dir = opendir(nixStore.c_str());
+            if (!dir) throw SysError(format("opening directory `%1%'") % nixStore);
+
+            /* Read the store and immediately delete all paths that
+               aren't valid.  When using --max-freed etc., deleting
+               invalid paths is preferred over deleting unreachable
+               paths, since unreachable paths could become reachable
+               again.  We don't use readDirectory() here so that GCing
+               can start faster. */
+            Paths entries;
+            struct dirent * dirent;
+            while (errno = 0, dirent = readdir(dir)) {
+                checkInterrupt();
+                string name = dirent->d_name;
+                if (name == "." || name == "..") continue;
+                Path path = nixStore + "/" + name;
+                if (isValidPath(path))
+                    entries.push_back(path);
+                else
+                    tryToDelete(state, path);
+            }
+
+	    dir.close();
+
+            /* Now delete the unreachable valid paths.  Randomise the
+               order in which we delete entries to make the collector
+               less biased towards deleting paths that come
+               alphabetically first (e.g. /nix/store/000...).  This
+               matters when using --max-freed etc. */
+            vector<Path> entries_(entries.begin(), entries.end());
+            random_shuffle(entries_.begin(), entries_.end());
+
             foreach (vector<Path>::iterator, i, entries_)
                 tryToDelete(state, nixStore + "/" + *i);
+
         } catch (GCLimitReached & e) {
         }
-    }        
+    }
 }
 
 
diff --git a/src/libutil/util.cc b/src/libutil/util.cc
index 9adaac40d..0352754f5 100644
--- a/src/libutil/util.cc
+++ b/src/libutil/util.cc
@@ -701,7 +701,7 @@ AutoCloseDir::AutoCloseDir(DIR * dir)
 
 AutoCloseDir::~AutoCloseDir()
 {
-    if (dir) closedir(dir);
+    close();
 }
 
 
@@ -717,6 +717,14 @@ AutoCloseDir::operator DIR *()
 }
 
 
+void AutoCloseDir::close()
+{
+    if (dir) {
+	closedir(dir);
+	dir = 0;
+    }
+}
+
 
 //////////////////////////////////////////////////////////////////////
 
diff --git a/src/libutil/util.hh b/src/libutil/util.hh
index f86290f31..a1cf68e69 100644
--- a/src/libutil/util.hh
+++ b/src/libutil/util.hh
@@ -223,6 +223,7 @@ public:
     ~AutoCloseDir();
     void operator =(DIR * dir);
     operator DIR *();
+    void close();
 };