* Put the garbage collector in nix-store: operation `--gc',

suboperations `--print-live', `--print-dead', and `--delete'.  The
  roots are not determined by nix-store; they are read from standard
  input.  This is to make it easy to customise what the roots are.

  The collector now no longer fails when store expressions are missing
  (which legally happens when using substitutes).  It never tries to
  fetch paths through substitutes.

  TODO: acquire a global lock on the store while garbage collecting.
  
* Removed `nix-store --delete'.

src/libstore/Makefile.am

@@ -4,7 +4,8 @@ libstore_a_SOURCES = \
  store.cc store.hh storeexpr.cc storeexpr.hh \
  normalise.cc misc.cc normalise.hh \
  globals.cc globals.hh db.cc db.hh \
- references.cc references.hh pathlocks.cc pathlocks.hh
+ references.cc references.hh pathlocks.cc pathlocks.hh \
+ gc.cc gc.hh
 
 AM_CXXFLAGS = -Wall \
  -I.. ${bdb_include} ${aterm_include} -I../libutil

src/libstore/gc.cc

@@ -0,0 +1,83 @@
+#include "normalise.hh"
+#include "globals.hh"
+
+
+void followLivePaths(Path nePath, PathSet & live)
+{
+    /* Just to be sure, canonicalise the path.  It is important to do
+       this here and in findDeadPath() to ensure that a live path is
+       not mistaken for a dead path due to some non-canonical
+       representation. */
+    nePath = canonPath(nePath);
+    
+    if (live.find(nePath) != live.end()) return;
+    live.insert(nePath);
+
+    startNest(nest, lvlDebug, format("following `%1%'") % nePath);
+    assertStorePath(nePath);
+
+    if (isValidPath(nePath)) {
+
+        /* !!! should make sure that no substitutes are used */
+        StoreExpr ne = storeExprFromPath(nePath);
+
+        /* !!! painfully similar to requisitesWorker() */
+        if (ne.type == StoreExpr::neClosure)
+            for (ClosureElems::iterator i = ne.closure.elems.begin();
+                 i != ne.closure.elems.end(); ++i)
+            {
+                Path p = canonPath(i->first);
+                if (live.find(p) == live.end()) {
+                    debug(format("found live `%1%'") % p);
+                    assertStorePath(p);
+                    live.insert(p);
+                }
+            }
+    
+        else if (ne.type == StoreExpr::neDerivation)
+            for (PathSet::iterator i = ne.derivation.inputs.begin();
+                 i != ne.derivation.inputs.end(); ++i)
+                followLivePaths(*i, live);
+
+        else abort();
+        
+    }
+
+    Path nfPath;
+    if (querySuccessor(nePath, nfPath))
+        followLivePaths(nfPath, live);
+}
+
+
+PathSet findLivePaths(const Paths & roots)
+{
+    PathSet live;
+
+    startNest(nest, lvlDebug, "finding live paths");
+
+    for (Paths::const_iterator i = roots.begin(); i != roots.end(); ++i)
+        followLivePaths(*i, live);
+
+    return live;
+}
+
+
+PathSet findDeadPaths(const PathSet & live)
+{
+    PathSet dead;
+
+    startNest(nest, lvlDebug, "finding dead paths");
+
+    Strings storeNames = readDirectory(nixStore);
+
+    for (Strings::iterator i = storeNames.begin(); i != storeNames.end(); ++i) {
+        Path p = canonPath(nixStore + "/" + *i);
+        if (live.find(p) == live.end()) {
+            debug(format("dead path `%1%'") % p);
+            dead.insert(p);
+        } else
+            debug(format("live path `%1%'") % p);
+    }
+    
+    return dead;
+}

src/libstore/gc.hh

@@ -0,0 +1,24 @@
+#ifndef __GC_H
+#define __GC_H
+
+#include "storeexpr.hh"
+
+
+/* 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). */
+PathSet findDeadPaths(const PathSet & live);
+
+
+#endif /* !__GC_H */

src/libstore/normalise.hh

@@ -40,9 +40,5 @@ PathSet storeExprRoots(const Path & nePath);
 PathSet storeExprRequisites(const Path & nePath,
     bool includeExprs, bool includeSuccessors);
 
-/* Return the list of the paths of all known store expressions whose
-   output paths are completely contained in the set `outputs'. */
-PathSet findGenerators(const PathSet & outputs);
-
 
 #endif /* !__NORMALISE_H */

src/libstore/store.cc

@@ -388,7 +388,7 @@ static void invalidatePath(const Path & path, Transaction & txn)
                 subs2.push_back(*j);
             else
                 found = true;
-        if (!found) throw Error("integrity error in substitutes mapping");
+        // !!!        if (!found) throw Error("integrity error in substitutes mapping");
         writeSubstitutes(txn, *i, subs);
 
 	/* If path *i now has no substitutes left, and is not valid,

src/nix-store/main.cc

@@ -3,6 +3,7 @@
 
 #include "globals.hh"
 #include "normalise.hh"
+#include "gc.hh"
 #include "archive.hh"
 #include "shared.hh"
 #include "dotgraph.hh"
@@ -32,17 +33,6 @@ static void opRealise(Strings opFlags, Strings opArgs)
 }
 
 
-/* Delete a path in the Nix store directory. */
-static void opDelete(Strings opFlags, Strings opArgs)
-{
-    if (!opFlags.empty()) throw UsageError("unknown flag");
-
-    for (Strings::iterator it = opArgs.begin();
-         it != opArgs.end(); it++)
-        deleteFromStore(*it);
-}
-
-
 /* Add paths to the Nix values directory and print the hashes of those
    paths. */
 static void opAdd(Strings opFlags, Strings opArgs)
@@ -220,6 +210,60 @@ static void opIsValid(Strings opFlags, Strings opArgs)
 }
 
 
+static void opGC(Strings opFlags, Strings opArgs)
+{
+    if (opFlags.size() != 1) throw UsageError("missing flag");
+    if (!opArgs.empty())
+        throw UsageError("no arguments expected");
+
+    /* Do what? */
+    string flag = opFlags.front();
+    enum { soPrintLive, soPrintDead, soDelete } subOp;
+    if (flag == "--print-live") subOp = soPrintLive;
+    else if (flag == "--print-dead") subOp = soPrintDead;
+    else if (flag == "--delete") subOp = soDelete;
+    else throw UsageError(format("bad sub-operation `%1% in GC") % flag);
+        
+    Paths roots;
+    while (1) {
+        Path root;
+        getline(cin, root);
+        if (cin.eof()) break;
+        roots.push_back(root);
+    }
+
+    PathSet live = findLivePaths(roots);
+
+    if (subOp == soPrintLive) {
+        for (PathSet::iterator i = live.begin(); i != live.end(); ++i)
+            cout << *i << endl;
+        return;
+    }
+
+    PathSet dead = findDeadPaths(live);
+
+    if (subOp == soPrintDead) {
+        for (PathSet::iterator i = dead.begin(); i != dead.end(); ++i)
+            cout << *i << endl;
+        return;
+    }
+
+    if (subOp == soDelete) {
+
+        /* !!! What happens if the garbage collector run is aborted
+           halfway through?  In particular, dead paths can always
+           become live again (through re-instantiation), and might
+           then refer to deleted paths. => check instantiation
+           invariants */
+
+        for (PathSet::iterator i = dead.begin(); i != dead.end(); ++i) {
+            printMsg(lvlInfo, format("deleting `%1%'") % *i);
+            deleteFromStore(*i);
+        }
+    }
+}
+
+
 /* A sink that writes dump output to stdout. */
 struct StdoutSink : DumpSink
 {
@@ -298,8 +342,6 @@ void run(Strings args)
 
         if (arg == "--realise" || arg == "-r")
             op = opRealise;
-        else if (arg == "--delete" || arg == "-d")
-            op = opDelete;
         else if (arg == "--add" || arg == "-A")
             op = opAdd;
         else if (arg == "--query" || arg == "-q")
@@ -312,6 +354,8 @@ void run(Strings args)
             op = opValidPath;
         else if (arg == "--isvalid")
             op = opIsValid;
+        else if (arg == "--gc")
+            op = opGC;
         else if (arg == "--dump")
             op = opDump;
         else if (arg == "--restore")