* Simplify the substitute mechanism:

- Drop the store expression.  So now a substitute is just a
    command-line invocation (a program name + arguments).  If you
    register a substitute you are responsible for registering the
    expression that built it (if any) as a root of the garbage
    collector.
  - Drop the substitutes-rev DB table.

src/libstore/normalise.cc

@@ -1416,9 +1416,6 @@ private:
     /* The current substitute. */
     Substitute sub;
 
-    /* The normal form of the substitute store expression. */
-    Path nfSub;
-
     /* Pipe for the substitute's standard output/error. */
     Pipe logPipe;
 
@@ -1440,8 +1437,6 @@ public:
     /* The states. */
     void init();
     void tryNext();
-    void exprNormalised();
-    void exprRealised();
     void tryToRun();
     void finished();
 
@@ -1506,26 +1501,7 @@ void SubstitutionGoal::tryNext()
     sub = subs.front();
     subs.pop_front();
 
-    /* Realise the substitute store expression. */
-    nrFailed = 0;
-    addWaitee(worker.makeRealisationGoal(sub.storeExpr));
-
-    state = &SubstitutionGoal::exprRealised;
-}
-
-
-void SubstitutionGoal::exprRealised()
-{
-    trace("substitute store expression realised");
-
-    if (nrFailed != 0) {
-        tryNext();
-        return;
-    }
-
-    /* !!! the storeExpr doesn't have to be a derivation, right? */
-    nfSub = queryNormalForm(sub.storeExpr);
-    
+    /* Wait until we can run the substitute program. */
     state = &SubstitutionGoal::tryToRun;
     worker.waitForBuildSlot(shared_from_this());
 }
@@ -1557,15 +1533,8 @@ void SubstitutionGoal::tryToRun()
 
     printMsg(lvlInfo,
         format("substituting path `%1%' using substituter `%2%'")
-        % storePath % sub.storeExpr);
+        % storePath % sub.program);
     
-    /* What's the substitute program? */
-    StoreExpr expr = storeExprFromPath(nfSub);
-    assert(expr.type == StoreExpr::neClosure);
-    assert(!expr.closure.roots.empty());
-    Path program =
-        canonPath(*expr.closure.roots.begin() + "/" + sub.program);
-
     logPipe.create();
 
     /* Remove the (stale) output path if it exists. */
@@ -1591,12 +1560,12 @@ void SubstitutionGoal::tryToRun()
             /* Fill in the arguments. */
             Strings args(sub.args);
             args.push_front(storePath);
-            args.push_front(baseNameOf(program));
+            args.push_front(baseNameOf(sub.program));
             const char * * argArr = strings2CharPtrs(args);
 
-            execv(program.c_str(), (char * *) argArr);
+            execv(sub.program.c_str(), (char * *) argArr);
             
-            throw SysError(format("executing `%1%'") % program);
+            throw SysError(format("executing `%1%'") % sub.program);
             
         } catch (exception & e) {
             cerr << format("substitute error: %1%\n") % e.what();
@@ -1648,7 +1617,7 @@ void SubstitutionGoal::finished()
 
         printMsg(lvlInfo,
             format("substitution of path `%1%' using substituter `%2%' failed: %3%")
-            % storePath % sub.storeExpr % e.msg());
+            % storePath % sub.program % e.msg());
         
         /* Try the next substitute. */
         state = &SubstitutionGoal::tryNext;

src/libstore/store.cc

@@ -41,12 +41,13 @@ static TableId dbSuccessors = 0;
 */
 static TableId dbSuccessorsRev = 0;
 
-/* dbSubstitutes :: Path -> [(Path, Path, [string])]
+/* dbSubstitutes :: Path -> [[Path]]
 
    Each pair $(p, subs)$ tells Nix that it can use any of the
-   substitutes in $subs$ to build path $p$.  Each substitute is a
-   tuple $(storeExpr, program, args)$ (see the type `Substitute' in
-   `store.hh').
+   substitutes in $subs$ to build path $p$.  Each substitute defines a
+   command-line invocation of a program (i.e., the first list element
+   is the full path to the program, the remaining elements are
+   arguments).
 
    The main purpose of this is for distributed caching of derivates.
    One system can compute a derivate and put it on a website (as a Nix
@@ -56,18 +57,10 @@ static TableId dbSuccessorsRev = 0;
 */
 static TableId dbSubstitutes = 0;
 
-/* dbSubstitutesRev :: Path -> [Path]
-
-   The reverse mapping of dbSubstitutes; it maps store expressions
-   back to the paths for which they are substitutes.
-*/
-static TableId dbSubstitutesRev = 0;
-
 
 bool Substitute::operator == (const Substitute & sub)
 {
-    return storeExpr == sub.storeExpr
-        && program == sub.program
+    return program == sub.program
         && args == sub.args;
 }
 
@@ -86,7 +79,6 @@ void openDB()
     dbSuccessors = nixDB.openTable("successors");
     dbSuccessorsRev = nixDB.openTable("successors-rev");
     dbSubstitutes = nixDB.openTable("substitutes");
-    dbSubstitutesRev = nixDB.openTable("substitutes-rev");
 }
 
 
@@ -300,10 +292,13 @@ static Substitutes readSubstitutes(const Transaction & txn,
             break;
         }
         Strings ss2 = unpackStrings(*i);
-        if (ss2.size() != 3) throw Error("malformed substitute");
+        if (ss2.size() == 3) {
+            /* Another old-style substitute. */
+            continue;
+        }
+        if (ss2.size() != 2) throw Error("malformed substitute");
         Strings::iterator j = ss2.begin();
         Substitute sub;
-        sub.storeExpr = *j++;
         sub.program = *j++;
         sub.args = unpackStrings(*j++);
         subs.push_back(sub);
@@ -322,7 +317,6 @@ static void writeSubstitutes(const Transaction & txn,
          i != subs.end(); ++i)
     {
         Strings ss2;
-        ss2.push_back(i->storeExpr);
         ss2.push_back(i->program);
         ss2.push_back(packStrings(i->args));
         ss.push_back(packStrings(ss2));
@@ -332,14 +326,9 @@ static void writeSubstitutes(const Transaction & txn,
 }
 
 
-typedef map<Path, Paths> SubstitutesRev;
-
-
 void registerSubstitutes(const Transaction & txn,
     const SubstitutePairs & subPairs)
 {
-    SubstitutesRev revMap;
-    
     for (SubstitutePairs::const_iterator i = subPairs.begin();
          i != subPairs.end(); ++i)
     {
@@ -347,7 +336,6 @@ void registerSubstitutes(const Transaction & txn,
         const Substitute & sub(i->second);
 
         assertStorePath(srcPath);
-        assertStorePath(sub.storeExpr);
     
         Substitutes subs = readSubstitutes(txn, srcPath);
 
@@ -357,19 +345,7 @@ void registerSubstitutes(const Transaction & txn,
         subs.push_front(sub);
         
         writeSubstitutes(txn, srcPath, subs);
-
-        Paths & revs = revMap[sub.storeExpr];
-        if (revs.empty())
-            nixDB.queryStrings(txn, dbSubstitutesRev, sub.storeExpr, revs);
-        if (find(revs.begin(), revs.end(), srcPath) == revs.end())
-            revs.push_back(srcPath);
     }
-
-    /* Re-write the reverse mapping in one go to prevent Theta(n^2)
-       performance.  (This would occur because the data fields of the
-       `substitutes-rev' table are lists). */
-    for (SubstitutesRev::iterator i = revMap.begin(); i != revMap.end(); ++i)
-        nixDB.setStrings(txn, dbSubstitutesRev, i->first, i->second);
 }
 
 
@@ -379,6 +355,12 @@ Substitutes querySubstitutes(const Path & srcPath)
 }
 
 
+void clearSubstitutes()
+{
+    
+}
+
+
 void registerValidPath(const Transaction & txn, const Path & _path)
 {
     Path path(canonPath(_path));
@@ -401,28 +383,6 @@ static void invalidatePath(const Path & path, Transaction & txn)
     for (Paths::iterator i = revs.begin(); i != revs.end(); ++i)
         nixDB.delPair(txn, dbSuccessors, *i);
     nixDB.delPair(txn, dbSuccessorsRev, path);
-
-    /* Remove any substitute mappings to this path. */
-    revs.clear();
-    nixDB.queryStrings(txn, dbSubstitutesRev, path, revs);
-    for (Paths::iterator i = revs.begin(); i != revs.end(); ++i) {
-        Substitutes subs = readSubstitutes(txn, *i), subs2;
-        bool found = false;
-        for (Substitutes::iterator j = subs.begin(); j != subs.end(); ++j)
-            if (j->storeExpr != path)
-                subs2.push_back(*j);
-            else
-                found = true;
-        // !!!        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,
-	   then it too should be invalidated.  This is because it may
-	   be a substitute or successor. */
-	if (subs.size() == 0 && !isValidPathTxn(*i, txn))
-	    invalidatePath(*i, txn);
-    }
-    nixDB.delPair(txn, dbSubstitutesRev, path);
 }
 
 
@@ -548,32 +508,11 @@ void verifyStore()
     Paths subKeys;
     nixDB.enumTable(txn, dbSubstitutes, subKeys);
     for (Paths::iterator i = subKeys.begin(); i != subKeys.end(); ++i) {
-        Substitutes subs = readSubstitutes(txn, *i), subs2;
-        for (Substitutes::iterator j = subs.begin(); j != subs.end(); ++j)
-            if (validPaths.find(j->storeExpr) == validPaths.end())
-                printMsg(lvlError,
-                    format("found substitute mapping to non-existent path `%1%'")
-                    % j->storeExpr);
-            else
-                subs2.push_back(*j);
-        if (subs.size() != subs2.size())
-            writeSubstitutes(txn, *i, subs2);
-	if (subs2.size() > 0)
+        Substitutes subs = readSubstitutes(txn, *i);
+	if (subs.size() > 0)
 	    usablePaths.insert(*i);
     }
 
-    /* Check that the keys of the reverse substitute mappings are
-       valid paths. */ 
-    Paths rsubKeys;
-    nixDB.enumTable(txn, dbSubstitutesRev, rsubKeys);
-    for (Paths::iterator i = rsubKeys.begin(); i != rsubKeys.end(); ++i) {
-        if (validPaths.find(*i) == validPaths.end()) {
-            printMsg(lvlError,
-                format("found reverse substitute mapping for non-existent path `%1%'") % *i);
-            nixDB.delPair(txn, dbSubstitutesRev, *i);
-        }
-    }
-
     /* Check that the values of the successor mappings are usable
        paths. */ 
     Paths sucKeys;

src/libstore/store.hh

@@ -14,10 +14,6 @@ using namespace std;
    network). */
 struct Substitute
 {
-    /* Store expression to be normalised and realised in order to
-       obtain `program'. */
-    Path storeExpr;
-
     /* Program to be executed to create the store path.  Must be in
        the output path of `storeExpr'. */
     Path program;
@@ -73,6 +69,9 @@ void registerSubstitutes(const Transaction & txn,
 /* Return the substitutes expression for the given path. */
 Substitutes querySubstitutes(const Path & srcPath);
 
+/* Deregister all substitutes. */
+void clearSubstitutes();
+
 /* Register the validity of a path. */
 void registerValidPath(const Transaction & txn, const Path & path);
 

src/nix-store/help.txt

@@ -11,6 +11,7 @@ Operations:
 
   --successor: register a successor expression (dangerous!)
   --substitute: register a substitute expression (dangerous!)
+  --clear-substitute: clear all substitutes
   --validpath: register path validity (dangerous!)
   --isvalid: check path validity
 

src/nix-store/main.cc

@@ -166,7 +166,6 @@ static void opSubstitute(Strings opFlags, Strings opArgs)
         Substitute sub;
         getline(cin, srcPath);
         if (cin.eof()) break;
-        getline(cin, sub.storeExpr);
         getline(cin, sub.program);
         string s;
         getline(cin, s);
@@ -186,6 +185,17 @@ static void opSubstitute(Strings opFlags, Strings opArgs)
 }
 
 
+static void opClearSubstitutes(Strings opFlags, Strings opArgs)
+{
+    if (!opFlags.empty()) throw UsageError("unknown flag");
+    if (!opArgs.empty())
+        throw UsageError("no arguments expected");
+
+    clearSubstitutes();
+}
+
+
+
 static void opValidPath(Strings opFlags, Strings opArgs)
 {
     if (!opFlags.empty()) throw UsageError("unknown flag");
@@ -354,6 +364,8 @@ void run(Strings args)
             op = opSuccessor;
         else if (arg == "--substitute")
             op = opSubstitute;
+        else if (arg == "--clear-substitutes")
+            op = opClearSubstitutes;
         else if (arg == "--validpath")
             op = opValidPath;
         else if (arg == "--isvalid")