* When obtaining derivations from Nix expressions, ignore all

expressions that cause an assertion failure (like `assert system ==
  "i686-linux"').  This allows all-packages.nix in Nixpkgs to be used
  on all platforms, even if some Nix expressions don't work on all
  platforms.

  Not sure if this is a good idea; it's a bit hacky.  In particular,
  due to laziness some derivations might appear in `nix-env -qa' but
  disappear in `nix-env -qas' or `nix-env -i'.

  Commit 5000!

src/libexpr/eval.cc

@@ -383,7 +383,13 @@ Expr evalExpr(EvalState & state, Expr e)
 
     /* Otherwise, evaluate and memoize. */
     state.normalForms.set(e, state.blackHole);
-    nf = evalExpr2(state, e);
+    try {
+        nf = evalExpr2(state, e);
+    } catch (Error & err) {
+        debug("removing black hole");
+        state.normalForms.remove(e);
+        throw;
+    }
     state.normalForms.set(e, nf);
     return nf;
 }

src/libexpr/get-drvs.cc

@@ -5,40 +5,51 @@
 typedef set<Expr> Exprs;
 
 
+/* Evaluate expression `e'.  If it evaluates to an attribute set of
+   type `derivation', then put information about it in `drvs' (unless
+   it's already in `doneExprs').  The result boolean indicates whether
+   it makes sense for the caller to recursively search for derivations
+   in `e'. */
 static bool getDerivation(EvalState & state, Expr e,
     DrvInfos & drvs, Exprs & doneExprs)
 {
-    ATermList es;
+    try {
-    e = evalExpr(state, e);
-    if (!matchAttrs(e, es)) return false;
         
-    ATermMap attrs;
+        ATermList es;
-    queryAllAttrs(e, attrs, false);
+        e = evalExpr(state, e);
+        if (!matchAttrs(e, es)) return true;
 
-    Expr a = attrs.get("type");
+        ATermMap attrs;
-    if (!a || evalString(state, a) != "derivation") return false;
+        queryAllAttrs(e, attrs, false);
     
-    /* Remove spurious duplicates (e.g., an attribute set like `rec {
+        Expr a = attrs.get("type");
-       x = derivation {...}; y = x;}'. */
+        if (!a || evalString(state, a) != "derivation") return true;
-    if (doneExprs.find(e) != doneExprs.end()) return true;
-    doneExprs.insert(e);
 
-    DrvInfo drv;
+        /* Remove spurious duplicates (e.g., an attribute set like
+           `rec { x = derivation {...}; y = x;}'. */
+        if (doneExprs.find(e) != doneExprs.end()) return false;
+        doneExprs.insert(e);
 
-    a = attrs.get("name");
+        DrvInfo drv;
-    if (!a) throw badTerm("derivation name missing", e);
-    drv.name = evalString(state, a);
     
-    a = attrs.get("system");
+        a = attrs.get("name");
-    if (!a)
+        if (!a) throw badTerm("derivation name missing", e);
-        drv.system = "unknown";
+        drv.name = evalString(state, a);
-    else
-        drv.system = evalString(state, a);
 
-    drv.attrs = attrs;
+        a = attrs.get("system");
+        if (!a)
+            drv.system = "unknown";
+        else
+            drv.system = evalString(state, a);
 
-    drvs.push_back(drv);
+        drv.attrs = attrs;
-    return true;
+
+        drvs.push_back(drv);
+        return false;
+    
+    } catch (AssertionError & e) {
+        return false;
+    }
 }
 
 
@@ -46,9 +57,10 @@ bool getDerivation(EvalState & state, Expr e, DrvInfo & drv)
 {
     Exprs doneExprs;
     DrvInfos drvs;
-    bool result = getDerivation(state, e, drvs, doneExprs);
+    getDerivation(state, e, drvs, doneExprs);
-    if (result) drv = drvs.front();
+    if (drvs.size() != 1) return false;
-    return result;
+    drv = drvs.front();
+    return true;
 }
 
 
@@ -101,26 +113,28 @@ static void getDerivations(EvalState & state, Expr e,
     ATermList es;
     DrvInfo drv;
 
-    e = evalExpr(state, e);
+    if (!getDerivation(state, e, drvs, doneExprs)) {
-
-    if (getDerivation(state, e, drvs, doneExprs)) {
         if (apType != apNone) throw attrError;
         return;
     }
 
+    e = evalExpr(state, e);
+
     if (matchAttrs(e, es)) {
         if (apType != apNone && apType != apAttr) throw attrError;
         ATermMap drvMap;
         queryAllAttrs(e, drvMap);
         if (apType == apNone) {
             for (ATermIterator i(drvMap.keys()); i; ++i) {
-                debug(format("evaluating attribute `%1%'") % aterm2String(*i));
+                startNest(nest, lvlDebug,
+                    format("evaluating attribute `%1%'") % aterm2String(*i));
                 getDerivation(state, drvMap.get(*i), drvs, doneExprs);
             }
         } else {
             Expr e2 = drvMap.get(attr);
             if (!e2) throw Error(format("attribute `%1%' in selection path not found") % attr);
-            debug(format("evaluating attribute `%1%'") % attr);
+            startNest(nest, lvlDebug,
+                format("evaluating attribute `%1%'") % attr);
             getDerivation(state, e2, drvs, doneExprs);
             if (!attrPath.empty())
                 getDerivations(state, e2, drvs, doneExprs, attrPathRest);
@@ -132,14 +146,16 @@ static void getDerivations(EvalState & state, Expr e,
         if (apType != apNone && apType != apIndex) throw attrError;
         if (apType == apNone) {
             for (ATermIterator i(es); i; ++i) {
-                debug(format("evaluating list element"));
+                startNest(nest, lvlDebug,
+                    format("evaluating list element"));
                 if (!getDerivation(state, *i, drvs, doneExprs))
                     getDerivations(state, *i, drvs, doneExprs, attrPathRest);
             }
         } else {
             Expr e2 = ATelementAt(es, attrIndex);
             if (!e2) throw Error(format("list index %1% in selection path not found") % attrIndex);
-            debug(format("evaluating list element"));
+            startNest(nest, lvlDebug,
+                format("evaluating list element"));
             if (!getDerivation(state, e2, drvs, doneExprs))
                 getDerivations(state, e2, drvs, doneExprs, attrPathRest);
         }

src/nix-env/main.cc

@@ -762,47 +762,53 @@ static void opQuery(Globals & globals,
     for (vector<DrvInfo>::iterator i = elems2.begin();
          i != elems2.end(); ++i)
     {
-        Strings columns;
+        try {
             
-        if (printStatus) {
+            Strings columns;
-            Substitutes subs = querySubstitutes(noTxn, i->queryOutPath(globals.state));
-            columns.push_back(
-                (string) (installed.find(i->queryOutPath(globals.state))
-                    != installed.end() ? "I" : "-")
-                + (isValidPath(i->queryOutPath(globals.state)) ? "P" : "-")
-                + (subs.size() > 0 ? "S" : "-"));
-        }
         
-        if (printName) columns.push_back(i->name);
+            if (printStatus) {
-
+                Substitutes subs = querySubstitutes(noTxn, i->queryOutPath(globals.state));
-        if (compareVersions) {
+                columns.push_back(
-            /* Compare this element against the versions of the same
+                    (string) (installed.find(i->queryOutPath(globals.state))
-               named packages in either the set of available elements,
+                        != installed.end() ? "I" : "-")
-               or the set of installed elements.  !!! This is O(N *
+                    + (isValidPath(i->queryOutPath(globals.state)) ? "P" : "-")
-               M), should be O(N * lg M). */
+                    + (subs.size() > 0 ? "S" : "-"));
-            string version;
-            VersionDiff diff = compareVersionAgainstSet(*i, otherElems, version);
-            char ch;
-            switch (diff) {
-                case cvLess: ch = '>'; break;
-                case cvEqual: ch = '='; break;
-                case cvGreater: ch = '<'; break;
-                case cvUnavail: ch = '-'; break;
-                default: abort();
             }
-            string column = (string) "" + ch + " " + version;
+
-            if (diff == cvGreater) column = colorString(column);
+            if (printName) columns.push_back(i->name);
-            columns.push_back(column);
+
+            if (compareVersions) {
+                /* Compare this element against the versions of the
+                   same named packages in either the set of available
+                   elements, or the set of installed elements.  !!!
+                   This is O(N * M), should be O(N * lg M). */
+                string version;
+                VersionDiff diff = compareVersionAgainstSet(*i, otherElems, version);
+                char ch;
+                switch (diff) {
+                    case cvLess: ch = '>'; break;
+                    case cvEqual: ch = '='; break;
+                    case cvGreater: ch = '<'; break;
+                    case cvUnavail: ch = '-'; break;
+                    default: abort();
+                }
+                string column = (string) "" + ch + " " + version;
+                if (diff == cvGreater) column = colorString(column);
+                columns.push_back(column);
+            }
+
+            if (printSystem) columns.push_back(i->system);
+
+            if (printDrvPath) columns.push_back(
+                i->queryDrvPath(globals.state) == ""
+                ? "-" : i->queryDrvPath(globals.state));
+        
+            if (printOutPath) columns.push_back(i->queryOutPath(globals.state));
+            
+            table.push_back(columns);
+        }
+        catch (AssertionError & e) {
         }
-
-        if (printSystem) columns.push_back(i->system);
-
-        if (printDrvPath) columns.push_back(
-            i->queryDrvPath(globals.state) == ""
-            ? "-" : i->queryDrvPath(globals.state));
-        
-        if (printOutPath) columns.push_back(i->queryOutPath(globals.state));
-        table.push_back(columns);
     }
 
     printTable(table);