* And yet another installation source: the ability to copy user

environment elements from one user environment to another, e.g.,

    $ nix-env -i --from-profile /nix/var/nix/profiles/other-profile aterm
    
  copies the `aterm' component installed in the `other-profile' to the
  user's current profile.

src/nix-env/main.cc

@@ -169,12 +169,12 @@ struct AddPos : TermFun
 };
 
 
-static void queryInstalled(EvalState & state, UserEnvElems & elems,
+static UserEnvElems queryInstalled(EvalState & state, const Path & userEnv)
-    const Path & userEnv)
 {
     Path path = userEnv + "/manifest";
 
-    if (!pathExists(path)) return; /* not an error, assume nothing installed */
+    if (!pathExists(path))
+        return UserEnvElems(); /* not an error, assume nothing installed */
 
     Expr e = ATreadFromNamedFile(path.c_str());
     if (!e) throw Error(format("cannot read Nix expression from `%1%'") % path);
@@ -183,7 +183,9 @@ static void queryInstalled(EvalState & state, UserEnvElems & elems,
     AddPos addPos;
     e = bottomupRewrite(addPos, e);
 
+    UserEnvElems elems;
     parseDerivations(state, e, elems);
+    return elems;
 }
 
 
@@ -273,32 +275,15 @@ static void createUserEnv(EvalState & state, const UserEnvElems & elems,
 }
 
 
-static void queryInstSources(EvalState & state,
+static UserEnvElems filterBySelector(const UserEnvElems & allElems,
-    const InstallSourceInfo & instSource, const Strings & args,
+    const Strings & args)
-    UserEnvElems & elems)
 {
-    InstallSourceType type = instSource.type;
-    if (type == srcUnknown && args.size() > 0 && args.front()[0] == '/')
-        type = srcStorePaths;
-    
-    switch (type) {
-
-        /* Get the available user environment elements from the
-           derivations specified in a Nix expression, including only
-           those with names matching any of the names in `args'. */
-        case srcUnknown:
-        case srcNixExprDrvs: {
-
     DrvNames selectors = drvNamesFromArgs(args);
 
-            /* Load the derivations from the (default or specified)
+    UserEnvElems elems;
-               Nix expression. */
-            UserEnvElems allElems;
-            loadDerivations(state, instSource.nixExprPath,
-                instSource.systemFilter, allElems);
 
     /* Filter out the ones we're not interested in. */
-            for (UserEnvElems::iterator i = allElems.begin();
+    for (UserEnvElems::const_iterator i = allElems.begin();
          i != allElems.end(); ++i)
     {
         DrvName drvName(i->second.name);
@@ -319,6 +304,34 @@ static void queryInstSources(EvalState & state,
             throw Error(format("selector `%1%' matches no derivations")
                 % i->fullName);
 
+    return elems;
+}
+
+
+static void queryInstSources(EvalState & state,
+    const InstallSourceInfo & instSource, const Strings & args,
+    UserEnvElems & elems)
+{
+    InstallSourceType type = instSource.type;
+    if (type == srcUnknown && args.size() > 0 && args.front()[0] == '/')
+        type = srcStorePaths;
+    
+    switch (type) {
+
+        /* Get the available user environment elements from the
+           derivations specified in a Nix expression, including only
+           those with names matching any of the names in `args'. */
+        case srcUnknown:
+        case srcNixExprDrvs: {
+
+            /* Load the derivations from the (default or specified)
+               Nix expression. */
+            UserEnvElems allElems;
+            loadDerivations(state, instSource.nixExprPath,
+                instSource.systemFilter, allElems);
+
+            elems = filterBySelector(allElems, args);
+    
             break;
         }
 
@@ -373,6 +386,8 @@ static void queryInstSources(EvalState & state,
             break;
 
         case srcProfile:
+            elems = filterBySelector(
+                queryInstalled(state, instSource.profile), args);
             break;
     }
 }
@@ -396,8 +411,7 @@ static void installDerivations(Globals & globals,
 
     /* Add in the already installed derivations, unless they have the
        same name as a to-be-installed element. */
-    UserEnvElems installedElems;
+    UserEnvElems installedElems = queryInstalled(globals.state, profile);
-    queryInstalled(globals.state, installedElems, profile);
 
     for (UserEnvElems::iterator i = installedElems.begin();
          i != installedElems.end(); ++i)
@@ -443,8 +457,7 @@ static void upgradeDerivations(Globals & globals,
        name and a higher version number. */
 
     /* Load the currently installed derivations. */
-    UserEnvElems installedElems;
+    UserEnvElems installedElems = queryInstalled(globals.state, profile);
-    queryInstalled(globals.state, installedElems, profile);
 
     /* Fetch all derivations from the input file. */
     UserEnvElems availElems;
@@ -519,8 +532,7 @@ static void opUpgrade(Globals & globals,
 static void uninstallDerivations(Globals & globals, DrvNames & selectors,
     Path & profile)
 {
-    UserEnvElems installedElems;
+    UserEnvElems installedElems = queryInstalled(globals.state, profile);
-    queryInstalled(globals.state, installedElems, profile);
 
     for (UserEnvElems::iterator i = installedElems.begin();
          i != installedElems.end(); ++i)
@@ -631,7 +643,7 @@ static void opQuery(Globals & globals,
     switch (source) {
 
         case sInstalled:
-            queryInstalled(globals.state, elems, globals.profile);
+            elems = queryInstalled(globals.state, globals.profile);
             break;
 
         case sAvailable: {
@@ -656,7 +668,7 @@ static void opQuery(Globals & globals,
     UserEnvElems installed; /* installed paths */
     
     if (printStatus)
-        queryInstalled(globals.state, installed, globals.profile);
+        installed = queryInstalled(globals.state, globals.profile);
             
     /* Print the desired columns. */
     Table table;