* nix-env -i: instead of breaking package ties by version, break them

by priority and version install.  That is, if there are multiple
  packages with the same name, then pick the package with the highest
  priority, and only use the version if there are multiple packages
  with the same priority.

  This makes it possible to mark specific versions/variant in Nixpkgs
  more or less desirable than others.  A typical example would be a
  beta version of some package (e.g., "gcc-4.2.0rc1") which should not
  be installed even though it is the highest version, except when it
  is explicitly selected (e.g., "nix-env -i gcc-4.2.0rc1").

* Idem for nix-env -u, only the semantics are a bit trickier since we
  also need to take into account the priority of the currently
  installed package (we never upgrade to a lower priority, unless
  --always is given).

doc/manual/release-notes.xml

@@ -70,10 +70,16 @@
   info about installed packages in user environments.  <option>-q
   --xml --meta</option> to show all meta info.</para></listitem>
 
+  
   <listitem><para>TODO: <command>nix-env</command>
   <option>--set-flag</option>.  Specific flags:
   <literal>active</literal>, <literal>priority</literal>,
   <literal>keep</literal>.</para></listitem>
+
+  
+  <listitem><para>TODO: <command>nix-env</command> <option>-i</option>
+  / <option>-u</option> take package priorities into
+  account.</para></listitem>
   
 
   <listitem><para><command>nix-env -q</command> now has a flag

src/libexpr/get-drvs.cc

@@ -60,6 +60,15 @@ MetaInfo DrvInfo::queryMetaInfo(EvalState & state) const
 }
 
 
+string DrvInfo::queryMetaInfo(EvalState & state, const string & name) const
+{
+    /* !!! evaluates all meta attributes => inefficient */
+    MetaInfo meta = queryMetaInfo(state);
+    MetaInfo::iterator i = meta.find(name);
+    return i == meta.end() ? "" : i->second;
+}
+
+
 void DrvInfo::setMetaInfo(const MetaInfo & meta)
 {
     ATermMap metaAttrs;

src/libexpr/get-drvs.hh

@@ -34,6 +34,7 @@ public:
     string queryDrvPath(EvalState & state) const;
     string queryOutPath(EvalState & state) const;
     MetaInfo queryMetaInfo(EvalState & state) const;
+    string queryMetaInfo(EvalState & state, const string & name) const;
 
     void setDrvPath(const string & s)
     {

src/nix-env/nix-env.cc

@@ -232,6 +232,16 @@ static void createUserEnv(EvalState & state, const DrvInfos & elems,
 }
 
 
+static int comparePriorities(EvalState & state,
+    const DrvInfo & drv1, const DrvInfo & drv2)
+{
+    int prio1, prio2;
+    if (!string2Int(drv1.queryMetaInfo(state, "priority"), prio1)) prio1 = 0;
+    if (!string2Int(drv2.queryMetaInfo(state, "priority"), prio2)) prio2 = 0;
+    return prio2 - prio1; /* higher number = lower priority, so negate */
+}
+
+
 static DrvInfos filterBySelector(EvalState & state,
     const DrvInfos & allElems,
     const Strings & args, bool newestOnly)
@@ -258,8 +268,10 @@ static DrvInfos filterBySelector(EvalState & state,
         }
 
         /* If `newestOnly', if a selector matches multiple derivations
-           with the same name, pick the one with the highest version.
-           If there are multiple derivations with the same name *and*
+           with the same name, pick the one with the highest priority.
+           If there are multiple derivations with the same priority,
+           pick the one with the highest version.  If there are
+           multiple derivations with the same priority and name and
            version, then pick the first one. */
         if (newestOnly) {
 
@@ -270,13 +282,22 @@ static DrvInfos filterBySelector(EvalState & state,
 
             for (Matches::iterator j = matches.begin(); j != matches.end(); ++j) {
                 DrvName drvName(j->first.name);
+                int d = 1;
+
                 Newest::iterator k = newest.find(drvName.name);
+                
                 if (k != newest.end()) {
-                    int d = compareVersions(drvName.version, DrvName(k->second.first.name).version);
-                    if (d > 0) newest[drvName.name] = *j;
-                    else if (d == 0) multiple.insert(j->first.name);
-                } else
+                    d = comparePriorities(state, j->first, k->second.first);
+                    if (d == 0)
+                        d = compareVersions(drvName.version, DrvName(k->second.first.name).version);
+                }
+
+                if (d > 0) {
                     newest[drvName.name] = *j;
+                    multiple.erase(j->first.name);
+                } else if (d == 0) {
+                    multiple.insert(j->first.name);
+                }
             }
 
             matches.clear();
@@ -549,9 +570,10 @@ static void upgradeDerivations(Globals & globals,
         if (meta["keep"] == "true") continue;
 
         /* Find the derivation in the input Nix expression with the
-           same name and satisfying the version constraints specified
+           same name that satisfies the version constraints specified
            by upgradeType.  If there are multiple matches, take the
-           one with highest version. */
+           one with the highest priority.  If there are still multiple
+           matches, take the one with the highest version. */
         DrvInfos::iterator bestElem = availElems.end();
         DrvName bestName;
         for (DrvInfos::iterator j = availElems.begin();
@@ -559,16 +581,19 @@ static void upgradeDerivations(Globals & globals,
         {
             DrvName newName(j->name);
             if (newName.name == drvName.name) {
-                int d = compareVersions(drvName.version, newName.version);
+                int d = comparePriorities(globals.state, *i, *j);
+                if (d == 0) d = compareVersions(drvName.version, newName.version);
                 if (upgradeType == utLt && d < 0 ||
                     upgradeType == utLeq && d <= 0 ||
                     upgradeType == utEq && d == 0 ||
                     upgradeType == utAlways)
                 {
-                    if ((bestElem == availElems.end() ||
-                         compareVersions(
-                             bestName.version, newName.version) < 0))
-                    {
+                    int d2 = -1;
+                    if (bestElem != availElems.end()) {
+                        d2 = comparePriorities(globals.state, *bestElem, *j);
+                        if (d2 == 0) d2 = compareVersions(bestName.version, newName.version);
+                    }
+                    if (d2 < 0) {
                         bestElem = j;
                         bestName = newName;
                     }