Make "nix-build -A <derivation>.<output>" do the right thing

For example, given a derivation with outputs "out", "man" and "bin":

  $ nix-build -A pkg

produces ./result pointing to the "out" output;

  $ nix-build -A pkg.man

produces ./result-man pointing to the "man" output;

  $ nix-build -A pkg.all

produces ./result, ./result-man and ./result-bin;

  $ nix-build -A pkg.all -A pkg2

produces ./result, ./result-man, ./result-bin and ./result-2.

src/libstore/build.cc

@@ -3197,11 +3197,13 @@ void LocalStore::buildPaths(const PathSet & drvPaths, bool repair)
     Worker worker(*this);
 
     Goals goals;
-    foreach (PathSet::const_iterator, i, drvPaths)
-        if (isDerivation(*i))
-            goals.insert(worker.makeDerivationGoal(*i, repair));
+    foreach (PathSet::const_iterator, i, drvPaths) {
+        DrvPathWithOutputs i2 = parseDrvPathWithOutputs(*i);
+        if (isDerivation(i2.first))
+            goals.insert(worker.makeDerivationGoal(i2.first, repair));
         else
             goals.insert(worker.makeSubstitutionGoal(*i, repair));
+    }
 
     worker.run(goals);
 

src/libstore/derivations.cc

@@ -252,4 +252,21 @@ Hash hashDerivationModulo(StoreAPI & store, Derivation drv)
 }
 
 
+DrvPathWithOutputs parseDrvPathWithOutputs(const string & s)
+{
+    size_t n = s.find("!");
+    return n == s.npos
+        ? DrvPathWithOutputs(s, std::set<string>())
+        : DrvPathWithOutputs(string(s, 0, n), tokenizeString<std::set<string> >(string(s, n + 1), ","));
+}
+
+
+Path makeDrvPathWithOutputs(const Path & drvPath, std::set<string> outputs)
+{
+    return outputs.empty()
+        ? drvPath
+        : drvPath + "!" + concatStringsSep(",", outputs);
+}
+
+
 }

src/libstore/derivations.hh

@@ -79,4 +79,13 @@ typedef std::map<Path, Hash> DrvHashes;
 
 extern DrvHashes drvHashes;
 
+/* Split a string specifying a derivation and a set of outputs
+   (/nix/store/hash-foo!out1,out2,...) into the derivation path and
+   the outputs. */
+typedef std::pair<string, std::set<string> > DrvPathWithOutputs;
+DrvPathWithOutputs parseDrvPathWithOutputs(const string & s);
+
+Path makeDrvPathWithOutputs(const Path & drvPath, std::set<string> outputs);
+
+
 }

src/libstore/misc.cc

@@ -82,20 +82,23 @@ void queryMissing(StoreAPI & store, const PathSet & targets,
             if (done.find(*i) != done.end()) continue;
             done.insert(*i);
 
-            if (isDerivation(*i)) {
-                if (!store.isValidPath(*i)) {
+            DrvPathWithOutputs i2 = parseDrvPathWithOutputs(*i);
+
+            if (isDerivation(i2.first)) {
+                if (!store.isValidPath(i2.first)) {
                     // FIXME: we could try to substitute p.
                     unknown.insert(*i);
                     continue;
                 }
-                Derivation drv = derivationFromPath(store, *i);
+                Derivation drv = derivationFromPath(store, i2.first);
 
                 PathSet invalid;
+                // FIXME: only fetch the desired outputs
                 foreach (DerivationOutputs::iterator, j, drv.outputs)
                     if (!store.isValidPath(j->second.path)) invalid.insert(j->second.path);
                 if (invalid.empty()) continue;
 
-                todoDrv.insert(*i);
+                todoDrv.insert(i2.first);
                 if (settings.useSubstitutes) query.insert(invalid.begin(), invalid.end());
             }
 

src/libutil/util.cc

@@ -997,13 +997,14 @@ template<class C> C tokenizeString(const string & s, const string & separators)
         string::size_type end = s.find_first_of(separators, pos + 1);
         if (end == string::npos) end = s.size();
         string token(s, pos, end - pos);
-        result.push_back(token);
+        result.insert(result.end(), token);
         pos = s.find_first_not_of(separators, end);
     }
     return result;
 }
 
 template Strings tokenizeString(const string & s, const string & separators);
+template StringSet tokenizeString(const string & s, const string & separators);
 template vector<string> tokenizeString(const string & s, const string & separators);
 
 
@@ -1018,6 +1019,17 @@ string concatStringsSep(const string & sep, const Strings & ss)
 }
 
 
+string concatStringsSep(const string & sep, const StringSet & ss)
+{
+    string s;
+    foreach (StringSet::const_iterator, i, ss) {
+        if (s.size() != 0) s += sep;
+        s += *i;
+    }
+    return s;
+}
+
+
 string chomp(const string & s)
 {
     size_t i = s.find_last_not_of(" \n\r\t");

src/libutil/util.hh

@@ -293,6 +293,7 @@ template<class C> C tokenizeString(const string & s, const string & separators =
 /* Concatenate the given strings with a separator between the
    elements. */
 string concatStringsSep(const string & sep, const Strings & ss);
+string concatStringsSep(const string & sep, const StringSet & ss);
 
 
 /* Remove trailing whitespace from a string. */

src/nix-instantiate/nix-instantiate.cc

@@ -8,6 +8,7 @@
 #include "util.hh"
 #include "store-api.hh"
 #include "common-opts.hh"
+#include "misc.hh"
 
 #include <map>
 #include <iostream>
@@ -59,6 +60,19 @@ void processExpr(EvalState & state, const Strings & attrPaths,
                 getDerivations(state, v, "", autoArgs, drvs, false);
                 foreach (DrvInfos::iterator, i, drvs) {
                     Path drvPath = i->queryDrvPath(state);
+
+                    /* What output do we want? */
+                    Path outPath = i->queryOutPath(state);
+                    Derivation drv = derivationFromPath(*store, drvPath);
+                    string outputName;
+                    foreach (DerivationOutputs::iterator, i, drv.outputs)
+                        if (i->second.path == outPath) {
+                            outputName = i->first;
+                            break;
+                        }
+                    if (outputName == "")
+                        throw Error(format("derivation `%1%' does not have an output `%2%'") % drvPath % outPath);
+
                     if (gcRoot == "")
                         printGCWarning();
                     else {
@@ -66,7 +80,7 @@ void processExpr(EvalState & state, const Strings & attrPaths,
                         if (++rootNr > 1) rootName += "-" + int2String(rootNr);
                         drvPath = addPermRoot(*store, drvPath, rootName, indirectRoot);
                     }
-                    std::cout << format("%1%\n") % drvPath;
+                    std::cout << format("%1%%2%\n") % drvPath % (outputName != "out" ? "!" + outputName : "");
                 }
             }
         }

src/nix-store/nix-store.cc

@@ -60,13 +60,21 @@ static Path useDeriver(Path path)
    other paths it means ensure their validity. */
 static PathSet realisePath(const Path & path, bool build = true)
 {
-    if (isDerivation(path)) {
+    DrvPathWithOutputs p = parseDrvPathWithOutputs(path);
+
+    if (isDerivation(p.first)) {
         if (build) store->buildPaths(singleton<PathSet>(path));
-        Derivation drv = derivationFromPath(*store, path);
+        Derivation drv = derivationFromPath(*store, p.first);
         rootNr++;
 
+        if (p.second.empty())
+            foreach (DerivationOutputs::iterator, i, drv.outputs) p.second.insert(i->first);
+
         PathSet outputs;
-        foreach (DerivationOutputs::iterator, i, drv.outputs) {
+        foreach (StringSet::iterator, j, p.second) {
+            DerivationOutputs::iterator i = drv.outputs.find(*j);
+            if (i == drv.outputs.end())
+                throw Error(format("derivation `%1%' does not have an output named `%2%'") % p.first % *j);
             Path outPath = i->second.path;
             if (gcRoot == "")
                 printGCWarning();
@@ -103,8 +111,10 @@ static void opRealise(Strings opFlags, Strings opArgs)
         else throw UsageError(format("unknown flag `%1%'") % *i);
 
     Paths paths;
-    foreach (Strings::iterator, i, opArgs)
-        paths.push_back(followLinksToStorePath(*i));
+    foreach (Strings::iterator, i, opArgs) {
+        DrvPathWithOutputs p = parseDrvPathWithOutputs(*i);
+        paths.push_back(makeDrvPathWithOutputs(followLinksToStorePath(p.first), p.second));
+    }
 
     unsigned long long downloadSize, narSize;
     PathSet willBuild, willSubstitute, unknown;