nix-env: respect meta.outputsToInstall

Discussed on https://github.com/NixOS/nixpkgs/pull/12653#discussion_r51601849

doc/manual/command-ref/nix-env.xml

@@ -367,6 +367,10 @@ number of possible ways:
   linkend="rsec-nix-store-realise">realised</link> and
   installed.</para></listitem>
 
+  <listitem><para>By default all outputs are installed for each derivation.
+  That can be reduced by setting <literal>meta.outputsToInstall</literal>.
+  </para></listitem> <!-- TODO: link nixpkgs docs on the ability to override those. -->
+
 </itemizedlist>
 
 </para>

src/libexpr/get-drvs.cc

@@ -30,7 +30,7 @@ string DrvInfo::queryOutPath()
 }
 
 
-DrvInfo::Outputs DrvInfo::queryOutputs()
+DrvInfo::Outputs DrvInfo::queryOutputs(bool onlyOutputsToInstall)
 {
     if (outputs.empty()) {
         /* Get the ‘outputs’ list. */
@@ -55,7 +55,23 @@ DrvInfo::Outputs DrvInfo::queryOutputs()
         } else
             outputs["out"] = queryOutPath();
     }
+    if (!onlyOutputsToInstall || !attrs)
         return outputs;
+
+    /* Check for `meta.outputsToInstall` and return `outputs` reduced to that. */
+    const Value * outTI = queryMeta("outputsToInstall");
+    if (!outTI) return outputs;
+    const auto errMsg = Error("this derivation has bad ‘meta.outputsToInstall’");
+        /* ^ this shows during `nix-env -i` right under the bad derivation */
+    if (!outTI->isList()) throw errMsg;
+    Outputs result;
+    for (auto i = outTI->listElems(); i != outTI->listElems() + outTI->listSize(); ++i) {
+        if ((*i)->type != tString) throw errMsg;
+        auto out = outputs.find((*i)->string.s);
+        if (out == outputs.end()) throw errMsg;
+        result.insert(*out);
+    }
+    return result;
 }
 
 
@@ -192,8 +208,8 @@ typedef set<Bindings *> Done;
 
 
 /* Evaluate value `v'.  If it evaluates to a set of type `derivation',
-   then put information about it in `drvs' (unless it's already in
+   then put information about it in `drvs' (unless it's already in `done').
-   `doneExprs').  The result boolean indicates whether it makes sense
+   The result boolean indicates whether it makes sense
    for the caller to recursively search for derivations in `v'. */
 static bool getDerivation(EvalState & state, Value & v,
     const string & attrPath, DrvInfos & drvs, Done & done,

src/libexpr/get-drvs.hh

@@ -42,7 +42,8 @@ public:
     string queryDrvPath();
     string queryOutPath();
     string queryOutputName();
-    Outputs queryOutputs();
+    /** Return the list of outputs. The "outputs to install" are determined by `mesa.outputsToInstall`. */
+    Outputs queryOutputs(bool onlyOutputsToInstall = false);
 
     StringSet queryMetaNames();
     Value * queryMeta(const string & name);

src/nix-env/user-env.cc

@@ -63,8 +63,8 @@ bool createUserEnv(EvalState & state, DrvInfos & elems,
         if (drvPath != "")
             mkString(*state.allocAttr(v, state.sDrvPath), i.queryDrvPath());
 
-        // Copy each output.
+        // Copy each output meant for installation.
-        DrvInfo::Outputs outputs = i.queryOutputs();
+        DrvInfo::Outputs outputs = i.queryOutputs(true);
         Value & vOutputs = *state.allocAttr(v, state.sOutputs);
         state.mkList(vOutputs, outputs.size());
         unsigned int m = 0;