nix-env: always print output names in JSON and XML

The current `--out-path` flag has two disadvantages when one is only
concerned with querying the names of outputs:
- it requires evaluating every output's `outPath`, which takes
  significantly more resources and runs into more failures
- it destroys the information of the order of outputs so we can't tell
  which one is the main output

This patch makes the output names always present (replacing paths with
`null` in JSON if `--out-path` isn't given), and adds an `outputName`
field.

4 files changed, 96 insertions, 87 deletions

diff --git a/src/libexpr/get-drvs.cc b/src/libexpr/get-drvs.cc
index 7630c5ff4..7f2ecb4f7 100644
--- a/src/libexpr/get-drvs.cc
+++ b/src/libexpr/get-drvs.cc
@@ -102,7 +102,7 @@ StorePath DrvInfo::queryOutPath() const
 }
 
 
-DrvInfo::Outputs DrvInfo::queryOutputs(bool onlyOutputsToInstall)
+DrvInfo::Outputs DrvInfo::queryOutputs(bool withPaths, bool onlyOutputsToInstall)
 {
     if (outputs.empty()) {
         /* Get the ‘outputs’ list. */
@@ -112,20 +112,24 @@ DrvInfo::Outputs DrvInfo::queryOutputs(bool onlyOutputsToInstall)
 
             /* For each output... */
             for (auto elem : i->value->listItems()) {
-                /* Evaluate the corresponding set. */
-                std::string name(state->forceStringNoCtx(*elem, *i->pos));
-                Bindings::iterator out = attrs->find(state->symbols.create(name));
-                if (out == attrs->end()) continue; // FIXME: throw error?
-                state->forceAttrs(*out->value, *i->pos);
-
-                /* And evaluate its ‘outPath’ attribute. */
-                Bindings::iterator outPath = out->value->attrs->find(state->sOutPath);
-                if (outPath == out->value->attrs->end()) continue; // FIXME: throw error?
-                PathSet context;
-                outputs.emplace(name, state->coerceToStorePath(*outPath->pos, *outPath->value, context));
+                std::string output(state->forceStringNoCtx(*elem, *i->pos));
+
+                if (withPaths) {
+                    /* Evaluate the corresponding set. */
+                    Bindings::iterator out = attrs->find(state->symbols.create(output));
+                    if (out == attrs->end()) continue; // FIXME: throw error?
+                    state->forceAttrs(*out->value, *i->pos);
+
+                    /* And evaluate its ‘outPath’ attribute. */
+                    Bindings::iterator outPath = out->value->attrs->find(state->sOutPath);
+                    if (outPath == out->value->attrs->end()) continue; // FIXME: throw error?
+                    PathSet context;
+                    outputs.emplace(output, state->coerceToStorePath(*outPath->pos, *outPath->value, context));
+                } else
+                    outputs.emplace(output, std::nullopt);
             }
         } else
-            outputs.emplace("out", queryOutPath());
+            outputs.emplace("out", withPaths ? std::optional{queryOutPath()} : std::nullopt);
     }
     if (!onlyOutputsToInstall || !attrs)
         return outputs;
diff --git a/src/libexpr/get-drvs.hh b/src/libexpr/get-drvs.hh
index 3ca6f1fca..7cc1abef2 100644
--- a/src/libexpr/get-drvs.hh
+++ b/src/libexpr/get-drvs.hh
@@ -13,7 +13,7 @@ namespace nix {
 struct DrvInfo
 {
 public:
-    typedef std::map<std::string, StorePath> Outputs;
+    typedef std::map<std::string, std::optional<StorePath>> Outputs;
 
 private:
     EvalState * state;
@@ -46,8 +46,9 @@ public:
     StorePath requireDrvPath() const;
     StorePath queryOutPath() const;
     std::string queryOutputName() const;
-    /** Return the list of outputs. The "outputs to install" are determined by `meta.outputsToInstall`. */
-    Outputs queryOutputs(bool onlyOutputsToInstall = false);
+    /** Return the unordered map of output names to (optional) output paths.
+     * The "outputs to install" are determined by `meta.outputsToInstall`. */
+    Outputs queryOutputs(bool withPaths = true, bool onlyOutputsToInstall = false);
 
     StringSet queryMetaNames();
     Value * queryMeta(const std::string & name);
diff --git a/src/nix-env/nix-env.cc b/src/nix-env/nix-env.cc
index 40c3c5d65..e68218d1c 100644
--- a/src/nix-env/nix-env.cc
+++ b/src/nix-env/nix-env.cc
@@ -918,12 +918,17 @@ static void queryJSON(Globals & globals, std::vector<DrvInfo> & elems, bool prin
             pkgObj.attr("pname", drvName.name);
             pkgObj.attr("version", drvName.version);
             pkgObj.attr("system", i.querySystem());
+            pkgObj.attr("outputName", i.queryOutputName());
 
-            if (printOutPath) {
-                DrvInfo::Outputs outputs = i.queryOutputs();
+            {
+                DrvInfo::Outputs outputs = i.queryOutputs(printOutPath);
                 JSONObject outputObj = pkgObj.object("outputs");
-                for (auto & j : outputs)
-                    outputObj.attr(j.first, globals.state->store->printStorePath(j.second));
+                for (auto & j : outputs) {
+                    if (j.second)
+                        outputObj.attr(j.first, globals.state->store->printStorePath(*j.second));
+                    else
+                        outputObj.attr(j.first, nullptr);
+                }
             }
 
             if (printMeta) {
@@ -1154,13 +1159,16 @@ static void opQuery(Globals & globals, Strings opFlags, Strings opArgs)
                     columns.push_back(drvPath ? store.printStorePath(*drvPath) : "-");
             }
 
+            if (xmlOutput)
+                attrs["outputName"] = i.queryOutputName();
+
             if (printOutPath && !xmlOutput) {
                 DrvInfo::Outputs outputs = i.queryOutputs();
                 std::string s;
                 for (auto & j : outputs) {
                     if (!s.empty()) s += ';';
                     if (j.first != "out") { s += j.first; s += "="; }
-                    s += store.printStorePath(j.second);
+                    s += store.printStorePath(*j.second);
                 }
                 columns.push_back(s);
             }
@@ -1174,71 +1182,67 @@ static void opQuery(Globals & globals, Strings opFlags, Strings opArgs)
             }
 
             if (xmlOutput) {
-                if (printOutPath || printMeta) {
-                    XMLOpenElement item(xml, "item", attrs);
-                    if (printOutPath) {
-                        DrvInfo::Outputs outputs = i.queryOutputs();
-                        for (auto & j : outputs) {
-                            XMLAttrs attrs2;
-                            attrs2["name"] = j.first;
-                            attrs2["path"] = store.printStorePath(j.second);
-                            xml.writeEmptyElement("output", attrs2);
-                        }
-                    }
-                    if (printMeta) {
-                        StringSet metaNames = i.queryMetaNames();
-                        for (auto & j : metaNames) {
-                            XMLAttrs attrs2;
-                            attrs2["name"] = j;
-                            Value * v = i.queryMeta(j);
-                            if (!v)
-                                printError(
-                                    "derivation '%s' has invalid meta attribute '%s'",
-                                    i.queryName(), j);
-                            else {
-                                if (v->type() == nString) {
-                                    attrs2["type"] = "string";
-                                    attrs2["value"] = v->string.s;
-                                    xml.writeEmptyElement("meta", attrs2);
-                                } else if (v->type() == nInt) {
-                                    attrs2["type"] = "int";
-                                    attrs2["value"] = (format("%1%") % v->integer).str();
-                                    xml.writeEmptyElement("meta", attrs2);
-                                } else if (v->type() == nFloat) {
-                                    attrs2["type"] = "float";
-                                    attrs2["value"] = (format("%1%") % v->fpoint).str();
-                                    xml.writeEmptyElement("meta", attrs2);
-                                } else if (v->type() == nBool) {
-                                    attrs2["type"] = "bool";
-                                    attrs2["value"] = v->boolean ? "true" : "false";
-                                    xml.writeEmptyElement("meta", attrs2);
-                                } else if (v->type() == nList) {
-                                    attrs2["type"] = "strings";
-                                    XMLOpenElement m(xml, "meta", attrs2);
-                                    for (auto elem : v->listItems()) {
-                                        if (elem->type() != nString) continue;
-                                        XMLAttrs attrs3;
-                                        attrs3["value"] = elem->string.s;
-                                        xml.writeEmptyElement("string", attrs3);
-                                    }
-                              } else if (v->type() == nAttrs) {
-                                  attrs2["type"] = "strings";
-                                  XMLOpenElement m(xml, "meta", attrs2);
-                                  Bindings & attrs = *v->attrs;
-                                  for (auto &i : attrs) {
-                                      Attr & a(*attrs.find(i.name));
-                                      if(a.value->type() != nString) continue;
-                                      XMLAttrs attrs3;
-                                      attrs3["type"] = i.name;
-                                      attrs3["value"] = a.value->string.s;
-                                      xml.writeEmptyElement("string", attrs3);
+                XMLOpenElement item(xml, "item", attrs);
+                DrvInfo::Outputs outputs = i.queryOutputs(printOutPath);
+                for (auto & j : outputs) {
+                    XMLAttrs attrs2;
+                    attrs2["name"] = j.first;
+                    if (j.second)
+                        attrs2["path"] = store.printStorePath(*j.second);
+                    xml.writeEmptyElement("output", attrs2);
+                }
+                if (printMeta) {
+                    StringSet metaNames = i.queryMetaNames();
+                    for (auto & j : metaNames) {
+                        XMLAttrs attrs2;
+                        attrs2["name"] = j;
+                        Value * v = i.queryMeta(j);
+                        if (!v)
+                            printError(
+                                "derivation '%s' has invalid meta attribute '%s'",
+                                i.queryName(), j);
+                        else {
+                            if (v->type() == nString) {
+                                attrs2["type"] = "string";
+                                attrs2["value"] = v->string.s;
+                                xml.writeEmptyElement("meta", attrs2);
+                            } else if (v->type() == nInt) {
+                                attrs2["type"] = "int";
+                                attrs2["value"] = (format("%1%") % v->integer).str();
+                                xml.writeEmptyElement("meta", attrs2);
+                            } else if (v->type() == nFloat) {
+                                attrs2["type"] = "float";
+                                attrs2["value"] = (format("%1%") % v->fpoint).str();
+                                xml.writeEmptyElement("meta", attrs2);
+                            } else if (v->type() == nBool) {
+                                attrs2["type"] = "bool";
+                                attrs2["value"] = v->boolean ? "true" : "false";
+                                xml.writeEmptyElement("meta", attrs2);
+                            } else if (v->type() == nList) {
+                                attrs2["type"] = "strings";
+                                XMLOpenElement m(xml, "meta", attrs2);
+                                for (auto elem : v->listItems()) {
+                                    if (elem->type() != nString) continue;
+                                    XMLAttrs attrs3;
+                                    attrs3["value"] = elem->string.s;
+                                    xml.writeEmptyElement("string", attrs3);
                                 }
-                              }
+                            } else if (v->type() == nAttrs) {
+                                attrs2["type"] = "strings";
+                                XMLOpenElement m(xml, "meta", attrs2);
+                                Bindings & attrs = *v->attrs;
+                                for (auto &i : attrs) {
+                                    Attr & a(*attrs.find(i.name));
+                                    if(a.value->type() != nString) continue;
+                                    XMLAttrs attrs3;
+                                    attrs3["type"] = i.name;
+                                    attrs3["value"] = a.value->string.s;
+                                    xml.writeEmptyElement("string", attrs3);
+                            }
                             }
                         }
                     }
-                } else
-                    xml.writeEmptyElement("item", attrs);
+                }
             } else
                 table.push_back(columns);
 
diff --git a/src/nix-env/user-env.cc b/src/nix-env/user-env.cc
index af4f350ff..bcc7736ac 100644
--- a/src/nix-env/user-env.cc
+++ b/src/nix-env/user-env.cc
@@ -56,7 +56,7 @@ bool createUserEnv(EvalState & state, DrvInfos & elems,
            output paths, and optionally the derivation path, as well
            as the meta attributes. */
         std::optional<StorePath> drvPath = keepDerivations ? i.queryDrvPath() : std::nullopt;
-        DrvInfo::Outputs outputs = i.queryOutputs(true);
+        DrvInfo::Outputs outputs = i.queryOutputs(true, true);
         StringSet metaNames = i.queryMetaNames();
 
         auto attrs = state.buildBindings(7 + outputs.size());
@@ -76,15 +76,15 @@ bool createUserEnv(EvalState & state, DrvInfos & elems,
         for (const auto & [m, j] : enumerate(outputs)) {
             (vOutputs.listElems()[m] = state.allocValue())->mkString(j.first);
             auto outputAttrs = state.buildBindings(2);
-            outputAttrs.alloc(state.sOutPath).mkString(state.store->printStorePath(j.second));
+            outputAttrs.alloc(state.sOutPath).mkString(state.store->printStorePath(*j.second));
             attrs.alloc(j.first).mkAttrs(outputAttrs);
 
             /* This is only necessary when installing store paths, e.g.,
                `nix-env -i /nix/store/abcd...-foo'. */
-            state.store->addTempRoot(j.second);
-            state.store->ensurePath(j.second);
+            state.store->addTempRoot(*j.second);
+            state.store->ensurePath(*j.second);
 
-            references.insert(j.second);
+            references.insert(*j.second);
         }
 
         // Copy the meta attributes.