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.

1 files changed, 17 insertions, 13 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;