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, 4 insertions, 3 deletions

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);