Fix bug, test more, document more

src/libstore/derived-path.cc

@@ -97,8 +97,12 @@ DerivedPath::Built DerivedPath::Built::parse(const Store & store, std::string_vi
 {
     auto drvPath = store.parseStorePath(drvS);
     std::set<std::string> outputs;
-    if (outputsS != "*")
+    if (outputsS != "*") {
         outputs = tokenizeString<std::set<std::string>>(outputsS, ",");
+        if (outputs.empty())
+            throw Error(
+                 "Explicit list of wanted outputs '%s' must not be empty. Consider using '*' as a wildcard meaning all outputs if no output in particular is wanted.", outputsS);
+	}
     return {drvPath, outputs};
 }
 

src/nix/nix.md

@@ -134,8 +134,9 @@ the Nix store. Here are the recognised types of installables:
 
   *(Experimental, part of by the `computed-derivations` experimental feature.)*
 
-  Store derivations can be indexed with a specific output name. This
-  allows finer control versus just specifying a derivation (without
+  Store derivations can be indexed with a non-empty comma-separated list
+  of specific output names, or `*` meaning all ouptuts. This allows
+  finer control versus just specifying a derivation (without
   `--derivation`) and getting all the outputs.
 
   This is especially useful for (currently unstable) floating content

tests/build-explicit-output.sh

@@ -1,14 +1,24 @@
 source common.sh
 
+set -o pipefail
+
 enableFeatures "computed-derivations"
 restartDaemon
 
 drv=$(nix eval -f multiple-outputs.nix --raw a.drvPath)
-if nix build "$drv^not-an-output" --json; then
+if nix build "$drv^not-an-output" --no-link --json; then
     fail "'not-an-output' should fail to build"
 fi
 
-nix build "$drv^first" --json | jq --exit-status '
+if nix build "$drv^" --no-link --json; then
+    fail "'empty outputs list' should fail to build"
+fi
+
+if nix build "$drv^*nope" --no-link --json; then
+    fail "'* must be entire string' should fail to build"
+fi
+
+nix build "$drv^first" --no-link --json | jq --exit-status '
   (.[0] |
     (.drvPath | match(".*multiple-outputs-a.drv")) and
     (.outputs |
@@ -16,3 +26,21 @@ nix build "$drv^first" --json | jq --exit-status '
       (.first | match(".*multiple-outputs-a-first")) and
       (has("second") | not)))
 '
+
+nix build "$drv^first,second" --no-link --json | jq --exit-status '
+  (.[0] |
+    (.drvPath | match(".*multiple-outputs-a.drv")) and
+    (.outputs |
+      (keys | length == 2) and
+      (.first | match(".*multiple-outputs-a-first")) and
+	  (.second | match(".*multiple-outputs-a-second"))))
+'
+
+nix build "$drv^*" --no-link --json | jq --exit-status '
+  (.[0] |
+    (.drvPath | match(".*multiple-outputs-a.drv")) and
+    (.outputs |
+      (keys | length == 2) and
+      (.first | match(".*multiple-outputs-a-first")) and
+	  (.second | match(".*multiple-outputs-a-second"))))
+'