Add builtins.appendContext.

A partner of builtins.getContext, useful for the same reasons.

src/libexpr/primops/context.cc

@@ -1,5 +1,6 @@
 #include "primops.hh"
 #include "eval-inline.hh"
+#include "derivations.hh"
 
 namespace nix {
 
@@ -61,8 +62,7 @@ static RegisterPrimOp r3("__unsafeDiscardOutputDependency", 1, prim_unsafeDiscar
               (i.e. the kind of context you get when referencing
               .outPath of some derivation). Empty list if missing.
    Note that for a given path any combination of the above attributes
-   may be present, but at least one must be set to something other
-   than the default.
+   may be present.
 */
 static void prim_getContext(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
@@ -129,4 +129,59 @@ static void prim_getContext(EvalState & state, const Pos & pos, Value * * args,
 
 static RegisterPrimOp r4("__getContext", 1, prim_getContext);
 
+
+/* Append the given context to a given string.
+
+   See the commentary above unsafeGetContext for details of the
+   context representation.
+*/
+static void prim_appendContext(EvalState & state, const Pos & pos, Value * * args, Value & v)
+{
+    PathSet context;
+    auto orig = state.forceString(*args[0], context, pos);
+
+    state.forceAttrs(*args[1], pos);
+
+    auto sPath = state.symbols.create("path");
+    auto sAllOutputs = state.symbols.create("allOutputs");
+    for (auto & i : *args[1]->attrs) {
+        if (!state.store->isStorePath(i.name))
+            throw EvalError("Context key '%s' is not a store path, at %s", i.name, i.pos);
+        if (!settings.readOnlyMode)
+            state.store->ensurePath(i.name);
+        state.forceAttrs(*i.value, *i.pos);
+        auto iter = i.value->attrs->find(sPath);
+        if (iter != i.value->attrs->end()) {
+            if (state.forceBool(*iter->value, *iter->pos))
+                context.insert(i.name);
+        }
+
+        iter = i.value->attrs->find(sAllOutputs);
+        if (iter != i.value->attrs->end()) {
+            if (state.forceBool(*iter->value, *iter->pos)) {
+                if (!isDerivation(i.name)) {
+                    throw EvalError("Tried to add all-outputs context of %s, which is not a derivation, to a string, at %s", i.name, i.pos);
+                }
+                context.insert("=" + string(i.name));
+            }
+        }
+
+        iter = i.value->attrs->find(state.sOutputs);
+        if (iter != i.value->attrs->end()) {
+            state.forceList(*iter->value, *iter->pos);
+            if (iter->value->listSize() && !isDerivation(i.name)) {
+                throw EvalError("Tried to add derivation output context of %s, which is not a derivation, to a string, at %s", i.name, i.pos);
+            }
+            for (unsigned int n = 0; n < iter->value->listSize(); ++n) {
+                auto name = state.forceStringNoCtx(*iter->value->listElems()[n], *iter->pos);
+                context.insert("!" + name + "!" + string(i.name));
+            }
+        }
+    }
+
+    mkString(v, orig, context);
+}
+
+static RegisterPrimOp r5("__appendContext", 2, prim_appendContext);
+
 }

tests/lang/eval-okay-context-introspection.nix

@@ -18,5 +18,7 @@ let
     };
   };
 
-  legit-context = "${path}${drv.outPath}${drv.foo.outPath}${drv.drvPath}";
-in builtins.getContext legit-context == desired-context
+  legit-context = builtins.getContext "${path}${drv.outPath}${drv.foo.outPath}${drv.drvPath}";
+
+  constructed-context = builtins.getContext (builtins.appendContext "" desired-context);
+in legit-context == constructed-context