cleanup and slightly refactor DrvInfo::queryOutputs

Change-Id: I00d9ff707fe61995737b86af6d2eaa1e4d8116ff

2 files changed, 118 insertions, 48 deletions

diff --git a/src/libexpr/get-drvs.cc b/src/libexpr/get-drvs.cc
index 8e3969aac..f5d0f0120 100644
--- a/src/libexpr/get-drvs.cc
+++ b/src/libexpr/get-drvs.cc
@@ -101,66 +101,134 @@ StorePath DrvInfo::queryOutPath()
     return *outPath;
 }
 
+void DrvInfo::fillOutputs(bool withPaths)
+{
+    auto fillDefault = [&]() {
+        std::optional<StorePath> outPath = std::nullopt;
+        if (withPaths) {
+            outPath.emplace(this->queryOutPath());
+        }
+        this->outputs.emplace("out", outPath);
+    };
+
+    // lol. lmao even.
+    if (this->attrs == nullptr) {
+        fillDefault();
+        return;
+    }
+
+    Attr * outputs = this->attrs->get(this->state->sOutputs);
+    if (outputs == nullptr) {
+        fillDefault();
+        return;
+    }
+
+    // NOTE(Qyriad): I don't think there is any codepath that can cause this to error.
+    this->state->forceList(
+        *outputs->value,
+        outputs->pos,
+        "while evaluating the 'outputs' attribute of a derivation"
+    );
+
+    for (auto [idx, elem] : enumerate(outputs->value->listItems())) {
+        // NOTE(Qyriad): This error should be *extremely* rare in practice.
+        // It is impossible to construct with `stdenv.mkDerivation`,
+        // `builtins.derivation`, or even `derivationStrict`. As far as we can tell,
+        // it is only possible by overriding a derivation attrset already created by
+        // one of those with `//` to introduce the failing `outputs` entry.
+        auto errMsg = fmt("while evaluating output %d of a derivation", idx);
+        std::string_view outputName = state->forceStringNoCtx(
+            *elem,
+            outputs->pos,
+            errMsg
+        );
+
+        if (withPaths) {
+            // Find the attr with this output's name...
+            Attr * out = this->attrs->get(this->state->symbols.create(outputName));
+            if (out == nullptr) {
+                // FIXME: throw error?
+                continue;
+            }
+
+            // Meanwhile we couldn't figure out any circumstances
+            // that cause this to error.
+            state->forceAttrs(*out->value, outputs->pos, errMsg);
+
+            // ...and evaluate its `outPath` attribute.
+            Attr * outPath = out->value->attrs->get(this->state->sOutPath);
+            if (outPath == nullptr) {
+                continue;
+                // FIXME: throw error?
+            }
+
+            NixStringContext context;
+            // And idk what could possibly cause this one to error
+            // that wouldn't error before here.
+            auto storePath = state->coerceToStorePath(
+                outPath->pos,
+                *outPath->value,
+                context,
+                errMsg
+            );
+            this->outputs.emplace(outputName, storePath);
+        } else {
+            this->outputs.emplace(outputName, std::nullopt);
+        }
+    }
+}
 
 DrvInfo::Outputs DrvInfo::queryOutputs(bool withPaths, bool onlyOutputsToInstall)
 {
+    // If we haven't already cached the outputs set, then do so now.
     if (outputs.empty()) {
-        /* Get the ‘outputs’ list. */
-        Bindings::iterator i;
-        if (attrs && (i = attrs->find(state->sOutputs)) != attrs->end()) {
-            state->forceList(*i->value, i->pos, "while evaluating the 'outputs' attribute of a derivation");
-
-            /* For each output... */
-            for (auto elem : i->value->listItems()) {
-                std::string output(state->forceStringNoCtx(*elem, i->pos, "while evaluating the name of an output of a derivation"));
-
-                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, "while evaluating an output of a derivation");
-
-                    /* And evaluate its ‘outPath’ attribute. */
-                    Bindings::iterator outPath = out->value->attrs->find(state->sOutPath);
-                    if (outPath == out->value->attrs->end()) continue; // FIXME: throw error?
-                    NixStringContext context;
-                    outputs.emplace(output, state->coerceToStorePath(outPath->pos, *outPath->value, context, "while evaluating an output path of a derivation"));
-                } else
-                    outputs.emplace(output, std::nullopt);
-            }
-        } else
-            outputs.emplace("out", withPaths ? std::optional{queryOutPath()} : std::nullopt);
+        // FIXME: this behavior seems kind of busted, since whether or not this
+        // DrvInfo will have paths is forever determined by the *first* call to
+        // this function??
+        fillOutputs(withPaths);
     }
 
+    // Things that operate on derivations like packages, like `nix-env` and `nix build`,
+    // allow derivations to specify which outputs should be used in those user-facing
+    // cases if the user didn't specify an output explicitly.
+    // If the caller just wanted all the outputs for this derivation, though,
+    // then we're done here.
     if (!onlyOutputsToInstall || !attrs)
         return outputs;
 
-    Bindings::iterator i;
-    if (attrs && (i = attrs->find(state->sOutputSpecified)) != attrs->end() && state->forceBool(*i->value, i->pos, "while evaluating the 'outputSpecified' attribute of a derivation")) {
-        Outputs result;
-        auto out = outputs.find(queryOutputName());
-        if (out == outputs.end())
-            throw Error("derivation does not have output '%s'", queryOutputName());
-        result.insert(*out);
-        return result;
+    // Regardless of `meta.outputsToInstall`, though, you can select into a derivation
+    // output by its attribute, e.g. `pkgs.lix.dev`, which (lol?) sets the magic
+    // attribute `outputSpecified = true`, and changes the `outputName` attr to the
+    // explicitly selected-into output.
+    if (Attr * outSpecAttr = attrs->get(state->sOutputSpecified)) {
+        bool outputSpecified = this->state->forceBool(
+            *outSpecAttr->value,
+            outSpecAttr->pos,
+            "while evaluating the 'outputSpecified' attribute of a derivation"
+        );
+        if (outputSpecified) {
+            auto maybeOut = outputs.find(queryOutputName());
+            if (maybeOut == outputs.end()) {
+                throw Error("derivation does not have output '%s'", queryOutputName());
+            }
+            return Outputs{*maybeOut};
+        }
     }
 
-    else {
-        /* Check for `meta.outputsToInstall` and return `outputs` reduced to that. */
-        const Value * outTI = queryMeta("outputsToInstall");
-        if (!outTI) return outputs;
-        auto errMsg = Error("this derivation has bad 'meta.outputsToInstall'");
-            /* ^ this shows during `nix-env -i` right under the bad derivation */
-        if (!outTI->isList()) throw errMsg;
-        Outputs result;
-        for (auto elem : outTI->listItems()) {
-            if (elem->type() != nString) throw errMsg;
-            auto out = outputs.find(elem->string.s);
-            if (out == outputs.end()) throw errMsg;
-            result.insert(*out);
-        }
-        return result;
+    /* Check for `meta.outputsToInstall` and return `outputs` reduced to that. */
+    const Value * outTI = queryMeta("outputsToInstall");
+    if (!outTI) return outputs;
+    auto errMsg = Error("this derivation has bad 'meta.outputsToInstall'");
+        /* ^ this shows during `nix-env -i` right under the bad derivation */
+    if (!outTI->isList()) throw errMsg;
+    Outputs result;
+    for (auto elem : outTI->listItems()) {
+        if (elem->type() != nString) throw errMsg;
+        auto out = outputs.find(elem->string.s);
+        if (out == outputs.end()) throw errMsg;
+        result.insert(*out);
     }
+    return result;
 }
 
 
diff --git a/src/libexpr/get-drvs.hh b/src/libexpr/get-drvs.hh
index 44aac3015..525ac9803 100644
--- a/src/libexpr/get-drvs.hh
+++ b/src/libexpr/get-drvs.hh
@@ -37,6 +37,8 @@ private:
 
     bool checkMeta(Value & v);
 
+    void fillOutputs(bool withPaths = true);
+
 public:
     /**
      * path towards the derivation