From 8e8e9d8705a68e1be63d6d8059c6c07127826525 Mon Sep 17 00:00:00 2001
From: Eelco Dolstra <edolstra@gmail.com>
Date: Mon, 30 May 2022 11:32:37 +0200
Subject: [PATCH] Respect the outputSpecified attribute

E.g. 'nix build nixpkgs#libxml2.dev' will build the 'dev' output.
---
 doc/manual/src/release-notes/rl-next.md |  3 --
 src/libcmd/installables.cc              |  9 +++++-
 src/libexpr/eval.cc                     |  1 +
 src/libexpr/eval.hh                     |  3 +-
 src/libexpr/get-drvs.cc                 | 37 +++++++++++++++++--------
 5 files changed, 36 insertions(+), 17 deletions(-)

diff --git a/doc/manual/src/release-notes/rl-next.md b/doc/manual/src/release-notes/rl-next.md
index 878916dc9..7151751dd 100644
--- a/doc/manual/src/release-notes/rl-next.md
+++ b/doc/manual/src/release-notes/rl-next.md
@@ -22,9 +22,6 @@
   `meta.outputsToInstall` attribute if it exists, or all outputs
   otherwise.
 
-  Selecting derivation outputs using the attribute selection syntax
-  (e.g. `nixpkgs#glibc.dev`) no longer works.
-
 * Running nix with the new `--debugger` flag will cause it to start a repl session if
   there is an exception thrown during eval, or if `builtins.break` is called.  From
   there one can inspect symbol values and evaluate nix expressions.  In debug mode
diff --git a/src/libcmd/installables.cc b/src/libcmd/installables.cc
index 635ce19b6..21db2b08b 100644
--- a/src/libcmd/installables.cc
+++ b/src/libcmd/installables.cc
@@ -623,7 +623,14 @@ std::tuple<std::string, FlakeRef, InstallableValue::DerivationInfo> InstallableF
     std::set<std::string> outputsToInstall;
     std::optional<NixInt> priority;
 
-    if (auto aMeta = attr->maybeGetAttr(state->sMeta)) {
+    if (auto aOutputSpecified = attr->maybeGetAttr(state->sOutputSpecified)) {
+        if (aOutputSpecified->getBool()) {
+            if (auto aOutputName = attr->maybeGetAttr("outputName"))
+                outputsToInstall = { aOutputName->getString() };
+        }
+    }
+
+    else if (auto aMeta = attr->maybeGetAttr(state->sMeta)) {
         if (auto aOutputsToInstall = aMeta->maybeGetAttr("outputsToInstall"))
             for (auto & s : aOutputsToInstall->getListOfStrings())
                 outputsToInstall.insert(s);
diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index 18baf1cb7..40462afdf 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -459,6 +459,7 @@ EvalState::EvalState(
     , sKey(symbols.create("key"))
     , sPath(symbols.create("path"))
     , sPrefix(symbols.create("prefix"))
+    , sOutputSpecified(symbols.create("outputSpecified"))
     , repair(NoRepair)
     , emptyBindings(0)
     , store(store)
diff --git a/src/libexpr/eval.hh b/src/libexpr/eval.hh
index 0a32d5885..7b8732169 100644
--- a/src/libexpr/eval.hh
+++ b/src/libexpr/eval.hh
@@ -103,7 +103,8 @@ public:
         sOutputHash, sOutputHashAlgo, sOutputHashMode,
         sRecurseForDerivations,
         sDescription, sSelf, sEpsilon, sStartSet, sOperator, sKey, sPath,
-        sPrefix;
+        sPrefix,
+        sOutputSpecified;
     Symbol sDerivationNix;
 
     /* If set, force copying files to the Nix store even if they
diff --git a/src/libexpr/get-drvs.cc b/src/libexpr/get-drvs.cc
index d616b3921..346741dd5 100644
--- a/src/libexpr/get-drvs.cc
+++ b/src/libexpr/get-drvs.cc
@@ -132,23 +132,36 @@ DrvInfo::Outputs DrvInfo::queryOutputs(bool withPaths, bool onlyOutputsToInstall
         } else
             outputs.emplace("out", withPaths ? std::optional{queryOutPath()} : std::nullopt);
     }
+
     if (!onlyOutputsToInstall || !attrs)
         return outputs;
 
-    /* Check for `meta.outputsToInstall` and return `outputs` reduced to that. */
-    const Value * outTI = queryMeta("outputsToInstall");
-    if (!outTI) return outputs;
-    const 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;
+    Bindings::iterator i;
+    if (attrs && (i = attrs->find(state->sOutputSpecified)) != attrs->end() && state->forceBool(*i->value, i->pos)) {
+        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;
+    }
+
+    else {
+        /* Check for `meta.outputsToInstall` and return `outputs` reduced to that. */
+        const Value * outTI = queryMeta("outputsToInstall");
+        if (!outTI) return outputs;
+        const 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;
     }
-    return result;
 }