post-build-hook: don't emit config settings that aren't overridden

This is a redesign from 9b1f3cbc13 where
this was introduced.

I deleted the AbstractConfig::toKeyValue since it was conspicuously and
obviously broken for two years since 450e5ec618.
I asked myself if anyone was using it, given that it only emitted
settings that were aliases (broken!), and found that nobody used it.

The motivation for this change is the same for only emitting overridden
settings to the protocol: the nix inside there may not be able to parse
our defaults, as is the case of CppNix since the consensual
accept-flake-config was added to Lix.

Fixes: https://git.lix.systems/lix-project/lix/issues/739
Change-Id: Ib9874a52137f1f22220c25bcfa2425a4802509c7

doc/manual/rl-next/post-build-hook-config.md

@@ -0,0 +1,10 @@
+---
+synopsis: "`post-build-hook` only receives settings that are set"
+cls: [2800]
+issues: [fj#739]
+category: Fixes
+credits: jade
+---
+If one is using `post-build-hook` to upload paths to a cache, it used to be broken if CppNix was used inside the script, since CppNix would fail about unsupported configuration option values in some of Lix's defaults.
+This is because `post-build-hook` receives the settings of the nix daemon in the `NIX_CONFIG` environment variable.
+Now Lix only emits overridden settings to `post-build-hook` invocations, which fixes this issue in the majority of cases: where the configuration is not explicitly incompatible.

lix/libstore/build/derivation-goal.cc

@@ -977,7 +977,7 @@ void runPostBuildHook(
     auto drvPathPretty = store.printStorePath(drvPath);
     hookEnvironment.emplace("DRV_PATH", drvPathPretty);
     hookEnvironment.emplace("OUT_PATHS", chomp(concatStringsSep(" ", store.printStorePathSet(outputPaths))));
-    hookEnvironment.emplace("NIX_CONFIG", globalConfig.toKeyValue());
+    hookEnvironment.emplace("NIX_CONFIG", globalConfig.toKeyValue(true));
 
     struct LogSink : Sink {
         Activity & act;

lix/libutil/config.cc

@@ -187,15 +187,6 @@ nlohmann::json Config::toJSON()
     return res;
 }
 
-std::string Config::toKeyValue()
-{
-    std::string res;
-    for (const auto & s : _settings)
-        if (s.second.isAlias)
-            res += fmt("%s = %s\n", s.first, s.second.setting->to_string());
-    return res;
-}
-
 void Config::convertToArgs(Args & args, const std::string & category)
 {
     for (auto & s : _settings) {
@@ -534,11 +525,11 @@ nlohmann::json GlobalConfig::toJSON()
     return res;
 }
 
-std::string GlobalConfig::toKeyValue()
+std::string GlobalConfig::toKeyValue(bool overriddenOnly)
 {
     std::string res;
     std::map<std::string, Config::SettingInfo> settings;
-    globalConfig.getSettings(settings);
+    globalConfig.getSettings(settings, overriddenOnly);
     for (const auto & s : settings)
         res += fmt("%s = %s\n", s.first, s.second.value);
     return res;

lix/libutil/config.hh

@@ -95,12 +95,6 @@ public:
      */
     virtual nlohmann::json toJSON() = 0;
 
-    /**
-     * Outputs all settings in a key-value pair format suitable to be used as
-     * `nix.conf`
-     */
-    virtual std::string toKeyValue() = 0;
-
     /**
      * Converts settings to `Args` to be used on the command line interface
      * - args: args to write to
@@ -166,8 +160,6 @@ public:
 
     nlohmann::json toJSON() override;
 
-    std::string toKeyValue() override;
-
     void convertToArgs(Args & args, const std::string & category) override;
 };
 
@@ -371,7 +363,11 @@ struct GlobalConfig : public AbstractConfig
 
     nlohmann::json toJSON() override;
 
-    std::string toKeyValue() override;
+    /**
+     * Outputs all settings in a key-value pair format suitable to be used as
+     * `nix.conf`
+     */
+    std::string toKeyValue(bool overriddenOnly = false);
 
     void convertToArgs(Args & args, const std::string & category) override;
 

tests/functional/post-hook.sh

@@ -29,3 +29,30 @@ clearStore
 nix copy --from "$REMOTE_STORE" --no-require-sigs -f dependencies.nix
 nix copy --from "$REMOTE_STORE" --no-require-sigs -f dependencies.nix input1_drv
 nix copy --from "$REMOTE_STORE" --no-require-sigs -f multiple-outputs.nix a^second
+
+clearStore
+
+# Should fail if the build hook fails
+cat > "$TEST_ROOT/fail.sh" <<-EOF
+#!${shell}
+false
+EOF
+chmod +x "$TEST_ROOT/fail.sh"
+expect 1 nix-build -o "$TEST_ROOT/result" dependencies.nix --post-build-hook "$TEST_ROOT/fail.sh"
+clearStore
+
+# Ensure that settings are passed into the post-build-hook, but only overridden
+# ones.
+rm -f "${TEST_ROOT}/nix-config"
+cat > "${TEST_ROOT}/settings.sh" <<-EOF
+#!${shell}
+echo "\$NIX_CONFIG" > "${TEST_ROOT}/nix-config"
+EOF
+chmod +x "${TEST_ROOT}/settings.sh"
+nix-build -o "${TEST_ROOT}/result" dependencies.nix --timeout 1337 --post-build-hook "${TEST_ROOT}/settings.sh"
+# Ensure pure-eval cannot become not a setting with the test passing.
+nix config show pure-eval
+# Defaulted setting does not appear.
+expect 1 grepQuiet pure-eval "${TEST_ROOT}/nix-config"
+# Overridden setting appears.
+grepQuiet "timeout = 1337" "${TEST_ROOT}/nix-config"