Generalize extra-* settings

This removes the extra-substituters and extra-sandbox-paths settings
and instead makes every array setting extensible by setting
"extra-<name> = <value>" in the configuration file or passing
"--<name> <value>" on the command line.

src/libstore/build/derivation-goal.cc

@@ -1332,13 +1332,9 @@ void DerivationGoal::startBuilder()
 
         /* Allow a user-configurable set of directories from the
            host file system. */
-        PathSet dirs = settings.sandboxPaths;
-        PathSet dirs2 = settings.extraSandboxPaths;
-        dirs.insert(dirs2.begin(), dirs2.end());
-
         dirsInChroot.clear();
 
-        for (auto i : dirs) {
+        for (auto i : settings.sandboxPaths.get()) {
             if (i.empty()) continue;
             bool optional = false;
             if (i[i.size() - 1] == '?') {

src/libstore/daemon.cc

@@ -231,8 +231,6 @@ struct ClientSettings
                     settings.set(name, value);
                 else if (setSubstituters(settings.substituters))
                     ;
-                else if (setSubstituters(settings.extraSubstituters))
-                    ;
                 else
                     debug("ignoring the client-specified setting '%s', because it is a restricted setting and you are not a trusted user", name);
             } catch (UsageError & e) {

src/libstore/globals.cc

@@ -160,7 +160,7 @@ NLOHMANN_JSON_SERIALIZE_ENUM(SandboxMode, {
     {SandboxMode::smDisabled, false},
 });
 
-template<> void BaseSetting<SandboxMode>::set(const std::string & str)
+template<> void BaseSetting<SandboxMode>::set(const std::string & str, bool append)
 {
     if (str == "true") value = smEnabled;
     else if (str == "relaxed") value = smRelaxed;
@@ -168,6 +168,11 @@ template<> void BaseSetting<SandboxMode>::set(const std::string & str)
     else throw UsageError("option '%s' has invalid value '%s'", name, str);
 }
 
+template<> bool BaseSetting<SandboxMode>::isAppendable()
+{
+    return false;
+}
+
 template<> std::string BaseSetting<SandboxMode>::to_string() const
 {
     if (value == smEnabled) return "true";
@@ -198,7 +203,7 @@ template<> void BaseSetting<SandboxMode>::convertToArg(Args & args, const std::s
     });
 }
 
-void MaxBuildJobsSetting::set(const std::string & str)
+void MaxBuildJobsSetting::set(const std::string & str, bool append)
 {
     if (str == "auto") value = std::max(1U, std::thread::hardware_concurrency());
     else if (!string2Int(str, value))

src/libstore/globals.hh

@@ -25,7 +25,7 @@ struct MaxBuildJobsSetting : public BaseSetting<unsigned int>
         options->addSetting(this);
     }
 
-    void set(const std::string & str) override;
+    void set(const std::string & str, bool append = false) override;
 };
 
 class Settings : public Config {
@@ -413,14 +413,6 @@ public:
     Setting<bool> sandboxFallback{this, true, "sandbox-fallback",
         "Whether to disable sandboxing when the kernel doesn't allow it."};
 
-    Setting<PathSet> extraSandboxPaths{
-        this, {}, "extra-sandbox-paths",
-        R"(
-          A list of additional paths appended to `sandbox-paths`. Useful if
-          you want to extend its default value.
-        )",
-        {"build-extra-chroot-dirs", "build-extra-sandbox-paths"}};
-
     Setting<size_t> buildRepeat{
         this, 0, "repeat",
         R"(
@@ -599,17 +591,6 @@ public:
         )",
         {"binary-caches"}};
 
-    // FIXME: provide a way to add to option values.
-    Setting<Strings> extraSubstituters{
-        this, {}, "extra-substituters",
-        R"(
-          Additional binary caches appended to those specified in
-          `substituters`. When used by unprivileged users, untrusted
-          substituters (i.e. those not listed in `trusted-substituters`) are
-          silently ignored.
-        )",
-        {"extra-binary-caches"}};
-
     Setting<StringSet> trustedSubstituters{
         this, {}, "trusted-substituters",
         R"(

src/libstore/store-api.cc

@@ -1114,9 +1114,6 @@ std::list<ref<Store>> getDefaultSubstituters()
         for (auto uri : settings.substituters.get())
             addStore(uri);
 
-        for (auto uri : settings.extraSubstituters.get())
-            addStore(uri);
-
         stores.sort([](ref<Store> & a, ref<Store> & b) {
             return a->priority < b->priority;
         });