9 files changed, 676 insertions, 73 deletions

diff --git a/src/libmain/common-args.cc b/src/libmain/common-args.cc
index 9e1d7cee6..051668e53 100644
--- a/src/libmain/common-args.cc
+++ b/src/libmain/common-args.cc
@@ -1,48 +1,61 @@
 #include "common-args.hh"
 #include "globals.hh"
+#include "loggers.hh"
 
 namespace nix {
 
 MixCommonArgs::MixCommonArgs(const string & programName)
     : programName(programName)
 {
-    mkFlag()
-        .longName("verbose")
-        .shortName('v')
-        .description("increase verbosity level")
-        .handler([]() { verbosity = (Verbosity) (verbosity + 1); });
-
-    mkFlag()
-        .longName("quiet")
-        .description("decrease verbosity level")
-        .handler([]() { verbosity = verbosity > lvlError ? (Verbosity) (verbosity - 1) : lvlError; });
-
-    mkFlag()
-        .longName("debug")
-        .description("enable debug output")
-        .handler([]() { verbosity = lvlDebug; });
-
-    mkFlag()
-        .longName("option")
-        .labels({"name", "value"})
-        .description("set a Nix configuration option (overriding nix.conf)")
-        .arity(2)
-        .handler([](std::vector<std::string> ss) {
+    addFlag({
+        .longName = "verbose",
+        .shortName = 'v',
+        .description = "increase verbosity level",
+        .handler = {[]() { verbosity = (Verbosity) (verbosity + 1); }},
+    });
+
+    addFlag({
+        .longName = "quiet",
+        .description = "decrease verbosity level",
+        .handler = {[]() { verbosity = verbosity > lvlError ? (Verbosity) (verbosity - 1) : lvlError; }},
+    });
+
+    addFlag({
+        .longName = "debug",
+        .description = "enable debug output",
+        .handler = {[]() { verbosity = lvlDebug; }},
+    });
+
+    addFlag({
+        .longName = "option",
+        .description = "set a Nix configuration option (overriding nix.conf)",
+        .labels = {"name", "value"},
+        .handler = {[](std::string name, std::string value) {
             try {
-                globalConfig.set(ss[0], ss[1]);
+                globalConfig.set(name, value);
             } catch (UsageError & e) {
                 warn(e.what());
             }
-        });
-
-    mkFlag()
-        .longName("max-jobs")
-        .shortName('j')
-        .label("jobs")
-        .description("maximum number of parallel builds")
-        .handler([=](std::string s) {
+        }},
+    });
+
+    addFlag({
+        .longName = "log-format",
+        .description = "format of log output; \"raw\", \"internal-json\", \"bar\" "
+                        "or \"bar-with-logs\"",
+        .labels = {"format"},
+        .handler = {[](std::string format) { setLogFormat(format); }},
+    });
+
+    addFlag({
+        .longName = "max-jobs",
+        .shortName = 'j',
+        .description = "maximum number of parallel builds",
+        .labels = Strings{"jobs"},
+        .handler = {[=](std::string s) {
             settings.set("max-jobs", s);
-        });
+        }}
+    });
 
     std::string cat = "config";
     globalConfig.convertToArgs(*this, cat);
diff --git a/src/libmain/local.mk b/src/libmain/local.mk
index 0c80f5a0a..a8eed6c65 100644
--- a/src/libmain/local.mk
+++ b/src/libmain/local.mk
@@ -6,6 +6,8 @@ libmain_DIR := $(d)
 
 libmain_SOURCES := $(wildcard $(d)/*.cc)
 
+libmain_CXXFLAGS += -I src/libutil -I src/libstore
+
 libmain_LDFLAGS = $(OPENSSL_LIBS)
 
 libmain_LIBS = libstore libutil
diff --git a/src/libmain/loggers.cc b/src/libmain/loggers.cc
new file mode 100644
index 000000000..c44bb6408
--- /dev/null
+++ b/src/libmain/loggers.cc
@@ -0,0 +1,52 @@
+#include "loggers.hh"
+#include "progress-bar.hh"
+
+namespace nix {
+
+LogFormat defaultLogFormat = LogFormat::raw;
+
+LogFormat parseLogFormat(const std::string & logFormatStr) {
+    if (logFormatStr == "raw")
+        return LogFormat::raw;
+    else if (logFormatStr == "raw-with-logs")
+        return LogFormat::rawWithLogs;
+    else if (logFormatStr == "internal-json")
+        return LogFormat::internalJson;
+    else if (logFormatStr == "bar")
+        return LogFormat::bar;
+    else if (logFormatStr == "bar-with-logs")
+        return LogFormat::barWithLogs;
+    throw Error("option 'log-format' has an invalid value '%s'", logFormatStr);
+}
+
+Logger * makeDefaultLogger() {
+    switch (defaultLogFormat) {
+    case LogFormat::raw:
+        return makeSimpleLogger(false);
+    case LogFormat::rawWithLogs:
+        return makeSimpleLogger(true);
+    case LogFormat::internalJson:
+        return makeJSONLogger(*makeSimpleLogger());
+    case LogFormat::bar:
+        return makeProgressBar();
+    case LogFormat::barWithLogs:
+        return makeProgressBar(true);
+    default:
+        abort();
+    }
+}
+
+void setLogFormat(const std::string & logFormatStr) {
+    setLogFormat(parseLogFormat(logFormatStr));
+}
+
+void setLogFormat(const LogFormat & logFormat) {
+    defaultLogFormat = logFormat;
+    createDefaultLogger();
+}
+
+void createDefaultLogger() {
+    logger = makeDefaultLogger();
+}
+
+}
diff --git a/src/libmain/loggers.hh b/src/libmain/loggers.hh
new file mode 100644
index 000000000..cada03110
--- /dev/null
+++ b/src/libmain/loggers.hh
@@ -0,0 +1,20 @@
+#pragma once
+
+#include "types.hh"
+
+namespace nix {
+
+enum class LogFormat {
+  raw,
+  rawWithLogs,
+  internalJson,
+  bar,
+  barWithLogs,
+};
+
+void setLogFormat(const std::string & logFormatStr);
+void setLogFormat(const LogFormat & logFormat);
+
+void createDefaultLogger();
+
+}
diff --git a/src/libmain/progress-bar.cc b/src/libmain/progress-bar.cc
new file mode 100644
index 000000000..95a9187de
--- /dev/null
+++ b/src/libmain/progress-bar.cc
@@ -0,0 +1,491 @@
+#include "progress-bar.hh"
+#include "util.hh"
+#include "sync.hh"
+#include "store-api.hh"
+#include "names.hh"
+
+#include <atomic>
+#include <map>
+#include <thread>
+#include <iostream>
+
+namespace nix {
+
+static std::string getS(const std::vector<Logger::Field> & fields, size_t n)
+{
+    assert(n < fields.size());
+    assert(fields[n].type == Logger::Field::tString);
+    return fields[n].s;
+}
+
+static uint64_t getI(const std::vector<Logger::Field> & fields, size_t n)
+{
+    assert(n < fields.size());
+    assert(fields[n].type == Logger::Field::tInt);
+    return fields[n].i;
+}
+
+static std::string_view storePathToName(std::string_view path)
+{
+    auto base = baseNameOf(path);
+    auto i = base.find('-');
+    return i == std::string::npos ? base.substr(0, 0) : base.substr(i + 1);
+}
+
+class ProgressBar : public Logger
+{
+private:
+
+    struct ActInfo
+    {
+        std::string s, lastLine, phase;
+        ActivityType type = actUnknown;
+        uint64_t done = 0;
+        uint64_t expected = 0;
+        uint64_t running = 0;
+        uint64_t failed = 0;
+        std::map<ActivityType, uint64_t> expectedByType;
+        bool visible = true;
+        ActivityId parent;
+        std::optional<std::string> name;
+    };
+
+    struct ActivitiesByType
+    {
+        std::map<ActivityId, std::list<ActInfo>::iterator> its;
+        uint64_t done = 0;
+        uint64_t expected = 0;
+        uint64_t failed = 0;
+    };
+
+    struct State
+    {
+        std::list<ActInfo> activities;
+        std::map<ActivityId, std::list<ActInfo>::iterator> its;
+
+        std::map<ActivityType, ActivitiesByType> activitiesByType;
+
+        uint64_t filesLinked = 0, bytesLinked = 0;
+
+        uint64_t corruptedPaths = 0, untrustedPaths = 0;
+
+        bool active = true;
+        bool haveUpdate = true;
+    };
+
+    Sync<State> state_;
+
+    std::thread updateThread;
+
+    std::condition_variable quitCV, updateCV;
+
+    bool printBuildLogs;
+    bool isTTY;
+
+public:
+
+    ProgressBar(bool printBuildLogs, bool isTTY)
+        : printBuildLogs(printBuildLogs)
+        , isTTY(isTTY)
+    {
+        state_.lock()->active = isTTY;
+        updateThread = std::thread([&]() {
+            auto state(state_.lock());
+            while (state->active) {
+                if (!state->haveUpdate)
+                    state.wait(updateCV);
+                draw(*state);
+                state.wait_for(quitCV, std::chrono::milliseconds(50));
+            }
+        });
+    }
+
+    ~ProgressBar()
+    {
+        stop();
+        updateThread.join();
+    }
+
+    void stop() override
+    {
+        auto state(state_.lock());
+        if (!state->active) return;
+        state->active = false;
+        writeToStderr("\r\e[K");
+        updateCV.notify_one();
+        quitCV.notify_one();
+    }
+
+    bool isVerbose() override {
+        return printBuildLogs;
+    }
+
+    void log(Verbosity lvl, const FormatOrString & fs) override
+    {
+        auto state(state_.lock());
+        log(*state, lvl, fs.s);
+    }
+
+    void logEI(const ErrorInfo &ei) override
+    {
+        auto state(state_.lock());
+
+        std::stringstream oss;
+        oss << ei;
+
+        log(*state, ei.level, oss.str());
+    }
+
+    void log(State & state, Verbosity lvl, const std::string & s)
+    {
+        if (state.active) {
+            writeToStderr("\r\e[K" + filterANSIEscapes(s, !isTTY) + ANSI_NORMAL "\n");
+            draw(state);
+        } else {
+            auto s2 = s + ANSI_NORMAL "\n";
+            if (!isTTY) s2 = filterANSIEscapes(s2, true);
+            writeToStderr(s2);
+        }
+    }
+
+    void startActivity(ActivityId act, Verbosity lvl, ActivityType type,
+        const std::string & s, const Fields & fields, ActivityId parent) override
+    {
+        auto state(state_.lock());
+
+        if (lvl <= verbosity && !s.empty() && type != actBuildWaiting)
+            log(*state, lvl, s + "...");
+
+        state->activities.emplace_back(ActInfo());
+        auto i = std::prev(state->activities.end());
+        i->s = s;
+        i->type = type;
+        i->parent = parent;
+        state->its.emplace(act, i);
+        state->activitiesByType[type].its.emplace(act, i);
+
+        if (type == actBuild) {
+            std::string name(storePathToName(getS(fields, 0)));
+            if (hasSuffix(name, ".drv"))
+                name = name.substr(0, name.size() - 4);
+            i->s = fmt("building " ANSI_BOLD "%s" ANSI_NORMAL, name);
+            auto machineName = getS(fields, 1);
+            if (machineName != "")
+                i->s += fmt(" on " ANSI_BOLD "%s" ANSI_NORMAL, machineName);
+            auto curRound = getI(fields, 2);
+            auto nrRounds = getI(fields, 3);
+            if (nrRounds != 1)
+                i->s += fmt(" (round %d/%d)", curRound, nrRounds);
+            i->name = DrvName(name).name;
+        }
+
+        if (type == actSubstitute) {
+            auto name = storePathToName(getS(fields, 0));
+            auto sub = getS(fields, 1);
+            i->s = fmt(
+                hasPrefix(sub, "local")
+                ? "copying " ANSI_BOLD "%s" ANSI_NORMAL " from %s"
+                : "fetching " ANSI_BOLD "%s" ANSI_NORMAL " from %s",
+                name, sub);
+        }
+
+        if (type == actPostBuildHook) {
+            auto name = storePathToName(getS(fields, 0));
+            if (hasSuffix(name, ".drv"))
+                name = name.substr(0, name.size() - 4);
+            i->s = fmt("post-build " ANSI_BOLD "%s" ANSI_NORMAL, name);
+            i->name = DrvName(name).name;
+        }
+
+        if (type == actQueryPathInfo) {
+            auto name = storePathToName(getS(fields, 0));
+            i->s = fmt("querying " ANSI_BOLD "%s" ANSI_NORMAL " on %s", name, getS(fields, 1));
+        }
+
+        if ((type == actFileTransfer && hasAncestor(*state, actCopyPath, parent))
+            || (type == actFileTransfer && hasAncestor(*state, actQueryPathInfo, parent))
+            || (type == actCopyPath && hasAncestor(*state, actSubstitute, parent)))
+            i->visible = false;
+
+        update(*state);
+    }
+
+    /* Check whether an activity has an ancestore with the specified
+       type. */
+    bool hasAncestor(State & state, ActivityType type, ActivityId act)
+    {
+        while (act != 0) {
+            auto i = state.its.find(act);
+            if (i == state.its.end()) break;
+            if (i->second->type == type) return true;
+            act = i->second->parent;
+        }
+        return false;
+    }
+
+    void stopActivity(ActivityId act) override
+    {
+        auto state(state_.lock());
+
+        auto i = state->its.find(act);
+        if (i != state->its.end()) {
+
+            auto & actByType = state->activitiesByType[i->second->type];
+            actByType.done += i->second->done;
+            actByType.failed += i->second->failed;
+
+            for (auto & j : i->second->expectedByType)
+                state->activitiesByType[j.first].expected -= j.second;
+
+            actByType.its.erase(act);
+            state->activities.erase(i->second);
+            state->its.erase(i);
+        }
+
+        update(*state);
+    }
+
+    void result(ActivityId act, ResultType type, const std::vector<Field> & fields) override
+    {
+        auto state(state_.lock());
+
+        if (type == resFileLinked) {
+            state->filesLinked++;
+            state->bytesLinked += getI(fields, 0);
+            update(*state);
+        }
+
+        else if (type == resBuildLogLine || type == resPostBuildLogLine) {
+            auto lastLine = trim(getS(fields, 0));
+            if (!lastLine.empty()) {
+                auto i = state->its.find(act);
+                assert(i != state->its.end());
+                ActInfo info = *i->second;
+                if (printBuildLogs) {
+                    auto suffix = "> ";
+                    if (type == resPostBuildLogLine) {
+                        suffix = " (post)> ";
+                    }
+                    log(*state, lvlInfo, ANSI_FAINT + info.name.value_or("unnamed") + suffix + ANSI_NORMAL + lastLine);
+                } else {
+                    state->activities.erase(i->second);
+                    info.lastLine = lastLine;
+                    state->activities.emplace_back(info);
+                    i->second = std::prev(state->activities.end());
+                    update(*state);
+                }
+            }
+        }
+
+        else if (type == resUntrustedPath) {
+            state->untrustedPaths++;
+            update(*state);
+        }
+
+        else if (type == resCorruptedPath) {
+            state->corruptedPaths++;
+            update(*state);
+        }
+
+        else if (type == resSetPhase) {
+            auto i = state->its.find(act);
+            assert(i != state->its.end());
+            i->second->phase = getS(fields, 0);
+            update(*state);
+        }
+
+        else if (type == resProgress) {
+            auto i = state->its.find(act);
+            assert(i != state->its.end());
+            ActInfo & actInfo = *i->second;
+            actInfo.done = getI(fields, 0);
+            actInfo.expected = getI(fields, 1);
+            actInfo.running = getI(fields, 2);
+            actInfo.failed = getI(fields, 3);
+            update(*state);
+        }
+
+        else if (type == resSetExpected) {
+            auto i = state->its.find(act);
+            assert(i != state->its.end());
+            ActInfo & actInfo = *i->second;
+            auto type = (ActivityType) getI(fields, 0);
+            auto & j = actInfo.expectedByType[type];
+            state->activitiesByType[type].expected -= j;
+            j = getI(fields, 1);
+            state->activitiesByType[type].expected += j;
+            update(*state);
+        }
+    }
+
+    void update(State & state)
+    {
+        state.haveUpdate = true;
+        updateCV.notify_one();
+    }
+
+    void draw(State & state)
+    {
+        state.haveUpdate = false;
+        if (!state.active) return;
+
+        std::string line;
+
+        std::string status = getStatus(state);
+        if (!status.empty()) {
+            line += '[';
+            line += status;
+            line += "]";
+        }
+
+        if (!state.activities.empty()) {
+            if (!status.empty()) line += " ";
+            auto i = state.activities.rbegin();
+
+            while (i != state.activities.rend() && (!i->visible || (i->s.empty() && i->lastLine.empty())))
+                ++i;
+
+            if (i != state.activities.rend()) {
+                line += i->s;
+                if (!i->phase.empty()) {
+                    line += " (";
+                    line += i->phase;
+                    line += ")";
+                }
+                if (!i->lastLine.empty()) {
+                    if (!i->s.empty()) line += ": ";
+                    line += i->lastLine;
+                }
+            }
+        }
+
+        auto width = getWindowSize().second;
+        if (width <= 0) width = std::numeric_limits<decltype(width)>::max();
+
+        writeToStderr("\r" + filterANSIEscapes(line, false, width) + "\e[K");
+    }
+
+    std::string getStatus(State & state)
+    {
+        auto MiB = 1024.0 * 1024.0;
+
+        std::string res;
+
+        auto renderActivity = [&](ActivityType type, const std::string & itemFmt, const std::string & numberFmt = "%d", double unit = 1) {
+            auto & act = state.activitiesByType[type];
+            uint64_t done = act.done, expected = act.done, running = 0, failed = act.failed;
+            for (auto & j : act.its) {
+                done += j.second->done;
+                expected += j.second->expected;
+                running += j.second->running;
+                failed += j.second->failed;
+            }
+
+            expected = std::max(expected, act.expected);
+
+            std::string s;
+
+            if (running || done || expected || failed) {
+                if (running)
+                    if (expected != 0)
+                        s = fmt(ANSI_BLUE + numberFmt + ANSI_NORMAL "/" ANSI_GREEN + numberFmt + ANSI_NORMAL "/" + numberFmt,
+                            running / unit, done / unit, expected / unit);
+                    else
+                        s = fmt(ANSI_BLUE + numberFmt + ANSI_NORMAL "/" ANSI_GREEN + numberFmt + ANSI_NORMAL,
+                            running / unit, done / unit);
+                else if (expected != done)
+                    if (expected != 0)
+                        s = fmt(ANSI_GREEN + numberFmt + ANSI_NORMAL "/" + numberFmt,
+                            done / unit, expected / unit);
+                    else
+                        s = fmt(ANSI_GREEN + numberFmt + ANSI_NORMAL, done / unit);
+                else
+                    s = fmt(done ? ANSI_GREEN + numberFmt + ANSI_NORMAL : numberFmt, done / unit);
+                s = fmt(itemFmt, s);
+
+                if (failed)
+                    s += fmt(" (" ANSI_RED "%d failed" ANSI_NORMAL ")", failed / unit);
+            }
+
+            return s;
+        };
+
+        auto showActivity = [&](ActivityType type, const std::string & itemFmt, const std::string & numberFmt = "%d", double unit = 1) {
+            auto s = renderActivity(type, itemFmt, numberFmt, unit);
+            if (s.empty()) return;
+            if (!res.empty()) res += ", ";
+            res += s;
+        };
+
+        showActivity(actBuilds, "%s built");
+
+        auto s1 = renderActivity(actCopyPaths, "%s copied");
+        auto s2 = renderActivity(actCopyPath, "%s MiB", "%.1f", MiB);
+
+        if (!s1.empty() || !s2.empty()) {
+            if (!res.empty()) res += ", ";
+            if (s1.empty()) res += "0 copied"; else res += s1;
+            if (!s2.empty()) { res += " ("; res += s2; res += ')'; }
+        }
+
+        showActivity(actFileTransfer, "%s MiB DL", "%.1f", MiB);
+
+        {
+            auto s = renderActivity(actOptimiseStore, "%s paths optimised");
+            if (s != "") {
+                s += fmt(", %.1f MiB / %d inodes freed", state.bytesLinked / MiB, state.filesLinked);
+                if (!res.empty()) res += ", ";
+                res += s;
+            }
+        }
+
+        // FIXME: don't show "done" paths in green.
+        showActivity(actVerifyPaths, "%s paths verified");
+
+        if (state.corruptedPaths) {
+            if (!res.empty()) res += ", ";
+            res += fmt(ANSI_RED "%d corrupted" ANSI_NORMAL, state.corruptedPaths);
+        }
+
+        if (state.untrustedPaths) {
+            if (!res.empty()) res += ", ";
+            res += fmt(ANSI_RED "%d untrusted" ANSI_NORMAL, state.untrustedPaths);
+        }
+
+        return res;
+    }
+
+    void writeToStdout(std::string_view s) override
+    {
+        auto state(state_.lock());
+        if (state->active) {
+            std::cerr << "\r\e[K";
+            Logger::writeToStdout(s);
+            draw(*state);
+        } else {
+            Logger::writeToStdout(s);
+        }
+    }
+};
+
+Logger * makeProgressBar(bool printBuildLogs)
+{
+    return new ProgressBar(
+        printBuildLogs,
+        isatty(STDERR_FILENO) && getEnv("TERM").value_or("dumb") != "dumb"
+    );
+}
+
+void startProgressBar(bool printBuildLogs)
+{
+    logger = makeProgressBar(printBuildLogs);
+}
+
+void stopProgressBar()
+{
+    auto progressBar = dynamic_cast<ProgressBar *>(logger);
+    if (progressBar) progressBar->stop();
+
+}
+
+}
diff --git a/src/libmain/progress-bar.hh b/src/libmain/progress-bar.hh
new file mode 100644
index 000000000..7f0dafecf
--- /dev/null
+++ b/src/libmain/progress-bar.hh
@@ -0,0 +1,13 @@
+#pragma once
+
+#include "logging.hh"
+
+namespace nix {
+
+Logger * makeProgressBar(bool printBuildLogs = false);
+
+void startProgressBar(bool printBuildLogs = false);
+
+void stopProgressBar();
+
+}
diff --git a/src/libmain/shared.cc b/src/libmain/shared.cc
index d41e772e9..dc6d5e413 100644
--- a/src/libmain/shared.cc
+++ b/src/libmain/shared.cc
@@ -2,6 +2,7 @@
 #include "shared.hh"
 #include "store-api.hh"
 #include "util.hh"
+#include "loggers.hh"
 
 #include <algorithm>
 #include <cctype>
@@ -75,7 +76,7 @@ string getArg(const string & opt,
     Strings::iterator & i, const Strings::iterator & end)
 {
     ++i;
-    if (i == end) throw UsageError(format("'%1%' requires an argument") % opt);
+    if (i == end) throw UsageError("'%1%' requires an argument", opt);
     return *i;
 }
 
@@ -155,7 +156,7 @@ void initNix()
        sshd). This breaks build users because they don't have access
        to the TMPDIR, in particular in ‘nix-store --serve’. */
 #if __APPLE__
-    if (getuid() == 0 && hasPrefix(getEnv("TMPDIR").value_or("/tmp"), "/var/folders/"))
+    if (hasPrefix(getEnv("TMPDIR").value_or("/tmp"), "/var/folders/"))
         unsetenv("TMPDIR");
 #endif
 }
@@ -165,28 +166,32 @@ LegacyArgs::LegacyArgs(const std::string & programName,
     std::function<bool(Strings::iterator & arg, const Strings::iterator & end)> parseArg)
     : MixCommonArgs(programName), parseArg(parseArg)
 {
-    mkFlag()
-        .longName("no-build-output")
-        .shortName('Q')
-        .description("do not show build output")
-        .set(&settings.verboseBuild, false);
-
-    mkFlag()
-        .longName("keep-failed")
-        .shortName('K')
-        .description("keep temporary directories of failed builds")
-        .set(&(bool&) settings.keepFailed, true);
-
-    mkFlag()
-        .longName("keep-going")
-        .shortName('k')
-        .description("keep going after a build fails")
-        .set(&(bool&) settings.keepGoing, true);
-
-    mkFlag()
-        .longName("fallback")
-        .description("build from source if substitution fails")
-        .set(&(bool&) settings.tryFallback, true);
+    addFlag({
+        .longName = "no-build-output",
+        .shortName = 'Q',
+        .description = "do not show build output",
+        .handler = {[&]() {setLogFormat(LogFormat::raw); }},
+    });
+
+    addFlag({
+        .longName = "keep-failed",
+        .shortName ='K',
+        .description = "keep temporary directories of failed builds",
+        .handler = {&(bool&) settings.keepFailed, true},
+    });
+
+    addFlag({
+        .longName = "keep-going",
+        .shortName ='k',
+        .description = "keep going after a build fails",
+        .handler = {&(bool&) settings.keepGoing, true},
+    });
+
+    addFlag({
+        .longName = "fallback",
+        .description = "build from source if substitution fails",
+        .handler = {&(bool&) settings.tryFallback, true},
+    });
 
     auto intSettingAlias = [&](char shortName, const std::string & longName,
         const std::string & description, const std::string & dest) {
@@ -205,11 +210,12 @@ LegacyArgs::LegacyArgs(const std::string & programName,
     mkFlag(0, "no-gc-warning", "disable warning about not using '--add-root'",
         &gcWarning, false);
 
-    mkFlag()
-        .longName("store")
-        .label("store-uri")
-        .description("URI of the Nix store to use")
-        .dest(&(std::string&) settings.storeUri);
+    addFlag({
+        .longName = "store",
+        .description = "URI of the Nix store to use",
+        .labels = {"store-uri"},
+        .handler = {&(std::string&) settings.storeUri},
+    });
 }
 
 
@@ -229,7 +235,7 @@ bool LegacyArgs::processArgs(const Strings & args, bool finish)
     Strings ss(args);
     auto pos = ss.begin();
     if (!parseArg(pos, ss.end()))
-        throw UsageError(format("unexpected argument '%1%'") % args.front());
+        throw UsageError("unexpected argument '%1%'", args.front());
     return true;
 }
 
@@ -260,7 +266,10 @@ void printVersion(const string & programName)
         cfg.push_back("signed-caches");
 #endif
         std::cout << "Features: " << concatStringsSep(", ", cfg) << "\n";
-        std::cout << "Configuration file: " << settings.nixConfDir + "/nix.conf" << "\n";
+        std::cout << "System configuration file: " << settings.nixConfDir + "/nix.conf" << "\n";
+        std::cout << "User configuration files: " <<
+            concatStringsSep(":", settings.nixUserConfFiles)
+            << "\n";
         std::cout << "Store directory: " << settings.nixStore << "\n";
         std::cout << "State directory: " << settings.nixStateDir << "\n";
     }
@@ -273,7 +282,7 @@ void showManPage(const string & name)
     restoreSignals();
     setenv("MANPATH", settings.nixManDir.c_str(), 1);
     execlp("man", "man", name.c_str(), nullptr);
-    throw SysError(format("command 'man %1%' failed") % name.c_str());
+    throw SysError("command 'man %1%' failed", name.c_str());
 }
 
 
@@ -281,6 +290,8 @@ int handleExceptions(const string & programName, std::function<void()> fun)
 {
     ReceiveInterrupts receiveInterrupts; // FIXME: need better place for this
 
+    ErrorInfo::programName = baseNameOf(programName);
+
     string error = ANSI_RED "error:" ANSI_NORMAL " ";
     try {
         try {
@@ -296,12 +307,13 @@ int handleExceptions(const string & programName, std::function<void()> fun)
     } catch (Exit & e) {
         return e.status;
     } catch (UsageError & e) {
-        printError(
-            format(error + "%1%\nTry '%2% --help' for more information.")
-            % e.what() % programName);
+        logError(e.info());
+        printError("Try '%1% --help' for more information.", programName);
         return 1;
     } catch (BaseError & e) {
-        printError(format(error + "%1%%2%") % (settings.showTrace ? e.prefix() : "") % e.msg());
+        if (settings.showTrace && e.prefix() != "")
+            printError(e.prefix());
+        logError(e.info());
         if (e.prefix() != "" && !settings.showTrace)
             printError("(use '--show-trace' to show detailed location information)");
         return e.status;
@@ -338,7 +350,7 @@ RunPager::RunPager()
         execlp("pager", "pager", nullptr);
         execlp("less", "less", nullptr);
         execlp("more", "more", nullptr);
-        throw SysError(format("executing '%1%'") % pager);
+        throw SysError("executing '%1%'", pager);
     });
 
     pid.setKillSignal(SIGINT);
diff --git a/src/libmain/shared.hh b/src/libmain/shared.hh
index b49574652..f558247c0 100644
--- a/src/libmain/shared.hh
+++ b/src/libmain/shared.hh
@@ -56,7 +56,7 @@ template<class N> N getIntArg(const string & opt,
     Strings::iterator & i, const Strings::iterator & end, bool allowUnit)
 {
     ++i;
-    if (i == end) throw UsageError(format("'%1%' requires an argument") % opt);
+    if (i == end) throw UsageError("'%1%' requires an argument", opt);
     string s = *i;
     N multiplier = 1;
     if (allowUnit && !s.empty()) {
@@ -66,13 +66,13 @@ template<class N> N getIntArg(const string & opt,
             else if (u == 'M') multiplier = 1ULL << 20;
             else if (u == 'G') multiplier = 1ULL << 30;
             else if (u == 'T') multiplier = 1ULL << 40;
-            else throw UsageError(format("invalid unit specifier '%1%'") % u);
+            else throw UsageError("invalid unit specifier '%1%'", u);
             s.resize(s.size() - 1);
         }
     }
     N n;
     if (!string2Int(s, n))
-        throw UsageError(format("'%1%' requires an integer argument") % opt);
+        throw UsageError("'%1%' requires an integer argument", opt);
     return n * multiplier;
 }
 
diff --git a/src/libmain/stack.cc b/src/libmain/stack.cc
index e6224de7d..b0a4a4c5d 100644
--- a/src/libmain/stack.cc
+++ b/src/libmain/stack.cc
@@ -1,4 +1,4 @@
-#include "types.hh"
+#include "error.hh"
 
 #include <cstring>
 #include <cstddef>