Add "nix verify-paths" command

Unlike "nix-store --verify-path", this command verifies signatures in
addition to store path contents, is multi-threaded (especially useful
when verifying binary caches), and has a progress indicator.

Example use:

$ nix verify-paths --store https://cache.nixos.org -r $(type -p thunderbird)
...
[17/132 checked] checking ‘/nix/store/rawakphadqrqxr6zri2rmnxh03gqkrl3-autogen-5.18.6’

5 files changed, 283 insertions, 0 deletions

diff --git a/src/nix/command.cc b/src/nix/command.cc
index 0c2103392f8e..a89246a937c1 100644
--- a/src/nix/command.cc
+++ b/src/nix/command.cc
@@ -69,4 +69,25 @@ void StoreCommand::run()
     run(openStoreAt(storeUri));
 }
 
+StorePathsCommand::StorePathsCommand()
+{
+    expectArgs("paths", &storePaths);
+    mkFlag('r', "recursive", "apply operation to closure of the specified paths", &recursive);
+}
+
+void StorePathsCommand::run(ref<Store> store)
+{
+    for (auto & storePath : storePaths)
+        storePath = followLinksToStorePath(storePath);
+
+    if (recursive) {
+        PathSet closure;
+        for (auto & storePath : storePaths)
+            store->computeFSClosure(storePath, closure, false, false);
+        storePaths = store->topoSortPaths(closure);
+    }
+
+    run(store, storePaths);
+}
+
 }
diff --git a/src/nix/command.hh b/src/nix/command.hh
index 5bf391d9367f..8397244ca177 100644
--- a/src/nix/command.hh
+++ b/src/nix/command.hh
@@ -24,6 +24,23 @@ struct StoreCommand : virtual Command
     virtual void run(ref<Store>) = 0;
 };
 
+/* A command that operates on zero or more store paths. */
+struct StorePathsCommand : public StoreCommand
+{
+private:
+
+    Paths storePaths;
+    bool recursive = false;
+
+public:
+
+    StorePathsCommand();
+
+    virtual void run(ref<Store> store, Paths storePaths) = 0;
+
+    void run(ref<Store> store) override;
+};
+
 typedef std::map<std::string, ref<Command>> Commands;
 
 /* An argument parser that supports multiple subcommands,
diff --git a/src/nix/progress-bar.cc b/src/nix/progress-bar.cc
new file mode 100644
index 000000000000..ed7b578e2f49
--- /dev/null
+++ b/src/nix/progress-bar.cc
@@ -0,0 +1,72 @@
+#include "progress-bar.hh"
+
+#include <iostream>
+
+namespace nix {
+
+ProgressBar::ProgressBar()
+{
+    _writeToStderr = [&](const unsigned char * buf, size_t count) {
+        auto state_(state.lock());
+        assert(!state_->done);
+        std::cerr << "\r\e[K" << std::string((const char *) buf, count);
+        render(*state_);
+    };
+}
+
+ProgressBar::~ProgressBar()
+{
+    done();
+}
+
+void ProgressBar::updateStatus(const std::string & s)
+{
+    auto state_(state.lock());
+    assert(!state_->done);
+    state_->status = s;
+    render(*state_);
+}
+
+void ProgressBar::done()
+{
+    auto state_(state.lock());
+    assert(state_->activities.empty());
+    state_->done = true;
+    std::cerr << "\r\e[K";
+    std::cerr.flush();
+    _writeToStderr = decltype(_writeToStderr)();
+}
+
+void ProgressBar::render(State & state_)
+{
+    std::cerr << '\r' << state_.status;
+    if (!state_.activities.empty()) {
+        if (!state_.status.empty()) std::cerr << ' ';
+        std::cerr << *state_.activities.rbegin();
+    }
+    std::cerr << "\e[K";
+    std::cerr.flush();
+}
+
+
+ProgressBar::Activity ProgressBar::startActivity(const FormatOrString & fs)
+{
+    return Activity(*this, fs);
+}
+
+ProgressBar::Activity::Activity(ProgressBar & pb, const FormatOrString & fs)
+    : pb(pb)
+{
+    auto state_(pb.state.lock());
+    state_->activities.push_back(fs.s);
+    it = state_->activities.end(); --it;
+    pb.render(*state_);
+}
+
+ProgressBar::Activity::~Activity()
+{
+    auto state_(pb.state.lock());
+    state_->activities.erase(it);
+}
+
+}
diff --git a/src/nix/progress-bar.hh b/src/nix/progress-bar.hh
new file mode 100644
index 000000000000..2dda24346c90
--- /dev/null
+++ b/src/nix/progress-bar.hh
@@ -0,0 +1,49 @@
+#pragma once
+
+#include "sync.hh"
+#include "util.hh"
+
+namespace nix {
+
+class ProgressBar
+{
+private:
+    struct State
+    {
+        std::string status;
+        bool done = false;
+        std::list<std::string> activities;
+    };
+
+    Sync<State> state;
+
+public:
+
+    ProgressBar();
+
+    ~ProgressBar();
+
+    void updateStatus(const std::string & s);
+
+    void done();
+
+    class Activity
+    {
+        friend class ProgressBar;
+    private:
+        ProgressBar & pb;
+        std::list<std::string>::iterator it;
+        Activity(ProgressBar & pb, const FormatOrString & fs);
+    public:
+        ~Activity();
+    };
+
+    Activity startActivity(const FormatOrString & fs);
+
+private:
+
+    void render(State & state_);
+
+};
+
+}
diff --git a/src/nix/verify.cc b/src/nix/verify.cc
new file mode 100644
index 000000000000..ef3e9fcc27ff
--- /dev/null
+++ b/src/nix/verify.cc
@@ -0,0 +1,124 @@
+#include "affinity.hh" // FIXME
+#include "command.hh"
+#include "progress-bar.hh"
+#include "shared.hh"
+#include "store-api.hh"
+#include "sync.hh"
+#include "thread-pool.hh"
+
+#include <atomic>
+
+using namespace nix;
+
+struct CmdVerifyPaths : StorePathsCommand
+{
+    bool noContents = false;
+    bool noSigs = false;
+
+    CmdVerifyPaths()
+    {
+        mkFlag(0, "no-contents", "do not verify the contents of each store path", &noContents);
+        mkFlag(0, "no-sigs", "do not verify whether each store path has a valid signature", &noSigs);
+    }
+
+    std::string name() override
+    {
+        return "verify-paths";
+    }
+
+    std::string description() override
+    {
+        return "verify the integrity of store paths";
+    }
+
+    void run(ref<Store> store, Paths storePaths) override
+    {
+        restoreAffinity(); // FIXME
+
+        auto publicKeys = getDefaultPublicKeys();
+
+        std::atomic<size_t> untrusted{0};
+        std::atomic<size_t> corrupted{0};
+        std::atomic<size_t> done{0};
+        std::atomic<size_t> failed{0};
+
+        ProgressBar progressBar;
+
+        auto showProgress = [&](bool final) {
+            std::string s;
+            if (final)
+                s = (format("checked %d paths") % storePaths.size()).str();
+            else
+                s = (format("[%d/%d checked") % done % storePaths.size()).str();
+            if (corrupted > 0)
+                s += (format(", %d corrupted") % corrupted).str();
+            if (untrusted > 0)
+                s += (format(", %d untrusted") % untrusted).str();
+            if (failed > 0)
+                s += (format(", %d failed") % failed).str();
+            if (!final) s += "]";
+            return s;
+        };
+
+        progressBar.updateStatus(showProgress(false));
+
+        ThreadPool pool;
+
+        auto doPath = [&](const Path & storePath) {
+            try {
+                progressBar.startActivity(format("checking ‘%s’") % storePath);
+
+                auto info = store->queryPathInfo(storePath);
+
+                if (!noContents) {
+
+                    HashSink sink(info.narHash.type);
+                    store->narFromPath(storePath, sink);
+
+                    auto hash = sink.finish();
+
+                    if (hash.first != info.narHash) {
+                        corrupted = 1;
+                        printMsg(lvlError,
+                            format("path ‘%s’ was modified! expected hash ‘%s’, got ‘%s’")
+                            % storePath % printHash(info.narHash) % printHash(hash.first));
+                    }
+
+                }
+
+                if (!noSigs) {
+
+                    if (!info.ultimate && !info.checkSignatures(publicKeys)) {
+                        untrusted++;
+                        printMsg(lvlError, format("path ‘%s’ is untrusted") % storePath);
+                    }
+
+                }
+
+                done++;
+
+                progressBar.updateStatus(showProgress(false));
+
+            } catch (Error & e) {
+                printMsg(lvlError, format(ANSI_RED "error:" ANSI_NORMAL " %s") % e.what());
+                failed++;
+            }
+        };
+
+        for (auto & storePath : storePaths)
+            pool.enqueue(std::bind(doPath, storePath));
+
+        pool.process();
+
+        progressBar.done();
+
+        printMsg(lvlInfo, showProgress(true));
+
+        throw Exit(
+            (corrupted ? 1 : 0) |
+            (untrusted ? 2 : 0) |
+            (failed ? 4 : 0));
+    }
+};
+
+static RegisterCommand r1(make_ref<CmdVerifyPaths>());