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’

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>());