Add command "nix why-depends"

This command shows why a package has another package in its runtime
closure. For example, to see why VLC has libdrm.dev in its closure:

  $ nix why-depends nixpkgs.vlc nixpkgs.libdrm.dev
  /nix/store/g901z9pcj0n5yy5n6ykxk3qm4ina1d6z-vlc-2.2.5.1:
    lib/libvlccore.so.8.0.0: …nfig:/nix/store/405lmx6jl8lp0ad1vrr6j498chrqhz8g-libdrm-2.4.75-d…

  /nix/store/s3nm7kd8hlcg0facn2q1ff2n7wrwdi2l-mesa-noglu-17.0.7-dev:
    nix-support/propagated-native-build-inputs: …-dev /nix/store/405lmx6jl8lp0ad1vrr6j498chrqhz8g-libdrm-2.4.75-d…

Thus, VLC's lib/libvlccore.so.8.0.0 as well as mesa-noglu's
nix-support/propagated-native-build-inputs cause the dependency.

src/nix/why-depends.cc

@@ -0,0 +1,133 @@
+#include "command.hh"
+#include "common-args.hh"
+#include "shared.hh"
+#include "store-api.hh"
+#include "progress-bar.hh"
+#include "fs-accessor.hh"
+
+using namespace nix;
+
+static std::string hilite(const std::string & s, size_t pos, size_t len)
+{
+    return
+        std::string(s, 0, pos)
+        + ANSI_RED
+        + std::string(s, pos, len)
+        + ANSI_NORMAL
+        + std::string(s, pos + len);
+}
+
+struct CmdWhyDepends : SourceExprCommand
+{
+    std::string _package, _dependency;
+
+    CmdWhyDepends()
+    {
+        expectArg("package", &_package);
+        expectArg("dependency", &_dependency);
+    }
+
+    std::string name() override
+    {
+        return "why-depends";
+    }
+
+    std::string description() override
+    {
+        return "show why a package has another package in its closure";
+    }
+
+    Examples examples() override
+    {
+        return {
+            Example{
+                "To show which files in Hello's closure depend on Glibc:",
+                "nix why-depends nixpkgs.hello nixpkgs.glibc"
+            },
+        };
+    }
+
+    void run(ref<Store> store) override
+    {
+        auto package = parseInstallable(*this, store, _package, false);
+        auto packagePath = toStorePath(store, Build, package);
+        auto dependency = parseInstallable(*this, store, _dependency, false);
+        auto dependencyPath = toStorePath(store, NoBuild, dependency);
+        auto dependencyPathHash = storePathToHash(dependencyPath);
+
+        PathSet closure;
+        store->computeFSClosure({packagePath}, closure, false, false);
+
+        if (!closure.count(dependencyPath)) {
+            printError("'%s' does not depend on '%s'", package->what(), dependency->what());
+            return;
+        }
+
+        stopProgressBar(); // FIXME
+
+        auto accessor = store->getFSAccessor();
+
+        // FIXME: show the path through the dependency graph.
+
+        bool first = true;
+
+        for (auto & path : closure) {
+
+            if (path == dependencyPath && packagePath != dependencyPath)
+                continue;
+
+            if (!store->queryPathInfo(path)->references.count(dependencyPath))
+                continue;
+
+            if (!first) std::cerr << "\n";
+            first = false;
+
+            std::cerr << fmt("%s:\n", path);
+
+            std::function<void(const Path &)> recurse;
+
+            recurse = [&](const Path & p) {
+                auto st = accessor->stat(p);
+
+                auto p2 = p == path ? "/" : std::string(p, path.size() + 1);
+
+                if (st.type == FSAccessor::Type::tDirectory) {
+                    auto names = accessor->readDirectory(p);
+                    for (auto & name : names)
+                        recurse(p + "/" + name);
+                }
+
+                else if (st.type == FSAccessor::Type::tRegular) {
+                    auto contents = accessor->readFile(p);
+                    auto pos = contents.find(dependencyPathHash);
+                    if (pos != std::string::npos) {
+                        size_t margin = 16;
+                        auto pos2 = pos >= margin ? pos - margin : 0;
+                        std::string fragment;
+                        for (char c : std::string(contents, pos2,
+                                pos - pos2 + dependencyPathHash.size() + margin))
+                        {
+                            fragment += isprint(c) ? c : '.';
+                        }
+
+                        std::cerr << fmt("  %s: …%s…\n",
+                            p2,
+                            hilite(fragment, pos - pos2, storePathHashLen));
+                    }
+                }
+
+                else if (st.type == FSAccessor::Type::tSymlink) {
+                    auto target = accessor->readLink(p);
+                    auto pos = target.find(dependencyPathHash);
+                    if (pos != std::string::npos) {
+                        std::cerr << fmt("  %s -> %s\n", p2, hilite(target, pos, storePathHashLen));
+                    }
+                }
+            };
+
+            recurse(path);
+        }
+    }
+};
+
+static RegisterCommand r1(make_ref<CmdWhyDepends>());