1 files changed, 265 insertions, 0 deletions
diff --git a/third_party/nix/src/nix/why-depends.cc b/third_party/nix/src/nix/why-depends.cc
new file mode 100644
index 000000000000..bbbfd1fe0182
--- /dev/null
+++ b/third_party/nix/src/nix/why-depends.cc
@@ -0,0 +1,265 @@
+#include <queue>
+
+#include <glog/logging.h>
+
+#include "command.hh"
+#include "fs-accessor.hh"
+#include "shared.hh"
+#include "store-api.hh"
+
+using namespace nix;
+
+static std::string hilite(const std::string& s, size_t pos, size_t len,
+                          const std::string& colour = ANSI_RED) {
+  return std::string(s, 0, pos) + colour + std::string(s, pos, len) +
+         ANSI_NORMAL + std::string(s, pos + len);
+}
+
+static std::string filterPrintable(const std::string& s) {
+  std::string res;
+  for (char c : s) {
+    res += isprint(c) != 0 ? c : '.';
+  }
+  return res;
+}
+
+struct CmdWhyDepends : SourceExprCommand {
+  std::string _package, _dependency;
+  bool all = false;
+
+  CmdWhyDepends() {
+    expectArg("package", &_package);
+    expectArg("dependency", &_dependency);
+
+    mkFlag()
+        .longName("all")
+        .shortName('a')
+        .description(
+            "show all edges in the dependency graph leading from 'package' to "
+            "'dependency', rather than just a shortest path")
+        .set(&all, true);
+  }
+
+  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 one path through the dependency graph leading from "
+                "Hello to Glibc:",
+                "nix why-depends nixpkgs.hello nixpkgs.glibc"},
+        Example{
+            "To show all files and paths in the dependency graph leading from "
+            "Thunderbird to libX11:",
+            "nix why-depends --all nixpkgs.thunderbird nixpkgs.xorg.libX11"},
+        Example{"To show why Glibc depends on itself:",
+                "nix why-depends nixpkgs.glibc 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) == 0u) {
+      LOG(WARNING) << "'" << package->what() << "' does not depend on '"
+                   << dependency->what() << "'";
+      return;
+    }
+
+    auto accessor = store->getFSAccessor();
+
+    auto const inf = std::numeric_limits<size_t>::max();
+
+    struct Node {
+      Path path;
+      PathSet refs;
+      PathSet rrefs;
+      size_t dist = inf;
+      Node* prev = nullptr;
+      bool queued = false;
+      bool visited = false;
+    };
+
+    std::map<Path, Node> graph;
+
+    for (auto& path : closure) {
+      graph.emplace(path, Node{path, store->queryPathInfo(path)->references});
+    }
+
+    // Transpose the graph.
+    for (auto& node : graph) {
+      for (auto& ref : node.second.refs) {
+        graph[ref].rrefs.insert(node.first);
+      }
+    }
+
+    /* Run Dijkstra's shortest path algorithm to get the distance
+       of every path in the closure to 'dependency'. */
+    graph[dependencyPath].dist = 0;
+
+    std::priority_queue<Node*> queue;
+
+    queue.push(&graph.at(dependencyPath));
+
+    while (!queue.empty()) {
+      auto& node = *queue.top();
+      queue.pop();
+
+      for (auto& rref : node.rrefs) {
+        auto& node2 = graph.at(rref);
+        auto dist = node.dist + 1;
+        if (dist < node2.dist) {
+          node2.dist = dist;
+          node2.prev = &node;
+          if (!node2.queued) {
+            node2.queued = true;
+            queue.push(&node2);
+          }
+        }
+      }
+    }
+
+    /* Print the subgraph of nodes that have 'dependency' in their
+       closure (i.e., that have a non-infinite distance to
+       'dependency'). Print every edge on a path between `package`
+       and `dependency`. */
+    std::function<void(Node&, const string&, const string&)> printNode;
+
+    const string treeConn = "╠═══";
+    const string treeLast = "╚═══";
+    const string treeLine = "║   ";
+    const string treeNull = "    ";
+
+    struct BailOut {};
+
+    printNode = [&](Node& node, const string& firstPad, const string& tailPad) {
+      assert(node.dist != inf);
+      std::cout << fmt("%s%s%s%s" ANSI_NORMAL "\n", firstPad,
+                       node.visited ? "\e[38;5;244m" : "",
+                       !firstPad.empty() ? "=> " : "", node.path);
+
+      if (node.path == dependencyPath && !all &&
+          packagePath != dependencyPath) {
+        throw BailOut();
+      }
+
+      if (node.visited) {
+        return;
+      }
+      node.visited = true;
+
+      /* Sort the references by distance to `dependency` to
+         ensure that the shortest path is printed first. */
+      std::multimap<size_t, Node*> refs;
+      std::set<std::string> hashes;
+
+      for (auto& ref : node.refs) {
+        if (ref == node.path && packagePath != dependencyPath) {
+          continue;
+        }
+        auto& node2 = graph.at(ref);
+        if (node2.dist == inf) {
+          continue;
+        }
+        refs.emplace(node2.dist, &node2);
+        hashes.insert(storePathToHash(node2.path));
+      }
+
+      /* For each reference, find the files and symlinks that
+         contain the reference. */
+      std::map<std::string, Strings> hits;
+
+      std::function<void(const Path&)> visitPath;
+
+      visitPath = [&](const Path& p) {
+        auto st = accessor->stat(p);
+
+        auto p2 = p == node.path ? "/" : std::string(p, node.path.size() + 1);
+
+        auto getColour = [&](const std::string& hash) {
+          return hash == dependencyPathHash ? ANSI_GREEN : ANSI_BLUE;
+        };
+
+        if (st.type == FSAccessor::Type::tDirectory) {
+          auto names = accessor->readDirectory(p);
+          for (auto& name : names) {
+            visitPath(p + "/" + name);
+          }
+        }
+
+        else if (st.type == FSAccessor::Type::tRegular) {
+          auto contents = accessor->readFile(p);
+
+          for (auto& hash : hashes) {
+            auto pos = contents.find(hash);
+            if (pos != std::string::npos) {
+              size_t margin = 32;
+              auto pos2 = pos >= margin ? pos - margin : 0;
+              hits[hash].emplace_back(fmt(
+                  "%s: …%s…\n", p2,
+                  hilite(
+                      filterPrintable(std::string(
+                          contents, pos2, pos - pos2 + hash.size() + margin)),
+                      pos - pos2, storePathHashLen, getColour(hash))));
+            }
+          }
+        }
+
+        else if (st.type == FSAccessor::Type::tSymlink) {
+          auto target = accessor->readLink(p);
+
+          for (auto& hash : hashes) {
+            auto pos = target.find(hash);
+            if (pos != std::string::npos) {
+              hits[hash].emplace_back(
+                  fmt("%s -> %s\n", p2,
+                      hilite(target, pos, storePathHashLen, getColour(hash))));
+            }
+          }
+        }
+      };
+
+      // FIXME: should use scanForReferences().
+
+      visitPath(node.path);
+
+      RunPager pager;
+      for (auto& ref : refs) {
+        auto hash = storePathToHash(ref.second->path);
+
+        bool last = all ? ref == *refs.rbegin() : true;
+
+        for (auto& hit : hits[hash]) {
+          bool first = hit == *hits[hash].begin();
+          std::cout << tailPad
+                    << (first ? (last ? treeLast : treeConn)
+                              : (last ? treeNull : treeLine))
+                    << hit;
+          if (!all) {
+            break;
+          }
+        }
+
+        printNode(*ref.second, tailPad + (last ? treeNull : treeLine),
+                  tailPad + (last ? treeNull : treeLine));
+      }
+    };
+
+    try {
+      printNode(graph.at(packagePath), "", "");
+    } catch (BailOut&) {
+    }
+  }
+};
+
+static RegisterCommand r1(make_ref<CmdWhyDepends>());