fetchTree: Support applying patches

You can now write

  fetchTree {
    type = "github";
    owner = "NixOS";
    repo = "nixpkgs";
    rev = "0f316e4d72daed659233817ffe52bf08e081b5de";
    patches = [ ./thunderbird-1.patch ./thunderbird-2.patch ];
  };

to apply a list of patches to a tree. These are applied lazily - the
patched tree is not materialized unless you do something that causes
the entire tree to be copied to the store (like 'src = fetchTree {
... }'). The equivalent of '-p1' is implied.

File additions/deletions/renames are not yet handled.

Issue NixOS#3920.

src/libexpr/primops/fetchTree.cc

@@ -105,6 +105,7 @@ static void fetchTree(
 ) {
     fetchers::Input input;
     PathSet context;
+    std::vector<std::string> patches;
 
     state.forceValue(*args[0], pos);
 
@@ -130,7 +131,22 @@ static void fetchTree(
 
         for (auto & attr : *args[0]->attrs) {
             if (attr.name == state.sType) continue;
+
+            if (attr.name == "patches") {
+                state.forceList(*attr.value, *attr.pos);
+
+                for (auto elem : attr.value->listItems()) {
+                    // FIXME: use realisePath
+                    PathSet context;
+                    auto patchFile = state.unpackPath(state.coerceToPath(pos, *elem, context));
+                    patches.push_back(patchFile.accessor->readFile(patchFile.path));
+                }
+
+                continue;
+            }
+
             state.forceValue(*attr.value, *attr.pos);
+
             if (attr.value->type() == nPath || attr.value->type() == nString) {
                 auto s = state.coerceToString(*attr.pos, *attr.value, context, false, false).toOwned();
                 attrs.emplace(attr.name,
@@ -178,6 +194,9 @@ static void fetchTree(
 
     auto [accessor, input2] = input.lazyFetch(state.store);
 
+    if (!patches.empty())
+        accessor = makePatchingInputAccessor(accessor, patches);
+
     //state.allowPath(tree.storePath);
 
     emitTreeAttrs(

src/libfetchers/input-accessor.hh

@@ -57,6 +57,10 @@ ref<MemoryInputAccessor> makeMemoryInputAccessor();
 
 ref<InputAccessor> makeZipInputAccessor(PathView path);
 
+ref<InputAccessor> makePatchingInputAccessor(
+    ref<InputAccessor> next,
+    const std::vector<std::string> & patches);
+
 struct SourcePath
 {
     ref<InputAccessor> accessor;

src/libfetchers/patching-input-accessor.cc

@@ -0,0 +1,115 @@
+#include "input-accessor.hh"
+
+namespace nix {
+
+// TODO: handle file creation / deletion.
+struct PatchingInputAccessor : InputAccessor
+{
+    ref<InputAccessor> next;
+
+    std::map<Path, std::vector<std::string>> patchesPerFile;
+
+    PatchingInputAccessor(
+        ref<InputAccessor> next,
+        const std::vector<std::string> & patches)
+        : next(next)
+    {
+        /* Extract the patches for each file. */
+        for (auto & patch : patches) {
+            std::string_view p = patch;
+            std::string_view start;
+            std::string_view fileName;
+
+            auto flush = [&]()
+            {
+                if (start.empty()) return;
+                auto contents = start.substr(0, p.data() - start.data());
+                start = "";
+                auto slash = fileName.find('/');
+                if (slash == fileName.npos) return;
+                fileName = fileName.substr(slash);
+                debug("found patch for '%s'", fileName);
+                patchesPerFile.emplace(Path(fileName), std::vector<std::string>())
+                    .first->second.push_back(std::string(contents));
+            };
+
+            while (!p.empty()) {
+                auto [line, rest] = getLine(p);
+
+                if (hasPrefix(line, "--- ")) {
+                    flush();
+                    start = p;
+                    fileName = line.substr(4);
+                }
+
+                if (!start.empty()) {
+                    if (!(hasPrefix(line, "+++ ")
+                            || hasPrefix(line, "@@")
+                            || hasPrefix(line, "+")
+                            || hasPrefix(line, "-")
+                            || hasPrefix(line, " ")))
+                    {
+                        flush();
+                    }
+                }
+
+                p = rest;
+            }
+
+            flush();
+        }
+    }
+
+    std::string readFile(PathView path) override
+    {
+        auto contents = next->readFile(path);
+
+        auto i = patchesPerFile.find((Path) path);
+        if (i != patchesPerFile.end()) {
+            for (auto & patch : i->second) {
+                auto tempDir = createTempDir();
+                AutoDelete del(tempDir);
+                auto sourceFile = tempDir + "/source";
+                auto rejFile = tempDir + "/source.rej";
+                writeFile(sourceFile, contents);
+                try {
+                    contents = runProgram("patch", true, {"--quiet", sourceFile, "--output=-", "-r", rejFile}, patch);
+                } catch (ExecError & e) {
+                    del.cancel();
+                    throw;
+                }
+            }
+        }
+
+        return contents;
+    }
+
+    bool pathExists(PathView path) override
+    {
+        return next->pathExists(path);
+    }
+
+    Stat lstat(PathView path) override
+    {
+        return next->lstat(path);
+    }
+
+    DirEntries readDirectory(PathView path) override
+    {
+        return next->readDirectory(path);
+    }
+
+    std::string readLink(PathView path) override
+    {
+        return next->readLink(path);
+    }
+};
+
+ref<InputAccessor> makePatchingInputAccessor(
+    ref<InputAccessor> next,
+    const std::vector<std::string> & patches)
+{
+    return make_ref<PatchingInputAccessor>(next, std::move(patches));
+}
+
+}

src/libutil/tests/tests.cc

@@ -311,6 +311,42 @@ namespace nix {
         ASSERT_THROW(base64Decode("cXVvZCBlcm_0IGRlbW9uc3RyYW5kdW0="), Error);
     }
 
+    /* ----------------------------------------------------------------------------
+     * getLine
+     * --------------------------------------------------------------------------*/
+
+    TEST(getLine, all) {
+        {
+            auto [line, rest] = getLine("foo\nbar\nxyzzy");
+            ASSERT_EQ(line, "foo");
+            ASSERT_EQ(rest, "bar\nxyzzy");
+        }
+
+        {
+            auto [line, rest] = getLine("foo\r\nbar\r\nxyzzy");
+            ASSERT_EQ(line, "foo");
+            ASSERT_EQ(rest, "bar\r\nxyzzy");
+        }
+
+        {
+            auto [line, rest] = getLine("foo\n");
+            ASSERT_EQ(line, "foo");
+            ASSERT_EQ(rest, "");
+        }
+
+        {
+            auto [line, rest] = getLine("foo");
+            ASSERT_EQ(line, "foo");
+            ASSERT_EQ(rest, "");
+        }
+
+        {
+            auto [line, rest] = getLine("");
+            ASSERT_EQ(line, "");
+            ASSERT_EQ(rest, "");
+        }
+    }
+
     /* ----------------------------------------------------------------------------
      * toLower
      * --------------------------------------------------------------------------*/

src/libutil/util.cc

@@ -1564,6 +1564,21 @@ std::string stripIndentation(std::string_view s)
 }
 
 
+std::pair<std::string_view, std::string_view> getLine(std::string_view s)
+{
+    auto newline = s.find('\n');
+
+    if (newline == s.npos) {
+        return {s, ""};
+    } else {
+        auto line = s.substr(0, newline);
+        if (!line.empty() && line[line.size() - 1] == '\r')
+            line = line.substr(0, line.size() - 1);
+        return {line, s.substr(newline + 1)};
+    }
+}
+
+
 //////////////////////////////////////////////////////////////////////
 
 

src/libutil/util.hh

@@ -537,6 +537,12 @@ std::string base64Decode(std::string_view s);
 std::string stripIndentation(std::string_view s);
 
 
+/* Get the prefix of 's' up to and excluding the next line break (LF
+   optionally preceded by CR), and the remainder following the line
+   break. */
+std::pair<std::string_view, std::string_view> getLine(std::string_view s);
+
+
 /* Get a value for the specified key from an associate container. */
 template <class T>
 std::optional<typename T::mapped_type> get(const T & map, const typename T::key_type & key)