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 #3920.

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)