patch: Support creation of a file

Previously patch would always expect the file that it was patching to
exist (even it were empty). If we know that the patch is creating a file
from nothing (i.e has a start line of '0'), then we treat a file that
doesn't exist as if it has no content lines.

Author: shannonbooth

Tests/Utilities/TestPatch.cpp

@@ -149,3 +149,19 @@ TEST_CASE(strip_path_partially)
 
     EXPECT_FILE_EQ(MUST(String::formatted("{}/to/basename", s_test_dir)), "Hello, friends!\n");
 }
+
+TEST_CASE(add_file_from_scratch)
+{
+    PatchSetup setup;
+
+    auto patch = R"(
+--- /dev/null
++++ a/file_to_add
+@@ -0,0 +1 @@
++Hello, friends!
+)"sv;
+
+    run_patch({}, patch, "patching file file_to_add\n"sv);
+
+    EXPECT_FILE_EQ(MUST(String::formatted("{}/file_to_add", s_test_dir)), "Hello, friends!\n");
+}

Userland/Utilities/patch.cpp

@@ -12,10 +12,31 @@
 #include <LibFileSystem/FileSystem.h>
 #include <LibMain/Main.h>
 
+static bool is_adding_file(Diff::Patch const& patch)
+{
+    return patch.hunks[0].location.old_range.start_line == 0;
+}
+
+static ErrorOr<ByteBuffer> read_content(StringView path_of_file_to_patch, Diff::Patch const& patch)
+{
+    auto file_to_patch_or_error = Core::File::open(path_of_file_to_patch, Core::File::OpenMode::Read);
+
+    // Trivial case - no error reading the file.
+    if (!file_to_patch_or_error.is_error())
+        return TRY(file_to_patch_or_error.release_value()->read_until_eof());
+
+    auto const& error = file_to_patch_or_error.error();
+
+    // If the patch is adding a file then it is fine for opening the file to error out if it did not exist.
+    if (!is_adding_file(patch) || !error.is_errno() || error.code() != ENOENT)
+        return file_to_patch_or_error.release_error();
+
+    return ByteBuffer {};
+}
+
 static ErrorOr<void> do_patch(StringView path_of_file_to_patch, Diff::Patch const& patch)
 {
-    auto file_to_patch = TRY(Core::File::open(path_of_file_to_patch, Core::File::OpenMode::Read));
-    auto content = TRY(file_to_patch->read_until_eof());
+    ByteBuffer content = TRY(read_content(path_of_file_to_patch, patch));
     auto lines = StringView(content).lines();
 
     // Apply patch to a temporary file in case one or more of the hunks fails.
@@ -54,7 +75,7 @@ ErrorOr<int> serenity_main(Main::Arguments arguments)
     StringView to_patch;
     if (FileSystem::is_regular_file(patch.header.old_file_path)) {
         to_patch = patch.header.old_file_path;
-    } else if (FileSystem::is_regular_file(patch.header.new_file_path)) {
+    } else if (is_adding_file(patch) || FileSystem::is_regular_file(patch.header.new_file_path)) {
         to_patch = patch.header.new_file_path;
     } else {
         warnln("Unable to determine file to patch");