LibDiff+patch: Support multiple patches in a single patch file

Multiple patches may be concatenated in the same patch file, such as git
commits which are changing multiple files at the same time. To handle
this, parse each patch in order in the patch file, and apply each patch
sequentially.

To determine whether we are at the end of a patch (and not just parsing
another hunk) the parser will look for a leading '@@ ' after every hunk.
If that is found, there is another hunk. Otherwise, we must be at the
end of this patch.

Tests/Utilities/TestPatch.cpp

@@ -165,3 +165,25 @@ TEST_CASE(add_file_from_scratch)
 
     EXPECT_FILE_EQ(MUST(String::formatted("{}/file_to_add", s_test_dir)), "Hello, friends!\n");
 }
+
+TEST_CASE(two_patches_in_single_patch_file)
+{
+    PatchSetup setup;
+
+    auto patch = R"(
+--- /dev/null
++++ a/first_file_to_add
+@@ -0,0 +1 @@
++Hello, friends!
+--- /dev/null
++++ a/second_file_to_add
+@@ -0,0 +1 @@
++Hello, friends!
+)"sv;
+
+    run_patch({}, patch, "patching file first_file_to_add\n"
+                         "patching file second_file_to_add\n"sv);
+
+    EXPECT_FILE_EQ(MUST(String::formatted("{}/first_file_to_add", s_test_dir)), "Hello, friends!\n");
+    EXPECT_FILE_EQ(MUST(String::formatted("{}/second_file_to_add", s_test_dir)), "Hello, friends!\n");
+}

Userland/Libraries/LibDiff/Hunks.cpp

@@ -87,6 +87,14 @@ ErrorOr<String> Parser::parse_file_line(Optional<size_t> const& strip_count)
     return stripped_path.to_string();
 }
 
+ErrorOr<Patch> Parser::parse_patch(Optional<size_t> const& strip_count)
+{
+    Patch patch;
+    patch.header = TRY(parse_header(strip_count));
+    patch.hunks = TRY(parse_hunks());
+    return patch;
+}
+
 ErrorOr<Header> Parser::parse_header(Optional<size_t> const& strip_count)
 {
     Header header;
@@ -111,20 +119,20 @@ ErrorOr<Header> Parser::parse_header(Optional<size_t> const& strip_count)
         consume_line();
     }
 
-    return Error::from_string_literal("Unable to find any patch");
+    return header;
 }
 
 ErrorOr<Vector<Hunk>> Parser::parse_hunks()
 {
     Vector<Hunk> hunks;
 
-    while (!is_eof()) {
+    while (next_is("@@ ")) {
         // Try an locate a hunk location in this hunk. It may be prefixed with information.
         auto maybe_location = consume_unified_location();
         consume_line();
 
         if (!maybe_location.has_value())
-            continue;
+            break;
 
         Hunk hunk { *maybe_location, {} };
 
@@ -178,6 +186,8 @@ ErrorOr<Vector<Hunk>> Parser::parse_hunks()
 ErrorOr<Vector<Hunk>> parse_hunks(StringView diff)
 {
     Parser lexer(diff);
+    while (!lexer.next_is("@@ ") && !lexer.is_eof())
+        lexer.consume_line();
     return lexer.parse_hunks();
 }
 }

Userland/Libraries/LibDiff/Hunks.h

@@ -80,11 +80,13 @@ class Parser : public GenericLexer {
 public:
     using GenericLexer::GenericLexer;
 
+    ErrorOr<Patch> parse_patch(Optional<size_t> const& strip_count = {});
+
     ErrorOr<Vector<Hunk>> parse_hunks();
 
+private:
     ErrorOr<Header> parse_header(Optional<size_t> const& strip_count);
 
-private:
     ErrorOr<String> parse_file_line(Optional<size_t> const& strip_count);
     Optional<HunkLocation> consume_unified_location();
     bool consume_line_number(size_t& number);

Userland/Utilities/patch.cpp

@@ -65,25 +65,28 @@ ErrorOr<int> serenity_main(Main::Arguments arguments)
 
     auto patch_content = TRY(input->read_until_eof());
 
-    // FIXME: Support multiple patches in the patch file.
     Diff::Parser parser(patch_content);
-    Diff::Patch patch;
-    patch.header = TRY(parser.parse_header(strip_count));
-    patch.hunks = TRY(parser.parse_hunks());
 
-    // FIXME: Support adding/removing a file, and asking for file to patch as fallback otherwise.
-    StringView to_patch;
-    if (FileSystem::is_regular_file(patch.header.old_file_path)) {
-        to_patch = patch.header.old_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");
-        return 1;
+    while (!parser.is_eof()) {
+        Diff::Patch patch = TRY(parser.parse_patch(strip_count));
+
+        if (patch.header.format == Diff::Format::Unknown)
+            break;
+
+        // FIXME: Support adding/removing a file, and asking for file to patch as fallback otherwise.
+        StringView to_patch;
+        if (FileSystem::is_regular_file(patch.header.old_file_path)) {
+            to_patch = patch.header.old_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");
+            return 1;
+        }
+
+        outln("patching file {}", to_patch);
+        TRY(do_patch(to_patch, patch));
     }
 
-    outln("patching file {}", to_patch);
-    TRY(do_patch(to_patch, patch));
-
     return 0;
 }