LibCore: Allow zero-size AnonymousBuffer creation

Previously, `AnonymousBuffer::create_with_size(0)` returned an error
because POSIX `mmap` rejects a zero length with `EINVAL`, and Windows
`CreateFileMapping` rejects a zero maximum size for an anonymous
mapping. This caused a crash when using `--headless=text` with zero
size pages like `about:blank`.

Libraries/LibCore/AnonymousBuffer.cpp

@@ -20,9 +20,13 @@ ErrorOr<AnonymousBuffer> AnonymousBuffer::create_with_size(size_t size)
 
 ErrorOr<NonnullRefPtr<AnonymousBufferImpl>> AnonymousBufferImpl::create(int fd, size_t size)
 {
-    auto* data = mmap(nullptr, round_up_to_power_of_two(size, PAGE_SIZE), PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
+    void* data = nullptr;
-    if (data == MAP_FAILED)
+    // POSIX mmap rejects a zero length with EINVAL, so leave m_data null for zero-size buffers.
-        return Error::from_errno(errno);
+    if (size > 0) {
+        data = mmap(nullptr, round_up_to_power_of_two(size, PAGE_SIZE), PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
+        if (data == MAP_FAILED)
+            return Error::from_errno(errno);
+    }
     return AK::adopt_nonnull_ref_or_enomem(new (nothrow) AnonymousBufferImpl(fd, size, data));
 }
 
@@ -32,8 +36,10 @@ AnonymousBufferImpl::~AnonymousBufferImpl()
         auto rc = close(m_fd);
         VERIFY(rc == 0);
     }
-    auto rc = munmap(m_data, round_up_to_power_of_two(m_size, PAGE_SIZE));
+    if (m_data) {
-    VERIFY(rc == 0);
+        auto rc = munmap(m_data, round_up_to_power_of_two(m_size, PAGE_SIZE));
+        VERIFY(rc == 0);
+    }
 }
 
 ErrorOr<AnonymousBuffer> AnonymousBuffer::create_from_anon_fd(int fd, size_t size)

Libraries/LibCore/AnonymousBufferWindows.cpp

@@ -30,7 +30,10 @@ AnonymousBufferImpl::~AnonymousBufferImpl()
 
 ErrorOr<NonnullRefPtr<AnonymousBufferImpl>> AnonymousBufferImpl::create(size_t size)
 {
-    HANDLE map_handle = CreateFileMapping(INVALID_HANDLE_VALUE, NULL, PAGE_READWRITE, size >> 32, size & 0xFFFFFFFF, NULL);
+    // CreateFileMapping rejects a zero-size mapping backed by INVALID_HANDLE_VALUE, so allocate at least one byte and
+    // track the logical size separately.
+    auto map_size = max(size, 1uz);
+    HANDLE map_handle = CreateFileMapping(INVALID_HANDLE_VALUE, NULL, PAGE_READWRITE, map_size >> 32, map_size & 0xFFFFFFFF, NULL);
     if (!map_handle)
         return Error::from_windows_error();
 
@@ -39,9 +42,12 @@ ErrorOr<NonnullRefPtr<AnonymousBufferImpl>> AnonymousBufferImpl::create(size_t s
 
 ErrorOr<NonnullRefPtr<AnonymousBufferImpl>> AnonymousBufferImpl::create(int fd, size_t size)
 {
-    void* ptr = MapViewOfFile(to_handle(fd), FILE_MAP_ALL_ACCESS, 0, 0, size);
+    void* ptr = nullptr;
-    if (!ptr)
+    if (size > 0) {
-        return Error::from_windows_error();
+        ptr = MapViewOfFile(to_handle(fd), FILE_MAP_ALL_ACCESS, 0, 0, size);
+        if (!ptr)
+            return Error::from_windows_error();
+    }
 
     return adopt_ref(*new AnonymousBufferImpl(fd, size, ptr));
 }

Services/WebContent/ConnectionFromClient.cpp

@@ -1070,7 +1070,8 @@ void ConnectionFromClient::request_internal_page_info(u64 page_id, WebView::Page
     }
 
     auto buffer = MUST(Core::AnonymousBuffer::create_with_size(builder.length()));
-    memcpy(buffer.data<void>(), builder.string_view().characters_without_null_termination(), builder.length());
+    if (builder.length() > 0)
+        memcpy(buffer.data<void>(), builder.string_view().characters_without_null_termination(), builder.length());
     async_did_get_internal_page_info(page_id, type, buffer);
 }
 

Tests/LibCore/TestLibCoreAnonymousBuffer.cpp

@@ -42,6 +42,20 @@ TEST_CASE(read_and_write_contents)
     EXPECT_EQ(read_back, payload);
 }
 
+TEST_CASE(create_with_zero_size)
+{
+    auto buffer = MUST(Core::AnonymousBuffer::create_with_size(0));
+    EXPECT(buffer.is_valid());
+    EXPECT_EQ(buffer.size(), 0u);
+    EXPECT(buffer.bytes().is_empty());
+
+    auto fd = MUST(Core::System::dup(buffer.fd()));
+    auto mirror = MUST(Core::AnonymousBuffer::create_from_anon_fd(fd, 0));
+    EXPECT(mirror.is_valid());
+    EXPECT_EQ(mirror.size(), 0u);
+    EXPECT(mirror.bytes().is_empty());
+}
+
 TEST_CASE(reconstruct_from_anon_fd_shares_memory)
 {
     auto original = MUST(Core::AnonymousBuffer::create_with_size(128));