lib: Return a valid MemoryMappedFile for zero-length files

mmap() will fail for a zero-length mapping.  A valid object should still
be returned to signal that the file was opened to the caller but it
should have a nullptr for data and zero for size.

Signed-off-by: Rick Altherr <kc8apf@kc8apf.net>
Signed-off-by: Tim 'mithro' Ansell <mithro@mithis.com>

lib/memory_mapped_file.cc

@@ -17,6 +17,16 @@ std::unique_ptr<MemoryMappedFile> MemoryMappedFile::InitWithFile(
 	struct stat statbuf;
 	if (fstat(fd, &statbuf) < 0) {
 		close(fd);
+		return nullptr;
+	}
+
+	// mmap() will fail with EINVAL if length==0. If this file is
+	// zero-length, return an object (to indicate the file exists) but
+	// load it with a nullptr and zero length.
+	if (statbuf.st_size == 0) {
+		close(fd);
+		return std::unique_ptr<MemoryMappedFile>(
+			new MemoryMappedFile(nullptr, 0));
 	}
 
 	void *file_map = mmap(NULL, statbuf.st_size, PROT_READ,

lib/memory_mapped_file_test.cc

@@ -6,6 +6,13 @@ TEST(MemoryMappedFileTest, NonExistantFile) {
 	EXPECT_FALSE(prjxray::MemoryMappedFile::InitWithFile("does_not_exist"));
 }
 
+TEST(MemoryMappedFileTest, ZeroLengthFileReturnObjectWithZeroLength) {
+	auto file = prjxray::MemoryMappedFile::InitWithFile("empty_file");
+	ASSERT_TRUE(file);
+	EXPECT_EQ(nullptr, file->data());
+	EXPECT_EQ(static_cast<size_t>(0), file->size());
+}
+
 TEST(MemoryMappedFileTest, ExistingFile) {
 	auto file = prjxray::MemoryMappedFile::InitWithFile("small_file");
 	ASSERT_TRUE(file);