chd: Handle short writes (#15902)

Check the number of bytes written by CHD file write operations and report an I/O error when a write completes without writing all requested data

Also, prevent the alignment padding loop from getting stuck if a write succeeds without making any progress

Used ChatGPT (GPT-5.6 Sol) to review the code, identify the short-write handling issues, and help prepare the fix.

Author: simzy39

src/lib/util/chd.cpp

@@ -194,7 +194,11 @@ inline std::error_condition chd_file::file_write(uint64_t offset, const void *so
 	err = m_file->seek(offset, SEEK_SET);
 	if (UNEXPECTED(err))
 		return err;
-	return write(*m_file, source, length).first;
+	size_t count;
+	std::tie(err, count) = write(*m_file, source, length);
+	if (UNEXPECTED(!err && (count != length)))
+		return std::error_condition(std::errc::io_error);
+	return err;
 }
 
 
@@ -235,6 +239,8 @@ inline uint64_t chd_file::file_append(const void *source, uint32_t length, uint3
 				std::tie(err, count) = write(*m_file, buffer, bytes_to_write);
 				if (UNEXPECTED(err))
 					throw err;
+				if (UNEXPECTED(!count))
+					throw std::error_condition(std::errc::io_error);
 				delta -= count;
 			}
 		}
@@ -245,9 +251,12 @@ inline uint64_t chd_file::file_append(const void *source, uint32_t length, uint3
 	err = m_file->tell(offset);
 	if (UNEXPECTED(err))
 		throw err;
-	std::tie(err, std::ignore) = write(*m_file, source, length);
+	size_t count;
+	std::tie(err, count) = write(*m_file, source, length);
 	if (UNEXPECTED(err))
 		throw err;
+	if (UNEXPECTED(count != length))
+		throw std::error_condition(std::errc::io_error);
 	return offset;
 }