Programming :: Cross Between Strace And Gdb

I want to run a program under ... something like strace, something like gdb. Let's call the something Fred. Every time I run a particular program under Fred, it detects when a system call is about to take place or a signal has occurred, and traps to Fred, which decides dynamically how to respond, whether the stimulus is a signal or an attempted open(), close(), read(), write(), socket(), connect(), listen(), select(), ioctl(), time(), or whatever. Although strace does this marvelously, what I'd like Fred to do is use code that I supply to doctor up what the subject program sees. In the case of write(), it should be able to modify what actually gets written. In other words, a hard shell environment around the program which completely mediates between the subject program and the outside world. I'll start with the source code for strace and gdb if I have to. But has this been done already?