New in libguestfs: Rewriting bits of the daemon in OCaml

libguestfs is a C library for creating and editing disk images. In the most common (but not the only) configuration, it uses KVM to sandbox access to disk images. The C library talks to a separate daemon running inside a KVM appliance, as in this Unicode-art diagram taken from the fine manual:

 ┌───────────────────┐
 │ main program      │
 │                   │
 │                   │           child process / appliance
 │                   │          ┌──────────────────────────┐
 │                   │          │ qemu                     │
 ├───────────────────┤   RPC    │      ┌─────────────────┐ │
 │ libguestfs  ◀╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍╍▶ guestfsd        │ │
 │                   │          │      ├─────────────────┤ │
 └───────────────────┘          │      │ Linux kernel    │ │
                                │      └────────┬────────┘ │
                                └───────────────│──────────┘
                                                │
                                                │ virtio-scsi
                                         ┌──────┴──────┐
                                         │  Device or  │
                                         │  disk image │
                                         └─────────────┘

The library has to be written in C because it needs to be linked to any main program. The daemon (guestfsd in the diagram) is also written in C. But there’s not so much a specific reason for that, except that’s what we did historically.

The daemon is essentially a big pile of functions, most corresponding to a libguestfs API. Writing the daemon in C is painful to say the least. Because it’s a long-running process running in a memory-constrained environment, we have to be very careful about memory management, religiously checking every return from malloc, strdup etc., making even the simplest task non-trivial and full of untested code paths.

So last week I modified libguestfs so you can now write APIs in OCaml if you want to. OCaml is a high level language that compiles down to object files, and it’s entirely possible to link the daemon from a mix of C object files and OCaml object files. Another advantage of OCaml is that you can call from C ↔ OCaml with relatively little glue code (although a disadvantage is that you still need to write that glue mostly by hand). Most simple calls turn into direct CALL instructions with just a simple bitshift required to convert between ints and bools on the C and OCaml sides. More complex calls passing strings and structures are not too difficult either.

OCaml also turns memory errors into a single exception, which unwinds the stack cleanly, so we don’t litter the code with memory handling. We can still run the mixed C/OCaml binary under valgrind.

Code gets quite a bit shorter. For example the case_sensitive_path API — all string handling and directory lookups — goes from 183 lines of C code to 56 lines of OCaml code (and much easier to understand too).

I’m reimplementing a few APIs in OCaml, but the plan is definitely not to convert them all. I think we’ll have C and OCaml APIs in the daemon for a very long time to come.

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