| Lift freeq's AV media plane out of sleek 90f8b89 nandi 20d ago | 1 | //! The rules every `.so` in this workspace keeps at its edge. |
| 2 | //! |
| 3 | //! A jolt library binds a shared object through `jolt.ffi/defcfn`: it declares |
| 4 | //! typed foreign functions and gets back integers, doubles, and borrowed UTF-8 |
| 5 | //! strings. That is the whole vocabulary. Everything a native crate here wants |
| 6 | //! to say has to be said in it, and the same three problems come up every time |
| 7 | //! — so they are solved once, here, rather than three times slightly |
| 8 | //! differently. |
| 9 | //! |
| 10 | //! # Unwinding |
| 11 | //! |
| 12 | //! A panic that crosses into a C caller is undefined behaviour, and the caller |
| 13 | //! on the other side of this boundary is Chez Scheme, which will not survive |
| 14 | //! it in any useful way. [`guard`] catches at the edge and answers a fallback, |
| 15 | //! so a bug in a decoder is a black tile rather than a dead process. |
| 16 | //! |
| 17 | //! # Strings out |
| 18 | //! |
| 19 | //! Nothing here can hand out memory the caller must free: jolt has no place to |
| 20 | //! put a free, and a leak per call is not a design. [`Scratch`] lends instead — |
| 21 | //! a returned string is valid until the next one, which is long enough because |
| 22 | //! jolt copies a `:string` return into a Scheme string as it crosses. |
| 23 | //! |
| 24 | //! # Asking, not telling |
| 25 | //! |
| 26 | //! None of these libraries call back. Native work happens on native threads, |
| 27 | //! and a callback into a foreign runtime from one of them is a rule about |
| 28 | //! threads that the caller has to keep and that nothing can check. So state |
| 29 | //! that arrives asynchronously is queued and *polled*, in the shape vidya's |
| 30 | //! event ABI already uses: a `poll` that answers 1 while there was something, |
| 31 | //! and accessors that describe whatever it last handed over. |
| 32 | |
| 33 | use std::ffi::{c_char, CString}; |
| 34 | use std::panic::AssertUnwindSafe; |
| 35 | use std::sync::Mutex; |
| 36 | |
| 37 | /// Run `f`, answering `fallback` if it panics. |
| 38 | /// |
| 39 | /// Every `#[no_mangle]` entry point in this workspace starts here. The panic |
| 40 | /// is logged rather than swallowed silently — a fallback that appears from |
| 41 | /// nowhere is worse to debug than a crash. |
| 42 | pub fn guard<R>(fallback: R, f: impl FnOnce() -> R) -> R { |
| 43 | match std::panic::catch_unwind(AssertUnwindSafe(f)) { |
| 44 | Ok(value) => value, |
| 45 | Err(_) => { |
| 46 | log::error!("jolt-abi: panic caught at the FFI boundary"); |
| 47 | fallback |
| 48 | } |
| 49 | } |
| 50 | } |
| 51 | |
| 52 | /// Read a caller's string. |
| 53 | /// |
| 54 | /// # Safety |
| 55 | /// `ptr` is null or a NUL-terminated string valid for the duration of the call. |
| 56 | pub unsafe fn borrowed(ptr: *const c_char) -> String { |
| 57 | if ptr.is_null() { |
| 58 | String::new() |
| 59 | } else { |
| 60 | std::ffi::CStr::from_ptr(ptr).to_string_lossy().into_owned() |
| 61 | } |
| 62 | } |
| 63 | |
| 64 | /// An empty string is how a C caller spells "no preference"; `None` is how |
| 65 | /// Rust spells it. Worth naming, because the two are not the same absence and |
| 66 | /// treating them as one is how a default device becomes a device called "". |
| 67 | pub fn preference(s: String) -> Option<String> { |
| 68 | if s.is_empty() { |
| 69 | None |
| 70 | } else { |
| 71 | Some(s) |
| 72 | } |
| 73 | } |
| 74 | |
| 75 | /// A place to keep the one string a library has most recently lent out. |
| 76 | /// |
| 77 | /// Hold one per library in a `static`. [`Scratch::lend`] answers a pointer good |
| 78 | /// until the next `lend` on the same scratch, which is the same contract |
| 79 | /// vidya's tree ABI states for `vidya_tree_event_text` and friends. |
| 80 | #[derive(Default)] |
| 81 | pub struct Scratch(Mutex<Option<CString>>); |
| 82 | |
| 83 | impl Scratch { |
| 84 | pub const fn new() -> Self { |
| 85 | Self(Mutex::new(None)) |
| 86 | } |
| 87 | |
| 88 | /// Hand `value` to the caller as a borrowed C string. |
| 89 | /// |
| 90 | /// Valid until the next call to `lend` on this scratch. An interior NUL — |
| 91 | /// which no string this workspace produces should have, but a device name |
| 92 | /// comes from the operating system — truncates rather than failing the |
| 93 | /// call, since a name is being shown to someone, not parsed. |
| 94 | pub fn lend(&self, value: impl Into<Vec<u8>>) -> *const c_char { |
| 95 | let bytes: Vec<u8> = value.into(); |
| 96 | let owned = CString::new(bytes.clone()).unwrap_or_else(|_| { |
| 97 | let cut = bytes.iter().position(|b| *b == 0).unwrap_or(0); |
| 98 | CString::new(&bytes[..cut]).unwrap_or_default() |
| 99 | }); |
| 100 | match self.0.lock() { |
| 101 | Ok(mut slot) => { |
| 102 | let stored = slot.insert(owned); |
| 103 | stored.as_ptr() |
| 104 | } |
| 105 | Err(_) => { |
| 106 | // The lock is poisoned, which means a panic happened while a |
| 107 | // string was being lent. Leak this one rather than hand back a |
| 108 | // dangling pointer: one leaked string beats one use-after-free. |
| 109 | let ptr = owned.as_ptr(); |
| 110 | std::mem::forget(owned); |
| 111 | ptr |
| 112 | } |
| 113 | } |
| 114 | } |
| 115 | } |
| 116 | |
| 117 | // A `Scratch` in a `static` is shared across threads by definition; the Mutex |
| 118 | // is what makes that sound, and the pointer it lends is only promised until the |
| 119 | // next lend, which is a contract the caller keeps, not a lifetime Rust checks. |
| 120 | unsafe impl Sync for Scratch {} |
| 121 | |
| 122 | /// The empty C string — what a string-returning entry point answers when there |
| 123 | /// is nothing to say. Static, so it outlives any scratch. |
| 124 | pub fn empty_str() -> *const c_char { |
| 125 | c"".as_ptr() |
| 126 | } |
| 127 | |
| 128 | #[cfg(test)] |
| 129 | mod tests { |
| 130 | use super::*; |
| 131 | use std::ffi::CStr; |
| 132 | |
| 133 | fn read(ptr: *const c_char) -> String { |
| Run the formatter over the tree 3e8c6f0 nandi 13d ago | 134 | unsafe { CStr::from_ptr(ptr) } |
| 135 | .to_string_lossy() |
| 136 | .into_owned() |
| Lift freeq's AV media plane out of sleek 90f8b89 nandi 20d ago | 137 | } |
| 138 | |
| 139 | #[test] |
| 140 | fn a_lent_string_lasts_until_the_next_one() { |
| 141 | let scratch = Scratch::new(); |
| 142 | let first = scratch.lend("one"); |
| 143 | assert_eq!(read(first), "one"); |
| 144 | let second = scratch.lend("two"); |
| 145 | assert_eq!(read(second), "two"); |
| 146 | } |
| 147 | |
| 148 | #[test] |
| 149 | fn two_scratches_do_not_tread_on_each_other() { |
| 150 | let a = Scratch::new(); |
| 151 | let b = Scratch::new(); |
| 152 | let from_a = a.lend("keys"); |
| 153 | let _ = b.lend("status"); |
| 154 | // b's lend must not have replaced a's — a library with two |
| 155 | // string-returning families keeps one scratch per family for exactly |
| 156 | // this reason. |
| 157 | assert_eq!(read(from_a), "keys"); |
| 158 | } |
| 159 | |
| 160 | #[test] |
| 161 | fn an_interior_nul_truncates_rather_than_failing() { |
| 162 | let scratch = Scratch::new(); |
| 163 | assert_eq!(read(scratch.lend(&b"we\0bcam"[..])), "we"); |
| 164 | } |
| 165 | |
| 166 | #[test] |
| 167 | fn guard_answers_the_fallback_instead_of_unwinding() { |
| 168 | assert_eq!(guard(7, || panic!("decoder")), 7); |
| 169 | assert_eq!(guard(7, || 1), 1); |
| 170 | } |
| 171 | |
| 172 | #[test] |
| 173 | fn an_empty_preference_is_no_preference() { |
| 174 | assert_eq!(preference(String::new()), None); |
| 175 | assert_eq!(preference("hw:1".to_owned()), Some("hw:1".to_owned())); |
| 176 | } |
| 177 | |
| 178 | #[test] |
| 179 | fn a_null_pointer_reads_as_the_empty_string() { |
| 180 | assert_eq!(unsafe { borrowed(std::ptr::null()) }, ""); |
| 181 | let owned = CString::new("nandi").unwrap(); |
| 182 | assert_eq!(unsafe { borrowed(owned.as_ptr()) }, "nandi"); |
| 183 | } |
| 184 | } |