diff options
Diffstat (limited to 'tvix/eval/src/value/string.rs')
| -rw-r--r-- | tvix/eval/src/value/string.rs | 389 |
1 files changed, 350 insertions, 39 deletions
diff --git a/tvix/eval/src/value/string.rs b/tvix/eval/src/value/string.rs index 5a7783b8fd..2f1592546b 100644 --- a/tvix/eval/src/value/string.rs +++ b/tvix/eval/src/value/string.rs @@ -5,11 +5,15 @@ //! paying the cost when creating new strings. use bstr::{BStr, BString, ByteSlice, Chars}; use rnix::ast; +use std::alloc::{alloc, dealloc, handle_alloc_error, Layout}; use std::borrow::{Borrow, Cow}; use std::collections::HashSet; -use std::fmt::Display; +use std::ffi::c_void; +use std::fmt::{self, Debug, Display}; use std::hash::Hash; use std::ops::Deref; +use std::ptr::{self, NonNull}; +use std::slice; use serde::de::{Deserializer, Visitor}; use serde::{Deserialize, Serialize}; @@ -80,7 +84,7 @@ impl NixContext { /// Copies from another [NixString] its context strings /// in this context. pub fn mimic(&mut self, other: &NixString) { - if let Some(ref context) = other.1 { + if let Some(context) = other.context() { self.0.extend(context.iter().cloned()); } } @@ -144,9 +148,274 @@ impl NixContext { } } -// FIXME: when serializing, ignore the context? -#[derive(Clone, Debug, Serialize)] -pub struct NixString(Box<BStr>, Option<NixContext>); +/// This type is never instantiated, but serves to document the memory layout of the actual heap +/// allocation for Nix strings. +#[allow(dead_code)] +struct NixStringInner { + /// The string context, if any. Note that this is boxed to take advantage of the null pointer + /// niche, otherwise this field ends up being very large: + /// + /// ```notrust + /// >> std::mem::size_of::<Option<HashSet<String>>>() + /// 48 + /// + /// >> std::mem::size_of::<Option<Box<HashSet<String>>>>() + /// 8 + /// ``` + context: Option<Box<NixContext>>, + /// The length of the data, stored *inline in the allocation* + length: usize, + /// The actual data for the string itself. Will always be `length` bytes long + data: [u8], +} + +#[allow(clippy::zst_offset)] +impl NixStringInner { + /// Construct a [`Layout`] for a nix string allocation with the given length. + /// + /// Returns a tuple of: + /// 1. The layout itself. + /// 2. The offset of [`Self::length`] within the allocation, assuming the allocation starts at 0 + /// 3. The offset of the data array within the allocation, assuming the allocation starts at 0 + fn layout(len: usize) -> (Layout, usize, usize) { + let layout = Layout::new::<Option<Box<NixContext>>>(); + let (layout, len_offset) = layout.extend(Layout::new::<usize>()).unwrap(); + let (layout, data_offset) = layout.extend(Layout::array::<u8>(len).unwrap()).unwrap(); + (layout, len_offset, data_offset) + } + + /// Returns the [`Layout`] for an *already-allocated* nix string, loading the length from the + /// pointer. + /// + /// Returns a tuple of: + /// 1. The layout itself. + /// 2. The offset of [`Self::length`] within the allocation, assuming the allocation starts at 0 + /// 3. The offset of the data array within the allocation, assuming the allocation starts at 0 + /// + /// # Safety + /// + /// This function must only be called on a pointer that has been properly initialized with + /// [`Self::alloc`]. The data buffer may not necessarily be initialized + unsafe fn layout_of(this: NonNull<c_void>) -> (Layout, usize, usize) { + let layout = Layout::new::<Option<Box<NixContext>>>(); + let (_, len_offset) = layout.extend(Layout::new::<usize>()).unwrap(); + // SAFETY: Layouts are linear, so even though we haven't involved data at all yet, we know + // the len_offset is a valid offset into the second field of the allocation + let len = *(this.as_ptr().add(len_offset) as *const usize); + Self::layout(len) + } + + /// Allocate an *uninitialized* nix string with the given length. Writes the length to the + /// length value in the pointer, but leaves both context and data uninitialized + /// + /// This function is safe to call (as constructing pointers of any sort of validity is always + /// safe in Rust) but it is unsafe to use the resulting pointer to do anything other than + /// + /// 1. Read the length + /// 2. Write the context + /// 3. Write the data + /// + /// until the string is fully initialized + fn alloc(len: usize) -> NonNull<c_void> { + let (layout, len_offset, _data_offset) = Self::layout(len); + debug_assert_ne!(layout.size(), 0); + unsafe { + // SAFETY: Layout has non-zero size, since the layout of the context and the + // layout of the len both have non-zero size + let ptr = alloc(layout); + + if let Some(this) = NonNull::new(ptr as *mut _) { + // SAFETY: We've allocated with a layout that causes the len_offset to be in-bounds + // and writeable, and if the allocation succeeded it won't wrap + ((this.as_ptr() as *mut u8).add(len_offset) as *mut usize).write(len); + debug_assert_eq!(Self::len(this), len); + this + } else { + handle_alloc_error(layout); + } + } + } + + /// Deallocate the Nix string at the given pointer + /// + /// # Safety + /// + /// This function must only be called with a pointer that has been properly initialized with + /// [`Self::alloc`] + unsafe fn dealloc(this: NonNull<c_void>) { + let (layout, _, _) = Self::layout_of(this); + // SAFETY: okay because of the safety guarantees of this method + dealloc(this.as_ptr() as *mut u8, layout) + } + + /// Return the length of the Nix string at the given pointer + /// + /// # Safety + /// + /// This function must only be called with a pointer that has been properly initialized with + /// [`Self::alloc`] + unsafe fn len(this: NonNull<c_void>) -> usize { + let (_, len_offset, _) = Self::layout_of(this); + // SAFETY: As long as the safety guarantees of this method are upheld, we've allocated with + // a layout that causes the len_offset to be in-bounds and writeable, and if the allocation + // succeeded it won't wrap + *(this.as_ptr().add(len_offset) as *const usize) + } + + /// Return a pointer to the context value within the given Nix string pointer + /// + /// # Safety + /// + /// This function must only be called with a pointer that has been properly initialized with + /// [`Self::alloc`] + unsafe fn context_ptr(this: NonNull<c_void>) -> *mut Option<Box<NixContext>> { + // SAFETY: The context is the first field in the layout of the allocation + this.as_ptr() as *mut Option<Box<NixContext>> + } + + /// Construct a shared reference to the context value within the given Nix string pointer + /// + /// # Safety + /// + /// This function must only be called with a pointer that has been properly initialized with + /// [`Self::alloc`], and where the context has been properly initialized (by writing to the + /// pointer returned from [`Self::context_ptr`]). + /// + /// Also, all the normal Rust rules about pointer-to-reference conversion apply. See + /// [`NonNull::as_ref`] for more. + unsafe fn context_ref<'a>(this: NonNull<c_void>) -> &'a Option<Box<NixContext>> { + Self::context_ptr(this).as_ref().unwrap() + } + + /// Construct a mutable reference to the context value within the given Nix string pointer + /// + /// # Safety + /// + /// This function must only be called with a pointer that has been properly initialized with + /// [`Self::alloc`], and where the context has been properly initialized (by writing to the + /// pointer returned from [`Self::context_ptr`]). + /// + /// Also, all the normal Rust rules about pointer-to-reference conversion apply. See + /// [`NonNull::as_mut`] for more. + unsafe fn context_mut<'a>(this: NonNull<c_void>) -> &'a mut Option<Box<NixContext>> { + Self::context_ptr(this).as_mut().unwrap() + } + + /// Return a pointer to the data array within the given Nix string pointer + /// + /// # Safety + /// + /// This function must only be called with a pointer that has been properly initialized with + /// [`Self::alloc`] + unsafe fn data_ptr(this: NonNull<c_void>) -> *mut u8 { + let (_, _, data_offset) = Self::layout_of(this); + // SAFETY: data is the third field in the layout of the allocation + this.as_ptr().add(data_offset) as *mut u8 + } + + /// Construct a shared reference to the data slice within the given Nix string pointer + /// + /// # Safety + /// + /// This function must only be called with a pointer that has been properly initialized with + /// [`Self::alloc`], and where the data array has been properly initialized (by writing to the + /// pointer returned from [`Self::data_ptr`]). + /// + /// Also, all the normal Rust rules about pointer-to-reference conversion apply. See + /// [`slice::from_raw_parts`] for more. + unsafe fn data_slice<'a>(this: NonNull<c_void>) -> &'a [u8] { + let len = Self::len(this); + let data = Self::data_ptr(this); + slice::from_raw_parts(data, len) + } + + /// Construct a mutable reference to the data slice within the given Nix string pointer + /// + /// # Safety + /// + /// This function must only be called with a pointer that has been properly initialized with + /// [`Self::alloc`], and where the data array has been properly initialized (by writing to the + /// pointer returned from [`Self::data_ptr`]). + /// + /// Also, all the normal Rust rules about pointer-to-reference conversion apply. See + /// [`slice::from_raw_parts_mut`] for more. + #[allow(dead_code)] + unsafe fn data_slice_mut<'a>(this: NonNull<c_void>) -> &'a mut [u8] { + let len = Self::len(this); + let data = Self::data_ptr(this); + slice::from_raw_parts_mut(data, len) + } + + /// Clone the Nix string pointed to by this pointer, and return a pointer to a new Nix string + /// containing the same data and context. + /// + /// # Safety + /// + /// This function must only be called with a pointer that has been properly initialized with + /// [`Self::alloc`], and where the context has been properly initialized (by writing to the + /// pointer returned from [`Self::context_ptr`]), and the data array has been properly + /// initialized (by writing to the pointer returned from [`Self::data_ptr`]). + unsafe fn clone(this: NonNull<c_void>) -> NonNull<c_void> { + let (layout, _, _) = Self::layout_of(this); + let ptr = alloc(layout); + if let Some(new) = NonNull::new(ptr as *mut _) { + ptr::copy_nonoverlapping(this.as_ptr(), new.as_ptr(), layout.size()); + Self::context_ptr(new).write(Self::context_ref(this).clone()); + new + } else { + handle_alloc_error(layout); + } + } +} + +/// Nix string values +/// +/// # Internals +/// +/// For performance reasons (to keep allocations small, and to avoid indirections), [`NixString`] is +/// represented as a single *thin* pointer to a packed data structure containing the +/// [context][NixContext] and the string data itself (which is a raw byte array, to match the Nix +/// string semantics that allow any array of bytes to be represented by a string). + +/// This memory representation is documented in [`NixStringInner`], but since Rust prefers to deal +/// with slices via *fat pointers* (pointers that include the length in the *pointer*, not in the +/// heap allocation), we have to do mostly manual layout management and allocation for this +/// representation. See the documentation for the methods of [`NixStringInner`] for more information +pub struct NixString(NonNull<c_void>); + +unsafe impl Send for NixString {} +unsafe impl Sync for NixString {} + +impl Drop for NixString { + fn drop(&mut self) { + // SAFETY: There's no way to construct a NixString that doesn't leave the allocation correct + // according to the rules of dealloc + unsafe { + NixStringInner::dealloc(self.0); + } + } +} + +impl Clone for NixString { + fn clone(&self) -> Self { + // SAFETY: There's no way to construct a NixString that doesn't leave the allocation correct + // according to the rules of clone + unsafe { Self(NixStringInner::clone(self.0)) } + } +} + +impl Debug for NixString { + fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { + if let Some(ctx) = self.context() { + f.debug_struct("NixString") + .field("context", ctx) + .field("data", &self.as_bstr()) + .finish() + } else { + write!(f, "{:?}", self.as_bstr()) + } + } +} impl PartialEq for NixString { fn eq(&self, other: &Self) -> bool { @@ -182,13 +451,13 @@ impl Ord for NixString { impl From<Box<BStr>> for NixString { fn from(value: Box<BStr>) -> Self { - Self(value, None) + Self::new(&value, None) } } impl From<BString> for NixString { fn from(value: BString) -> Self { - Self(Vec::<u8>::from(value).into_boxed_slice().into(), None) + Self::new(&value, None) } } @@ -212,7 +481,7 @@ impl From<Vec<u8>> for NixString { impl From<Box<[u8]>> for NixString { fn from(value: Box<[u8]>) -> Self { - Self(value.into(), None) + Self::new(&value, None) } } @@ -228,12 +497,12 @@ impl From<String> for NixString { } } -impl<T> From<(T, Option<NixContext>)> for NixString +impl<T> From<(T, Option<Box<NixContext>>)> for NixString where NixString: From<T>, { - fn from((s, ctx): (T, Option<NixContext>)) -> Self { - NixString(NixString::from(s).0, ctx) + fn from((s, ctx): (T, Option<Box<NixContext>>)) -> Self { + Self::new(NixString::from(s).as_ref(), ctx) } } @@ -251,25 +520,19 @@ impl From<ast::Ident> for NixString { impl<'a> From<&'a NixString> for &'a BStr { fn from(s: &'a NixString) -> Self { - BStr::new(&*s.0) - } -} - -impl From<NixString> for Box<BStr> { - fn from(s: NixString) -> Self { - s.0 + s.as_bstr() } } impl From<NixString> for BString { fn from(s: NixString) -> Self { - s.0.to_vec().into() + s.as_bstr().to_owned() } } impl AsRef<[u8]> for NixString { fn as_ref(&self) -> &[u8] { - &self.0 + self.as_bytes() } } @@ -322,7 +585,7 @@ impl Deref for NixString { type Target = BStr; fn deref(&self) -> &Self::Target { - &self.0 + self.as_bstr() } } @@ -344,37 +607,61 @@ mod arbitrary { } impl NixString { + fn new(contents: &[u8], context: Option<Box<NixContext>>) -> Self { + // SAFETY: We're always fully initializing a NixString here: + // + // 1. NixStringInner::alloc sets up the len for us + // 2. We set the context, using ptr::write to make sure that the uninitialized memory isn't + // read or dropped + // 3. We set the data, using copy_from_nonoverlapping to make sure that the uninitialized + // memory isn't read or dropped + // + // Only *then* can we construct a NixString + unsafe { + let inner = NixStringInner::alloc(contents.len()); + NixStringInner::context_ptr(inner).write(context); + NixStringInner::data_ptr(inner) + .copy_from_nonoverlapping(contents.as_ptr(), contents.len()); + Self(inner) + } + } + pub fn new_inherit_context_from<T>(other: &NixString, new_contents: T) -> Self where NixString: From<T>, { - Self(Self::from(new_contents).0, other.1.clone()) + Self::new( + Self::from(new_contents).as_ref(), + other.context().map(|c| Box::new(c.clone())), + ) } pub fn new_context_from<T>(context: NixContext, contents: T) -> Self where NixString: From<T>, { - Self( - Self::from(contents).0, + Self::new( + Self::from(contents).as_ref(), if context.is_empty() { None } else { - Some(context) + Some(Box::new(context)) }, ) } pub fn as_bstr(&self) -> &BStr { - self + BStr::new(self.as_bytes()) } pub fn as_bytes(&self) -> &[u8] { - &self.0 + // SAFETY: There's no way to construct an uninitialized NixString (see the SAFETY comment in + // `new`) + unsafe { NixStringInner::data_slice(self.0) } } pub fn into_bstring(self) -> BString { - (*self.0).to_owned() + self.as_bstr().to_owned() } /// Return a displayable representation of the string as an @@ -409,7 +696,7 @@ impl NixString { let mut s = self.to_vec(); s.extend(&(***other)); - let context = [&self.1, &other.1] + let context = [self.context(), other.context()] .into_iter() .flatten() .fold(NixContext::new(), |acc_ctx, new_ctx| { @@ -418,42 +705,59 @@ impl NixString { Self::new_context_from(context, s) } + pub(crate) fn context(&self) -> Option<&NixContext> { + // SAFETY: There's no way to construct an uninitialized or invalid NixString (see the SAFETY + // comment in `new`). + // + // Also, we're using the same lifetime and mutability as self, to fit the + // pointer-to-reference conversion rules + unsafe { NixStringInner::context_ref(self.0).as_deref() } + } + pub(crate) fn context_mut(&mut self) -> Option<&mut NixContext> { - return self.1.as_mut(); + // SAFETY: There's no way to construct an uninitialized or invalid NixString (see the SAFETY + // comment in `new`). + // + // Also, we're using the same lifetime and mutability as self, to fit the + // pointer-to-reference conversion rules + unsafe { NixStringInner::context_mut(self.0).as_deref_mut() } } pub fn iter_context(&self) -> impl Iterator<Item = &NixContext> { - return self.1.iter(); + self.context().into_iter() } pub fn iter_plain(&self) -> impl Iterator<Item = &str> { - return self.1.iter().flat_map(|context| context.iter_plain()); + self.iter_context().flat_map(|context| context.iter_plain()) } pub fn iter_derivation(&self) -> impl Iterator<Item = &str> { - return self.1.iter().flat_map(|context| context.iter_derivation()); + return self + .iter_context() + .flat_map(|context| context.iter_derivation()); } pub fn iter_single_outputs(&self) -> impl Iterator<Item = (&str, &str)> { return self - .1 - .iter() + .iter_context() .flat_map(|context| context.iter_single_outputs()); } /// Returns whether this Nix string possess a context or not. pub fn has_context(&self) -> bool { - self.1.is_some() + self.context().is_some() } /// This clears the context of that string, losing /// all dependency tracking information. pub fn clear_context(&mut self) { - self.1 = None; + // SAFETY: There's no way to construct an uninitialized or invalid NixString (see the SAFETY + // comment in `new`). + *unsafe { NixStringInner::context_mut(self.0) } = None; } pub fn chars(&self) -> Chars<'_> { - self.0.chars() + self.as_bstr().chars() } } @@ -539,13 +843,20 @@ impl Display for NixString { #[cfg(test)] mod tests { + use test_strategy::proptest; + use super::*; use crate::properties::{eq_laws, hash_laws, ord_laws}; #[test] fn size() { - assert_eq!(std::mem::size_of::<NixString>(), 64); + assert_eq!(std::mem::size_of::<NixString>(), 8); + } + + #[proptest] + fn clone_strings(s: NixString) { + drop(s.clone()) } eq_laws!(NixString); |