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//! Resolution of the entire dependency graph for a crate.
//!
//! This module implements the core logic in taking the world of crates and
//! constraints and creating a resolved graph with locked versions for all
//! crates and their dependencies. This is separate from the registry module
//! which is more worried about discovering crates from various sources, this
//! module just uses the Registry trait as a source to learn about crates from.
//!
//! Actually solving a constraint graph is an NP-hard problem. This algorithm
//! is basically a nice heuristic to make sure we get roughly the best answer
//! most of the time. The constraints that we're working with are:
//!
//! 1. Each crate can have any number of dependencies. Each dependency can
//! declare a version range that it is compatible with.
//! 2. Crates can be activated with multiple version (e.g., show up in the
//! dependency graph twice) so long as each pairwise instance have
//! semver-incompatible versions.
//!
//! The algorithm employed here is fairly simple, we simply do a DFS, activating
//! the "newest crate" (highest version) first and then going to the next
//! option. The heuristics we employ are:
//!
//! * Never try to activate a crate version which is incompatible. This means we
//! only try crates which will actually satisfy a dependency and we won't ever
//! try to activate a crate that's semver compatible with something else
//! activated (as we're only allowed to have one) nor try to activate a crate
//! that has the same links attribute as something else
//! activated.
//! * Always try to activate the highest version crate first. The default
//! dependency in Cargo (e.g., when you write `foo = "0.1.2"`) is
//! semver-compatible, so selecting the highest version possible will allow us
//! to hopefully satisfy as many dependencies at once.
//!
//! Beyond that, what's implemented below is just a naive backtracking version
//! which should in theory try all possible combinations of dependencies and
//! versions to see if one works. The first resolution that works causes
//! everything to bail out immediately and return success, and only if *nothing*
//! works do we actually return an error up the stack.
//!
//! Resolution is currently performed twice
//! 1. With all features enabled (this is what gets saved to `Cargo.lock`)
//! 2. With only the specific features the user selected on the command-line. Ideally this
//! run will get removed in the future when transitioning to the new feature resolver.
//!
//! A new feature-specific resolver was added in 2020 which adds more sophisticated feature
//! resolution. It is located in the [`features`] module. The original dependency resolver still
//! performs feature unification, as it can help reduce the dependencies it has to consider during
//! resolution (rather than assuming every optional dependency of every package is enabled).
//! Checking if a feature is enabled must go through the new feature resolver.
//!
//! ## Performance
//!
//! Note that this is a relatively performance-critical portion of Cargo. The
//! data that we're processing is proportional to the size of the dependency
//! graph, which can often be quite large (e.g., take a look at Servo). To make
//! matters worse the DFS algorithm we're implemented is inherently quite
//! inefficient. When we add the requirement of backtracking on top it means
//! that we're implementing something that probably shouldn't be allocating all
//! over the place.
use std::collections::{BTreeMap, HashMap, HashSet};
use std::mem;
use std::rc::Rc;
use std::time::{Duration, Instant};
use tracing::{debug, trace};
use crate::core::PackageIdSpec;
use crate::core::{Dependency, PackageId, Registry, Summary};
use crate::util::context::GlobalContext;
use crate::util::errors::CargoResult;
use crate::util::network::PollExt;
use self::context::ResolverContext;
use self::dep_cache::RegistryQueryer;
use self::features::RequestedFeatures;
use self::types::{ConflictMap, ConflictReason, DepsFrame};
use self::types::{FeaturesSet, RcVecIter, RemainingDeps, ResolverProgress};
pub use self::encode::Metadata;
pub use self::encode::{EncodableDependency, EncodablePackageId, EncodableResolve};
pub use self::errors::{ActivateError, ActivateResult, ResolveError};
pub use self::features::{CliFeatures, ForceAllTargets, HasDevUnits};
pub use self::resolve::{Resolve, ResolveVersion};
pub use self::types::{ResolveBehavior, ResolveOpts};
pub use self::version_prefs::{VersionOrdering, VersionPreferences};
mod conflict_cache;
mod context;
mod dep_cache;
pub(crate) mod encode;
pub(crate) mod errors;
pub mod features;
mod resolve;
mod types;
mod version_prefs;
/// Builds the list of all packages required to build the first argument.
///
/// * `summaries` - the list of package summaries along with how to resolve
/// their features. This is a list of all top-level packages that are intended
/// to be part of the lock file (resolve output). These typically are a list
/// of all workspace members.
///
/// * `replacements` - this is a list of `[replace]` directives found in the
/// root of the workspace. The list here is a `PackageIdSpec` of what to
/// replace and a `Dependency` to replace that with. In general it's not
/// recommended to use `[replace]` any more and use `[patch]` instead, which
/// is supported elsewhere.
///
/// * `registry` - this is the source from which all package summaries are
/// loaded. It's expected that this is extensively configured ahead of time
/// and is idempotent with our requests to it (aka returns the same results
/// for the same query every time). Typically this is an instance of a
/// `PackageRegistry`.
///
/// * `version_prefs` - this represents a preference for some versions over others,
/// based on the lock file or other reasons such as `[patch]`es.
///
/// * `resolve_version` - this controls how the lockfile will be serialized.
///
/// * `config` - a location to print warnings and such, or `None` if no warnings
/// should be printed
#[tracing::instrument(skip_all)]
pub fn resolve(
summaries: &[(Summary, ResolveOpts)],
replacements: &[(PackageIdSpec, Dependency)],
registry: &mut dyn Registry,
version_prefs: &VersionPreferences,
resolve_version: ResolveVersion,
gctx: Option<&GlobalContext>,
) -> CargoResult<Resolve> {
let first_version = match gctx {
Some(config) if config.cli_unstable().direct_minimal_versions => {
Some(VersionOrdering::MinimumVersionsFirst)
}
_ => None,
};
let mut registry = RegistryQueryer::new(registry, replacements, version_prefs);
let resolver_ctx = loop {
let resolver_ctx = ResolverContext::new();
let resolver_ctx =
activate_deps_loop(resolver_ctx, &mut registry, summaries, first_version, gctx)?;
if registry.reset_pending() {
break resolver_ctx;
} else {
registry.registry.block_until_ready()?;
}
};
let mut cksums = HashMap::new();
for (summary, _) in resolver_ctx.activations.values() {
let cksum = summary.checksum().map(|s| s.to_string());
cksums.insert(summary.package_id(), cksum);
}
let graph = resolver_ctx.graph();
let replacements = resolver_ctx.resolve_replacements(®istry);
let features = resolver_ctx
.resolve_features
.iter()
.map(|(k, v)| (*k, v.iter().cloned().collect()))
.collect();
let summaries = resolver_ctx
.activations
.into_iter()
.map(|(_key, (summary, _age))| (summary.package_id(), summary))
.collect();
let resolve = Resolve::new(
graph,
replacements,
features,
cksums,
BTreeMap::new(),
Vec::new(),
resolve_version,
summaries,
);
check_cycles(&resolve)?;
check_duplicate_pkgs_in_lockfile(&resolve)?;
trace!("resolved: {:?}", resolve);
Ok(resolve)
}
/// Recursively activates the dependencies for `summaries`, in depth-first order,
/// backtracking across possible candidates for each dependency as necessary.
///
/// If all dependencies can be activated and resolved to a version in the
/// dependency graph, `cx` is returned.
fn activate_deps_loop(
mut resolver_ctx: ResolverContext,
registry: &mut RegistryQueryer<'_>,
summaries: &[(Summary, ResolveOpts)],
first_version: Option<VersionOrdering>,
gctx: Option<&GlobalContext>,
) -> CargoResult<ResolverContext> {
let mut backtrack_stack = Vec::new();
let mut remaining_deps = RemainingDeps::new();
// `past_conflicting_activations` is a cache of the reasons for each time we
// backtrack.
let mut past_conflicting_activations = conflict_cache::ConflictCache::new();
// Activate all the initial summaries to kick off some work.
for (summary, opts) in summaries {
debug!("initial activation: {}", summary.package_id());
let res = activate(
&mut resolver_ctx,
registry,
None,
summary.clone(),
first_version,
opts,
);
match res {
Ok(Some((frame, _))) => remaining_deps.push(frame),
Ok(None) => (),
Err(ActivateError::Fatal(e)) => return Err(e),
Err(ActivateError::Conflict(_, _)) => panic!("bad error from activate"),
}
}
let mut printed = ResolverProgress::new();
// Main resolution loop, this is the workhorse of the resolution algorithm.
//
// You'll note that a few stacks are maintained on the side, which might
// seem odd when this algorithm looks like it could be implemented
// recursively. While correct, this is implemented iteratively to avoid
// blowing the stack (the recursion depth is proportional to the size of the
// input).
//
// The general sketch of this loop is to run until there are no dependencies
// left to activate, and for each dependency to attempt to activate all of
// its own dependencies in turn. The `backtrack_stack` is a side table of
// backtracking states where if we hit an error we can return to in order to
// attempt to continue resolving.
while let Some((just_here_for_the_error_messages, frame)) =
remaining_deps.pop_most_constrained()
{
let (mut parent, (mut dep, candidates, mut features)) = frame;
// If we spend a lot of time here (we shouldn't in most cases) then give
// a bit of a visual indicator as to what we're doing.
printed.shell_status(gctx)?;
trace!(
"{}[{}]>{} {} candidates",
parent.name(),
resolver_ctx.age,
dep.package_name(),
candidates.len()
);
let just_here_for_the_error_messages = just_here_for_the_error_messages
&& past_conflicting_activations
.conflicting(&resolver_ctx, &dep)
.is_some();
let mut remaining_candidates = RemainingCandidates::new(&candidates);
// `conflicting_activations` stores all the reasons we were unable to
// activate candidates. One of these reasons will have to go away for
// backtracking to find a place to restart. It is also the list of
// things to explain in the error message if we fail to resolve.
//
// This is a map of package ID to a reason why that packaged caused a
// conflict for us.
let mut conflicting_activations = ConflictMap::new();
// When backtracking we don't fully update `conflicting_activations`
// especially for the cases that we didn't make a backtrack frame in the
// first place. This `backtracked` var stores whether we are continuing
// from a restored backtrack frame so that we can skip caching
// `conflicting_activations` in `past_conflicting_activations`
let mut backtracked = false;
loop {
let next = remaining_candidates.next(&mut conflicting_activations, &resolver_ctx);
let (candidate, has_another) = next.ok_or(()).or_else(|_| {
// If we get here then our `remaining_candidates` was just
// exhausted, so `dep` failed to activate.
//
// It's our job here to backtrack, if possible, and find a
// different candidate to activate. If we can't find any
// candidates whatsoever then it's time to bail entirely.
trace!(
"{}[{}]>{} -- no candidates",
parent.name(),
resolver_ctx.age,
dep.package_name()
);
// Use our list of `conflicting_activations` to add to our
// global list of past conflicting activations, effectively
// globally poisoning `dep` if `conflicting_activations` ever
// shows up again. We'll use the `past_conflicting_activations`
// below to determine if a dependency is poisoned and skip as
// much work as possible.
//
// If we're only here for the error messages then there's no
// need to try this as this dependency is already known to be
// bad.
//
// As we mentioned above with the `backtracked` variable if this
// local is set to `true` then our `conflicting_activations` may
// not be right, so we can't push into our global cache.
let mut generalize_conflicting_activations = None;
if !just_here_for_the_error_messages && !backtracked {
past_conflicting_activations.insert(&dep, &conflicting_activations);
if let Some(c) = generalize_conflicting(
&resolver_ctx,
registry,
&mut past_conflicting_activations,
&parent,
&dep,
&conflicting_activations,
) {
generalize_conflicting_activations = Some(c);
}
}
match find_candidate(
&resolver_ctx,
&mut backtrack_stack,
&parent,
backtracked,
generalize_conflicting_activations
.as_ref()
.unwrap_or(&conflicting_activations),
) {
Some((candidate, has_another, frame)) => {
// Reset all of our local variables used with the
// contents of `frame` to complete our backtrack.
resolver_ctx = frame.context;
remaining_deps = frame.remaining_deps;
remaining_candidates = frame.remaining_candidates;
parent = frame.parent;
dep = frame.dep;
features = frame.features;
conflicting_activations = frame.conflicting_activations;
backtracked = true;
Ok((candidate, has_another))
}
None => {
debug!("no candidates found");
Err(errors::activation_error(
&resolver_ctx,
registry.registry,
&parent,
&dep,
&conflicting_activations,
&candidates,
gctx,
))
}
}
})?;
// If we're only here for the error messages then we know that this
// activation will fail one way or another. To that end if we've got
// more candidates we want to fast-forward to the last one as
// otherwise we'll just backtrack here anyway (helping us to skip
// some work).
if just_here_for_the_error_messages && !backtracked && has_another {
continue;
}
// We have a `candidate`. Create a `BacktrackFrame` so we can add it
// to the `backtrack_stack` later if activation succeeds.
//
// Note that if we don't actually have another candidate then there
// will be nothing to backtrack to so we skip construction of the
// frame. This is a relatively important optimization as a number of
// the `clone` calls below can be quite expensive, so we avoid them
// if we can.
let backtrack = if has_another {
Some(BacktrackFrame {
context: ResolverContext::clone(&resolver_ctx),
remaining_deps: remaining_deps.clone(),
remaining_candidates: remaining_candidates.clone(),
parent: Summary::clone(&parent),
dep: Dependency::clone(&dep),
features: Rc::clone(&features),
conflicting_activations: conflicting_activations.clone(),
})
} else {
None
};
let pid = candidate.package_id();
let opts = ResolveOpts {
dev_deps: false,
features: RequestedFeatures::DepFeatures {
features: Rc::clone(&features),
uses_default_features: dep.uses_default_features(),
},
};
trace!(
"{}[{}]>{} trying {}",
parent.name(),
resolver_ctx.age,
dep.package_name(),
candidate.version()
);
let first_version = None; // this is an indirect dependency
let res = activate(
&mut resolver_ctx,
registry,
Some((&parent, &dep)),
candidate,
first_version,
&opts,
);
let successfully_activated = match res {
// Success! We've now activated our `candidate` in our context
// and we're almost ready to move on. We may want to scrap this
// frame in the end if it looks like it's not going to end well,
// so figure that out here.
Ok(Some((mut frame, dur))) => {
printed.elapsed(dur);
// Our `frame` here is a new package with its own list of
// dependencies. Do a sanity check here of all those
// dependencies by cross-referencing our global
// `past_conflicting_activations`. Recall that map is a
// global cache which lists sets of packages where, when
// activated, the dependency is unresolvable.
//
// If any our frame's dependencies fit in that bucket,
// aka known unresolvable, then we extend our own set of
// conflicting activations with theirs. We can do this
// because the set of conflicts we found implies the
// dependency can't be activated which implies that we
// ourselves can't be activated, so we know that they
// conflict with us.
let mut has_past_conflicting_dep = just_here_for_the_error_messages;
if !has_past_conflicting_dep {
if let Some(conflicting) = frame
.remaining_siblings
.clone()
.filter_map(|(ref new_dep, _, _)| {
past_conflicting_activations.conflicting(&resolver_ctx, new_dep)
})
.next()
{
// If one of our deps is known unresolvable
// then we will not succeed.
// How ever if we are part of the reason that
// one of our deps conflicts then
// we can make a stronger statement
// because we will definitely be activated when
// we try our dep.
conflicting_activations.extend(
conflicting
.iter()
.filter(|&(p, _)| p != &pid)
.map(|(&p, r)| (p, r.clone())),
);
has_past_conflicting_dep = true;
}
}
// If any of `remaining_deps` are known unresolvable with
// us activated, then we extend our own set of
// conflicting activations with theirs and its parent. We can do this
// because the set of conflicts we found implies the
// dependency can't be activated which implies that we
// ourselves are incompatible with that dep, so we know that deps
// parent conflict with us.
if !has_past_conflicting_dep {
if let Some(known_related_bad_deps) =
past_conflicting_activations.dependencies_conflicting_with(pid)
{
if let Some((other_parent, conflict)) = remaining_deps
.iter()
// for deps related to us
.filter(|(_, other_dep)| known_related_bad_deps.contains(other_dep))
.filter_map(|(other_parent, other_dep)| {
past_conflicting_activations
.find_conflicting(&resolver_ctx, &other_dep, Some(pid))
.map(|con| (other_parent, con))
})
.next()
{
let rel = conflict.get(&pid).unwrap().clone();
// The conflict we found is
// "other dep will not succeed if we are activated."
// We want to add
// "our dep will not succeed if other dep is in remaining_deps"
// but that is not how the cache is set up.
// So we add the less general but much faster,
// "our dep will not succeed if other dep's parent is activated".
conflicting_activations.extend(
conflict
.iter()
.filter(|&(p, _)| p != &pid)
.map(|(&p, r)| (p, r.clone())),
);
conflicting_activations.insert(other_parent, rel);
has_past_conflicting_dep = true;
}
}
}
// Ok if we're in a "known failure" state for this frame we
// may want to skip it altogether though. We don't want to
// skip it though in the case that we're displaying error
// messages to the user!
//
// Here we need to figure out if the user will see if we
// skipped this candidate (if it's known to fail, aka has a
// conflicting dep and we're the last candidate). If we're
// here for the error messages, we can't skip it (but we can
// prune extra work). If we don't have any candidates in our
// backtrack stack then we're the last line of defense, so
// we'll want to present an error message for sure.
let activate_for_error_message = has_past_conflicting_dep && !has_another && {
just_here_for_the_error_messages || {
find_candidate(
&resolver_ctx,
&mut backtrack_stack.clone(),
&parent,
backtracked,
&conflicting_activations,
)
.is_none()
}
};
// If we're only here for the error messages then we know
// one of our candidate deps will fail, meaning we will
// fail and that none of the backtrack frames will find a
// candidate that will help. Consequently let's clean up the
// no longer needed backtrack frames.
if activate_for_error_message {
backtrack_stack.clear();
}
// If we don't know for a fact that we'll fail or if we're
// just here for the error message then we push this frame
// onto our list of to-be-resolve, which will generate more
// work for us later on.
//
// Otherwise we're guaranteed to fail and were not here for
// error messages, so we skip work and don't push anything
// onto our stack.
frame.just_for_error_messages = has_past_conflicting_dep;
if !has_past_conflicting_dep || activate_for_error_message {
remaining_deps.push(frame);
true
} else {
trace!(
"{}[{}]>{} skipping {} ",
parent.name(),
resolver_ctx.age,
dep.package_name(),
pid.version()
);
false
}
}
// This candidate's already activated, so there's no extra work
// for us to do. Let's keep going.
Ok(None) => true,
// We failed with a super fatal error (like a network error), so
// bail out as quickly as possible as we can't reliably
// backtrack from errors like these
Err(ActivateError::Fatal(e)) => return Err(e),
// We failed due to a bland conflict, bah! Record this in our
// frame's list of conflicting activations as to why this
// candidate failed, and then move on.
Err(ActivateError::Conflict(id, reason)) => {
conflicting_activations.insert(id, reason);
false
}
};
// If we've successfully activated then save off the backtrack frame
// if one was created, and otherwise break out of the inner
// activation loop as we're ready to move to the next dependency
if successfully_activated {
backtrack_stack.extend(backtrack);
break;
}
// We've failed to activate this dependency, oh dear! Our call to
// `activate` above may have altered our `cx` local variable, so
// restore it back if we've got a backtrack frame.
//
// If we don't have a backtrack frame then we're just using the `cx`
// for error messages anyway so we can live with a little
// imprecision.
if let Some(b) = backtrack {
resolver_ctx = b.context;
}
}
// Ok phew, that loop was a big one! If we've broken out then we've
// successfully activated a candidate. Our stacks are all in place that
// we're ready to move on to the next dependency that needs activation,
// so loop back to the top of the function here.
}
Ok(resolver_ctx)
}
/// Attempts to activate the summary `candidate` in the context `cx`.
///
/// This function will pull dependency summaries from the registry provided, and
/// the dependencies of the package will be determined by the `opts` provided.
/// If `candidate` was activated, this function returns the dependency frame to
/// iterate through next.
fn activate(
cx: &mut ResolverContext,
registry: &mut RegistryQueryer<'_>,
parent: Option<(&Summary, &Dependency)>,
candidate: Summary,
first_version: Option<VersionOrdering>,
opts: &ResolveOpts,
) -> ActivateResult<Option<(DepsFrame, Duration)>> {
let candidate_pid = candidate.package_id();
cx.age += 1;
if let Some((parent, dep)) = parent {
let parent_pid = parent.package_id();
// add an edge from candidate to parent in the parents graph
cx.parents
.link(candidate_pid, parent_pid)
// and associate dep with that edge
.insert(dep.clone());
}
let activated = cx.flag_activated(&candidate, opts, parent)?;
let candidate = match registry.replacement_summary(candidate_pid) {
Some(replace) => {
// Note the `None` for parent here since `[replace]` is a bit wonky
// and doesn't activate the same things that `[patch]` typically
// does. TBH it basically cause panics in the test suite if
// `parent` is passed through here and `[replace]` is otherwise
// on life support so it's not critical to fix bugs anyway per se.
if cx.flag_activated(replace, opts, None)? && activated {
return Ok(None);
}
trace!(
"activating {} (replacing {})",
replace.package_id(),
candidate_pid
);
replace.clone()
}
None => {
if activated {
return Ok(None);
}
trace!("activating {}", candidate_pid);
candidate
}
};
let now = Instant::now();
let (used_features, deps) = &*registry.build_deps(
cx,
parent.map(|p| p.0.package_id()),
&candidate,
opts,
first_version,
)?;
// Record what list of features is active for this package.
if !used_features.is_empty() {
Rc::make_mut(
cx.resolve_features
.entry(candidate.package_id())
.or_insert_with(Rc::default),
)
.extend(used_features);
}
let frame = DepsFrame {
parent: candidate,
just_for_error_messages: false,
remaining_siblings: RcVecIter::new(Rc::clone(deps)),
};
Ok(Some((frame, now.elapsed())))
}
#[derive(Clone)]
struct BacktrackFrame {
context: ResolverContext,
remaining_deps: RemainingDeps,
remaining_candidates: RemainingCandidates,
parent: Summary,
dep: Dependency,
features: FeaturesSet,
conflicting_activations: ConflictMap,
}
/// A helper "iterator" used to extract candidates within a current `Context` of
/// a dependency graph.
///
/// This struct doesn't literally implement the `Iterator` trait (requires a few
/// more inputs) but in general acts like one. Each `RemainingCandidates` is
/// created with a list of candidates to choose from. When attempting to iterate
/// over the list of candidates only *valid* candidates are returned. Validity
/// is defined within a `Context`.
///
/// Candidates passed to `new` may not be returned from `next` as they could be
/// filtered out, and as they are filtered the causes will be added to `conflicting_prev_active`.
#[derive(Clone)]
struct RemainingCandidates {
remaining: RcVecIter<Summary>,
// This is an inlined peekable generator
has_another: Option<Summary>,
}
impl RemainingCandidates {
fn new(candidates: &Rc<Vec<Summary>>) -> RemainingCandidates {
RemainingCandidates {
remaining: RcVecIter::new(Rc::clone(candidates)),
has_another: None,
}
}
/// Attempts to find another candidate to check from this list.
///
/// This method will attempt to move this iterator forward, returning a
/// candidate that's possible to activate. The `cx` argument is the current
/// context which determines validity for candidates returned, and the `dep`
/// is the dependency listing that we're activating for.
///
/// If successful a `(Candidate, bool)` pair will be returned. The
/// `Candidate` is the candidate to attempt to activate, and the `bool` is
/// an indicator of whether there are remaining candidates to try of if
/// we've reached the end of iteration.
///
/// If we've reached the end of the iterator here then `Err` will be
/// returned. The error will contain a map of package ID to conflict reason,
/// where each package ID caused a candidate to be filtered out from the
/// original list for the reason listed.
fn next(
&mut self,
conflicting_prev_active: &mut ConflictMap,
cx: &ResolverContext,
) -> Option<(Summary, bool)> {
for b in self.remaining.by_ref() {
let b_id = b.package_id();
// The `links` key in the manifest dictates that there's only one
// package in a dependency graph, globally, with that particular
// `links` key. If this candidate links to something that's already
// linked to by a different package then we've gotta skip this.
if let Some(link) = b.links() {
if let Some(&a) = cx.links.get(&link) {
if a != b_id {
conflicting_prev_active
.entry(a)
.or_insert_with(|| ConflictReason::Links(link));
continue;
}
}
}
// Otherwise the condition for being a valid candidate relies on
// semver. Cargo dictates that you can't duplicate multiple
// semver-compatible versions of a crate. For example we can't
// simultaneously activate `foo 1.0.2` and `foo 1.2.0`. We can,
// however, activate `1.0.2` and `2.0.0`.
//
// Here we throw out our candidate if it's *compatible*, yet not
// equal, to all previously activated versions.
if let Some((a, _)) = cx.activations.get(&b_id.as_activations_key()) {
if *a != b {
conflicting_prev_active
.entry(a.package_id())
.or_insert(ConflictReason::Semver);
continue;
}
}
// Well if we made it this far then we've got a valid dependency. We
// want this iterator to be inherently "peekable" so we don't
// necessarily return the item just yet. Instead we stash it away to
// get returned later, and if we replaced something then that was
// actually the candidate to try first so we return that.
if let Some(r) = mem::replace(&mut self.has_another, Some(b)) {
return Some((r, true));
}
}
// Alright we've entirely exhausted our list of candidates. If we've got
// something stashed away return that here (also indicating that there's
// nothing else).
self.has_another.take().map(|r| (r, false))
}
}
/// Attempts to find a new conflict that allows a `find_candidate` better then the input one.
/// It will add the new conflict to the cache if one is found.
fn generalize_conflicting(
cx: &ResolverContext,
registry: &mut RegistryQueryer<'_>,
past_conflicting_activations: &mut conflict_cache::ConflictCache,
parent: &Summary,
dep: &Dependency,
conflicting_activations: &ConflictMap,
) -> Option<ConflictMap> {
// We need to determine the `ContextAge` that this `conflicting_activations` will jump to, and why.
let (backtrack_critical_age, backtrack_critical_id) = shortcircuit_max(
conflicting_activations
.keys()
.map(|&c| cx.is_active(c).map(|a| (a, c))),
)?;
let backtrack_critical_reason: ConflictReason =
conflicting_activations[&backtrack_critical_id].clone();
if cx
.parents
.is_path_from_to(&parent.package_id(), &backtrack_critical_id)
{
// We are a descendant of the trigger of the problem.
// The best generalization of this is to let things bubble up
// and let `backtrack_critical_id` figure this out.
return None;
}
// What parents does that critical activation have
for (critical_parent, critical_parents_deps) in
cx.parents.edges(&backtrack_critical_id).filter(|(p, _)| {
// it will only help backjump further if it is older then the critical_age
cx.is_active(**p).expect("parent not currently active!?") < backtrack_critical_age
})
{
for critical_parents_dep in critical_parents_deps.iter() {
// We only want `first_version.is_some()` for direct dependencies of workspace
// members which isn't the case here as this has a `parent`
let first_version = None;
// A dep is equivalent to one of the things it can resolve to.
// Thus, if all the things it can resolve to have already ben determined
// to be conflicting, then we can just say that we conflict with the parent.
if let Some(others) = registry
.query(critical_parents_dep, first_version)
.expect("an already used dep now error!?")
.expect("an already used dep now pending!?")
.iter()
.rev() // the last one to be tried is the least likely to be in the cache, so start with that.
.map(|other| {
past_conflicting_activations
.find(
dep,
&|id| {
if id == other.package_id() {
// we are imagining that we used other instead
Some(backtrack_critical_age)
} else {
cx.is_active(id)
}
},
Some(other.package_id()),
// we only care about things that are newer then critical_age
backtrack_critical_age,
)
.map(|con| (other.package_id(), con))
})
.collect::<Option<Vec<(PackageId, &ConflictMap)>>>()
{
let mut con = conflicting_activations.clone();
// It is always valid to combine previously inserted conflicts.
// A, B are both known bad states each that can never be activated.
// A + B is redundant but can't be activated, as if
// A + B is active then A is active and we know that is not ok.
for (_, other) in &others {
con.extend(other.iter().map(|(&id, re)| (id, re.clone())));
}
// Now that we have this combined conflict, we can do a substitution:
// A dep is equivalent to one of the things it can resolve to.
// So we can remove all the things that it resolves to and replace with the parent.
for (other_id, _) in &others {
con.remove(other_id);
}
con.insert(*critical_parent, backtrack_critical_reason);
if cfg!(debug_assertions) {
// the entire point is to find an older conflict, so let's make sure we did
let new_age = con
.keys()
.map(|&c| cx.is_active(c).expect("not currently active!?"))
.max()
.unwrap();
assert!(
new_age < backtrack_critical_age,
"new_age {} < backtrack_critical_age {}",
new_age,
backtrack_critical_age
);
}
past_conflicting_activations.insert(dep, &con);
return Some(con);
}
}
}
None
}
/// Returns Some of the largest item in the iterator.
/// Returns None if any of the items are None or the iterator is empty.
fn shortcircuit_max<I: Ord>(iter: impl Iterator<Item = Option<I>>) -> Option<I> {
let mut out = None;
for i in iter {
if i.is_none() {
return None;
}
out = std::cmp::max(out, i);
}
out
}
/// Looks through the states in `backtrack_stack` for dependencies with
/// remaining candidates. For each one, also checks if rolling back
/// could change the outcome of the failed resolution that caused backtracking
/// in the first place. Namely, if we've backtracked past the parent of the
/// failed dep, or any of the packages flagged as giving us trouble in
/// `conflicting_activations`.
///
/// Read <https://github.com/rust-lang/cargo/pull/4834>
/// For several more detailed explanations of the logic here.
fn find_candidate(
cx: &ResolverContext,
backtrack_stack: &mut Vec<BacktrackFrame>,
parent: &Summary,
backtracked: bool,
conflicting_activations: &ConflictMap,
) -> Option<(Summary, bool, BacktrackFrame)> {
// When we're calling this method we know that `parent` failed to
// activate. That means that some dependency failed to get resolved for
// whatever reason. Normally, that means that all of those reasons
// (plus maybe some extras) are listed in `conflicting_activations`.
//
// The abnormal situations are things that do not put all of the reasons in `conflicting_activations`:
// If we backtracked we do not know how our `conflicting_activations` related to
// the cause of that backtrack, so we do not update it.
let age = if !backtracked {
// we don't have abnormal situations. So we can ask `cx` for how far back we need to go.
// If the `conflicting_activations` does not apply to `cx`,
// we will just fall back to laboriously trying all possibilities witch
// will give us the correct answer.
cx.is_conflicting(Some(parent.package_id()), conflicting_activations)
} else {
None
};
while let Some(mut frame) = backtrack_stack.pop() {
let next = frame
.remaining_candidates
.next(&mut frame.conflicting_activations, &frame.context);
let Some((candidate, has_another)) = next else {
continue;
};
// If all members of `conflicting_activations` are still
// active in this back up we know that we're guaranteed to not actually
// make any progress. As a result if we hit this condition we can
// completely skip this backtrack frame and move on to the next.
if let Some(age) = age {
if frame.context.age >= age {
trace!(
"{} = \"{}\" skip as not solving {}: {:?}",
frame.dep.package_name(),
frame.dep.version_req(),
parent.package_id(),
conflicting_activations
);
// above we use `cx` to determine that this is still going to be conflicting.
// but lets just double check.
debug_assert!(
frame
.context
.is_conflicting(Some(parent.package_id()), conflicting_activations)
== Some(age)
);
continue;
} else {
// above we use `cx` to determine that this is not going to be conflicting.
// but lets just double check.
debug_assert!(frame
.context
.is_conflicting(Some(parent.package_id()), conflicting_activations)
.is_none());
}
}
return Some((candidate, has_another, frame));
}
None
}
fn check_cycles(resolve: &Resolve) -> CargoResult<()> {
// Perform a simple cycle check by visiting all nodes.
// We visit each node at most once and we keep
// track of the path through the graph as we walk it. If we walk onto the
// same node twice that's a cycle.
let mut checked = HashSet::with_capacity(resolve.len());
let mut path = Vec::with_capacity(4);
let mut visited = HashSet::with_capacity(4);
for pkg in resolve.iter() {
if !checked.contains(&pkg) {
visit(&resolve, pkg, &mut visited, &mut path, &mut checked)?
}
}
return Ok(());
fn visit(
resolve: &Resolve,
id: PackageId,
visited: &mut HashSet<PackageId>,
path: &mut Vec<PackageId>,
checked: &mut HashSet<PackageId>,
) -> CargoResult<()> {
if !visited.insert(id) {
// We found a cycle and need to construct an error. Performance is no longer top priority.
let iter = path.iter().rev().scan(id, |child, parent| {
let dep = resolve.transitive_deps_not_replaced(*parent).find_map(
|(dep_id, transitive_dep)| {
(*child == dep_id || Some(*child) == resolve.replacement(dep_id))
.then_some(transitive_dep)
},
);
*child = *parent;
Some((parent, dep))
});
let iter = std::iter::once((&id, None)).chain(iter);
let describe_path = errors::describe_path(iter);
anyhow::bail!(
"cyclic package dependency: package `{id}` depends on itself. Cycle:\n{describe_path}"
);
}
if checked.insert(id) {
path.push(id);
for (dep_id, _transitive_dep) in resolve.transitive_deps_not_replaced(id) {
visit(resolve, dep_id, visited, path, checked)?;
if let Some(replace_id) = resolve.replacement(dep_id) {
visit(resolve, replace_id, visited, path, checked)?;
}
}
path.pop();
}
visited.remove(&id);
Ok(())
}
}
/// Checks that packages are unique when written to lock file.
///
/// When writing package ID's to lock file, we apply lossy encoding. In
/// particular, we don't store paths of path dependencies. That means that
/// *different* packages may collide in the lock file, hence this check.
fn check_duplicate_pkgs_in_lockfile(resolve: &Resolve) -> CargoResult<()> {
let mut unique_pkg_ids = HashMap::new();
let state = encode::EncodeState::new(resolve);
for pkg_id in resolve.iter() {
let encodable_pkd_id = encode::encodable_package_id(pkg_id, &state, resolve.version());
if let Some(prev_pkg_id) = unique_pkg_ids.insert(encodable_pkd_id, pkg_id) {
anyhow::bail!(
"package collision in the lockfile: packages {} and {} are different, \
but only one can be written to lockfile unambiguously",
prev_pkg_id,
pkg_id
)
}
}
Ok(())
}