docs: doc comments for remote registry

Author: weihanglo

src/cargo/sources/registry/remote.rs

@@ -1,3 +1,5 @@
+//! Access to a Git index based registry. See [`RemoteRegistry`] for details.
+
 use crate::core::{GitReference, PackageId, SourceId};
 use crate::sources::git;
 use crate::sources::git::fetch::RemoteKind;
@@ -21,29 +23,73 @@ use std::task::{ready, Poll};
 /// A remote registry is a registry that lives at a remote URL (such as
 /// crates.io). The git index is cloned locally, and `.crate` files are
 /// downloaded as needed and cached locally.
+///
+/// This type is primarily accessed through the [`RegistryData`] trait.
+///
+/// See the [module-level documentation](super) for the index format and layout.
+///
+/// ## History of Git-based index registry
+///
+/// Using Git to host this index used to be quite efficient. The full index can
+/// be stored efficiently locally on disk, and once it is downloaded, all
+/// queries of a registry can happen locally and needn't touch the network.
+/// Git-based index was a reasonable design choice at the time when HTTP/2
+/// was just introduced.
+///
+/// However, the full index keeps growing as crates.io grows. It becomes
+/// relatively big and slows down the first use of Cargo. Git (specifically
+/// libgit2) is not efficient at handling huge amounts of small files either.
+/// On the other hand, newer protocols like HTTP/2 are prevalent and capable to
+/// serve a bunch of tiny files. Today, it is encouraged to use [`HttpRegistry`],
+/// which is the default from 1.70.0. That being said, Cargo will continue
+/// supporting Git-based index for a pretty long while.
+///
+/// [`HttpRegistry`]: super::http_remote::HttpRegistry
 pub struct RemoteRegistry<'cfg> {
+    /// Path to the registry index (`$CARGO_HOME/registry/index/$REG-HASH`).
     index_path: Filesystem,
-    /// Path to the cache of `.crate` files (`$CARGO_HOME/registry/path/$REG-HASH`).
+    /// Path to the cache of `.crate` files (`$CARGO_HOME/registry/cache/$REG-HASH`).
     cache_path: Filesystem,
+    /// The unique identifier of this registry source.
     source_id: SourceId,
+    /// This reference is stored so that when a registry needs update, it knows
+    /// where to fetch from.
     index_git_ref: GitReference,
     config: &'cfg Config,
+    /// A Git [tree object] to help this registry find crate metadata from the
+    /// underlying Git repository.
+    ///
+    /// This is stored here to prevent Git from repeatly creating a tree object
+    /// during each call into `load()`.
+    ///
+    /// [tree object]: https://git-scm.com/book/en/v2/Git-Internals-Git-Objects#_tree_objects
     tree: RefCell<Option<git2::Tree<'static>>>,
+    /// A Git repository that contains the actual index we want.
     repo: LazyCell<git2::Repository>,
+    /// The current HEAD commit of the underlying Git repository.
     head: Cell<Option<git2::Oid>>,
+    /// This stores sha value of the current HEAD commit for convenience.
     current_sha: Cell<Option<InternedString>>,
-    needs_update: bool, // Does this registry need to be updated?
+    /// Whether this registry needs to update package informations.
+    ///
+    /// See [`RemoteRegistry::mark_updated`] on how to make sure a registry
+    /// index is updated only once per session.
+    needs_update: bool,
+    /// Disables status messages.
     quiet: bool,
 }
 
 impl<'cfg> RemoteRegistry<'cfg> {
+    /// Creates a Git-rebased remote registry for `source_id`.
+    ///
+    /// * `name` --- Name of a path segment where `.crate` tarballs and the
+    ///   registry index are stored. Expect to be unique.
     pub fn new(source_id: SourceId, config: &'cfg Config, name: &str) -> RemoteRegistry<'cfg> {
         RemoteRegistry {
             index_path: config.registry_index_path().join(name),
             cache_path: config.registry_cache_path().join(name),
             source_id,
             config,
-            // TODO: we should probably make this configurable
             index_git_ref: GitReference::DefaultBranch,
             tree: RefCell::new(None),
             repo: LazyCell::new(),
@@ -54,17 +100,16 @@ impl<'cfg> RemoteRegistry<'cfg> {
         }
     }
 
+    /// Creates intermediate dirs and initialize the repository.
     fn repo(&self) -> CargoResult<&git2::Repository> {
         self.repo.try_borrow_with(|| {
             let path = self.config.assert_package_cache_locked(&self.index_path);
 
-            // Fast path without a lock
             if let Ok(repo) = git2::Repository::open(&path) {
                 trace!("opened a repo without a lock");
                 return Ok(repo);
             }
 
-            // Ok, now we need to lock and try the whole thing over again.
             trace!("acquiring registry index lock");
             match git2::Repository::open(&path) {
                 Ok(repo) => Ok(repo),
@@ -97,6 +142,7 @@ impl<'cfg> RemoteRegistry<'cfg> {
         })
     }
 
+    /// Get the object ID of the HEAD commit from the underlying Git repository.
     fn head(&self) -> CargoResult<git2::Oid> {
         if self.head.get().is_none() {
             let repo = self.repo()?;
@@ -106,6 +152,8 @@ impl<'cfg> RemoteRegistry<'cfg> {
         Ok(self.head.get().unwrap())
     }
 
+    /// Returns a [`git2::Tree`] object of the current HEAD commit of the
+    /// underlying Git repository.
     fn tree(&self) -> CargoResult<Ref<'_, git2::Tree<'_>>> {
         {
             let tree = self.tree.borrow();
@@ -117,6 +165,7 @@ impl<'cfg> RemoteRegistry<'cfg> {
         let commit = repo.find_commit(self.head()?)?;
         let tree = commit.tree()?;
 
+        // SAFETY:
         // Unfortunately in libgit2 the tree objects look like they've got a
         // reference to the repository object which means that a tree cannot
         // outlive the repository that it came from. Here we want to cache this
@@ -134,6 +183,9 @@ impl<'cfg> RemoteRegistry<'cfg> {
         Ok(Ref::map(self.tree.borrow(), |s| s.as_ref().unwrap()))
     }
 
+    /// Gets the current version of the registry index.
+    ///
+    /// It is usually sha of the HEAD commit from the underlying Git repository.
     fn current_version(&self) -> Option<InternedString> {
         if let Some(sha) = self.current_sha.get() {
             return Some(sha);
@@ -143,10 +195,16 @@ impl<'cfg> RemoteRegistry<'cfg> {
         Some(sha)
     }
 
+    /// Whether the registry is up-to-date. See [`Self::mark_updated`] for more.
     fn is_updated(&self) -> bool {
         self.config.updated_sources().contains(&self.source_id)
     }
 
+    /// Marks this registry as up-to-date.
+    ///
+    /// This makes sure the index is only updated once per session since it is
+    /// an expensive operation. This generally only happens when the resolver
+    /// is run multiple times, such as during `cargo publish`.
     fn mark_updated(&self) {
         self.config.updated_sources().insert(self.source_id);
     }
@@ -156,7 +214,7 @@ const LAST_UPDATED_FILE: &str = ".last-updated";
 
 impl<'cfg> RegistryData for RemoteRegistry<'cfg> {
     fn prepare(&self) -> CargoResult<()> {
-        self.repo()?; // create intermediate dirs and initialize the repo
+        self.repo()?;
         Ok(())
     }
 
@@ -168,13 +226,20 @@ impl<'cfg> RegistryData for RemoteRegistry<'cfg> {
         self.config.assert_package_cache_locked(path)
     }
 
-    // `index_version` Is a string representing the version of the file used to construct the cached copy.
-    // Older versions of Cargo used the single value of the hash of the HEAD commit as a `index_version`.
-    // This is technically correct but a little too conservative. If a new commit is fetched all cached
-    // files need to be regenerated even if a particular file was not changed.
-    // However if an old cargo has written such a file we still know how to read it, as long as we check for that hash value.
-    //
-    // Cargo now uses a hash of the file's contents as provided by git.
+    /// Read the general concept for `load()` on [`RegistryData::load`].
+    ///
+    /// `index_version` is a string representing the version of the file used
+    /// to construct the cached copy.
+    ///
+    /// Older versions of Cargo used the single value of the hash of the HEAD
+    /// commit as a `index_version`. This is technically correct but a little
+    /// too conservative. If a new commit is fetched all cached files need to
+    /// be regenerated even if a particular file was not changed.
+    ///
+    /// However if an old cargo has written such a file we still know how to
+    /// read it, as long as we check for that hash value.
+    ///
+    /// Cargo now uses a hash of the file's contents as provided by git.
     fn load(
         &mut self,
         _root: &Path,
@@ -187,7 +252,8 @@ impl<'cfg> RegistryData for RemoteRegistry<'cfg> {
         // Check if the cache is valid.
         let git_commit_hash = self.current_version();
         if index_version.is_some() && index_version == git_commit_hash.as_deref() {
-            // This file was written by an old version of cargo, but it is still up-to-date.
+            // This file was written by an old version of cargo, but it is
+            // still up-to-date.
             return Poll::Ready(Ok(LoadResponse::CacheValid));
         }
         // Note that the index calls this method and the filesystem is locked
@@ -224,8 +290,8 @@ impl<'cfg> RegistryData for RemoteRegistry<'cfg> {
         match load_helper(&self, path, index_version) {
             Ok(result) => Poll::Ready(Ok(result)),
             Err(_) if !self.is_updated() => {
-                // If git returns an error and we haven't updated the repo, return
-                // pending to allow an update to try again.
+                // If git returns an error and we haven't updated the repo,
+                // return pending to allow an update to try again.
                 self.needs_update = true;
                 Poll::Pending
             }
@@ -265,9 +331,6 @@ impl<'cfg> RegistryData for RemoteRegistry<'cfg> {
 
         self.needs_update = false;
 
-        // Make sure the index is only updated once per session since it is an
-        // expensive operation. This generally only happens when the resolver
-        // is run multiple times, such as during `cargo publish`.
         if self.is_updated() {
             return Ok(());
         }
@@ -321,8 +384,11 @@ impl<'cfg> RegistryData for RemoteRegistry<'cfg> {
         Ok(())
     }
 
+    /// Read the general concept for `invalidate_cache()` on
+    /// [`RegistryData::invalidate_cache`].
+    ///
+    /// To fully invalidate, undo [`RemoteRegistry::mark_updated`]'s work.
     fn invalidate_cache(&mut self) {
-        // To fully invalidate, undo `mark_updated`s work
         self.needs_update = true;
     }
 
@@ -365,9 +431,10 @@ impl<'cfg> RegistryData for RemoteRegistry<'cfg> {
     }
 }
 
+/// Implemented to just be sure to drop `tree` field before our other fields.
+/// See SAFETY inside [`RemoteRegistry::tree()`] for more.
 impl<'cfg> Drop for RemoteRegistry<'cfg> {
     fn drop(&mut self) {
-        // Just be sure to drop this before our other fields
         self.tree.borrow_mut().take();
     }
 }