fix(git): make checkout names independent of git config (#17289)

### What does this PR try to resolve?

Previously the length followed user's `core.abbrev`.
Cargo cache layout should be stable and independent from user settings,
hence this fix.

Downside of this is that if people already had a system/user-wide
git config `core.abbrev` set to number other than 7,
it may lead to re-downloads.

### How to test and review this PR?

Commit by commit.

This was found during `-Ztrim-paths` implementation,
which we would like to embed revision in the remapped object files,
and we want it to be stable.
(but probably we don't need to embed)

Author: epage

src/sources/git/utils.rs

@@ -34,18 +34,6 @@ use std::time::{Duration, Instant};
 /// checkout is ready to go. See [`GitCheckout::reset`] for why we need this.
 const CHECKOUT_READY_LOCK: &str = ".cargo-ok";
 
-/// A short abbreviated OID.
-///
-/// Exists for avoiding extra allocations in [`GitDatabase::to_short_id`].
-pub struct GitShortID(git2::Buf);
-
-impl GitShortID {
-    /// Views the short ID as a `str`.
-    pub fn as_str(&self) -> &str {
-        self.0.as_str().unwrap()
-    }
-}
-
 /// A remote repository. It gets cloned into a local [`GitDatabase`].
 #[derive(PartialEq, Clone, Debug)]
 pub struct GitRemote {
@@ -205,10 +193,26 @@ impl GitDatabase {
         Ok(checkout)
     }
 
-    /// Get a short OID for a `revision`, usually 7 chars or more if ambiguous.
-    pub fn to_short_id(&self, revision: git2::Oid) -> CargoResult<GitShortID> {
-        let obj = self.repo.find_object(revision, None)?;
-        Ok(GitShortID(obj.short_id()?))
+    /// Get a short OID for a `revision`, 7 chars or more if ambiguous.
+    ///
+    /// Like [`git2::Object::short_id`]
+    /// but ignores the user's `core.abbrev` git config.
+    pub fn to_short_id(&self, rev: git2::Oid) -> CargoResult<String> {
+        const MIN_ABBREV_LEN: usize = 7; // this is git/libgit2's default
+        let odb = self.repo.odb()?;
+        let mut len = MIN_ABBREV_LEN;
+        let mut hex = rev.to_string();
+        // quasi- re-implementation of
+        // https://github.com/libgit2/libgit2/blob/26055f5af74ab/src/libgit2/object.c#L523-L573
+        while len < hex.len() {
+            match odb.exists_prefix(rev, len) {
+                Ok(_) => break,
+                Err(err) if err.code() == git2::ErrorCode::Ambiguous => len += 1,
+                Err(err) => return Err(err.into()),
+            }
+        }
+        hex.truncate(len);
+        Ok(hex)
     }
 
     /// Checks if the database contains the object of this `oid`..

tests/testsuite/git.rs

@@ -3163,6 +3163,49 @@ fn templatedir_doesnt_cause_problems() {
     p.cargo("check").run();
 }
 
+#[cargo_test]
+fn checkout_name_with_core_abbrev_config() {
+    let git_project = git::new("dep1", |project| {
+        project
+            .file("Cargo.toml", &basic_manifest("dep1", "0.5.0"))
+            .file("src/lib.rs", "")
+    });
+
+    let p = project()
+        .file(
+            "Cargo.toml",
+            &format!(
+                r#"
+                    [package]
+                    name = "foo"
+                    version = "0.1.0"
+                    edition = "2015"
+
+                    [dependencies]
+                    dep1 = {{ git = "{}" }}
+                "#,
+                git_project.url()
+            ),
+        )
+        .file("src/lib.rs", "")
+        .build();
+
+    fs::write(paths::home().join(".gitconfig"), "[core]\n\tabbrev = 4\n").unwrap();
+
+    p.cargo("fetch").run();
+
+    let mut co_paths = t!(glob::glob(
+        paths::home()
+            .join(".cargo/git/checkouts/dep1-*/*")
+            .to_str()
+            .unwrap()
+    ));
+    let co_path = co_paths.next().unwrap().unwrap();
+    let rev = co_path.file_name().unwrap().to_str().unwrap();
+    // The checkout directory name ignores the user's `core.abbrev`.
+    assert_eq!(rev.len(), 7);
+}
+
 #[cargo_test(requires = "git")]
 fn git_with_cli_force() {
     // Supports a force-pushed repo.