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mergebase.rs
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mergebase.rs
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//! Persistent storage to cache merge-base queries.
//!
//! A "merge-base" can be described as the common ancestor of two commits.
//! Merge-bases are calculated to determine
//!
//! 1) Whether a commit is a branch off of the main branch.
//! 2) How to order two commits topologically.
//!
//! In a large repository, merge-base queries can be quite expensive when
//! comparing commits which are far away from each other. This can happen, for
//! example, whenever you do a `git pull` to update the main branch, but you
//! haven't yet updated any of your lines of work. Your lines of work are now far
//! away from the current main branch commit, so the merge-base calculation may
//! take a while. It can also happen when simply checking out an old commit to
//! examine it.
use eyre::Context;
use rusqlite::OptionalExtension;
use tracing::instrument;
use crate::git::{NonZeroOid, Repo};
use crate::tui::Effects;
/// On-disk cache for merge-base queries.
pub struct MergeBaseDb<'conn> {
conn: &'conn rusqlite::Connection,
}
impl std::fmt::Debug for MergeBaseDb<'_> {
fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
write!(f, "<MergeBaseDb>")
}
}
#[instrument]
fn init_tables(conn: &rusqlite::Connection) -> eyre::Result<()> {
conn.execute(
"
CREATE TABLE IF NOT EXISTS merge_base_oids (
lhs_oid TEXT NOT NULL,
rhs_oid TEXT NOT NULL,
merge_base_oid TEXT,
UNIQUE (lhs_oid, rhs_oid)
)
",
rusqlite::params![],
)
.wrap_err("Creating tables")?;
Ok(())
}
impl<'conn> MergeBaseDb<'conn> {
/// Constructor.
#[instrument]
pub fn new(conn: &'conn rusqlite::Connection) -> eyre::Result<Self> {
init_tables(conn).wrap_err("Initializing tables")?;
Ok(MergeBaseDb { conn })
}
/// Get the merge-base for two given commits.
///
/// If the query is already in the cache, return the cached result. If
/// not, it is computed, cached, and returned.
///
/// Args:
/// * `repo`: The Git repo.
/// * `lhs_oid`: The first OID (ordering is arbitrary).
/// * `rhs_oid`: The second OID (ordering is arbitrary).
///
/// Returns: The merge-base OID for these two commits. Returns `None` if no
/// merge-base could be found.
#[instrument]
pub fn get_merge_base_oid(
&self,
effects: &Effects,
repo: &Repo,
lhs_oid: NonZeroOid,
rhs_oid: NonZeroOid,
) -> eyre::Result<Option<NonZeroOid>> {
let (_effects, _progress) =
effects.start_operation(crate::tui::OperationType::GetMergeBase);
let (lhs_oid, rhs_oid) = if lhs_oid < rhs_oid {
(lhs_oid, rhs_oid)
} else {
(rhs_oid, lhs_oid)
};
let merge_base_oid: Option<Option<String>> = self
.conn
.query_row(
"
SELECT merge_base_oid
FROM merge_base_oids
WHERE lhs_oid = :lhs_oid
AND rhs_oid = :rhs_oid
",
rusqlite::named_params! {
":lhs_oid": lhs_oid.to_string(),
":rhs_oid": rhs_oid.to_string(),
},
|row| row.get("merge_base_oid"),
)
.optional()
.wrap_err("Querying merge-base DB")?;
match merge_base_oid {
// Cached and non-NULL.
Some(Some(merge_base_oid)) => {
let merge_base_oid: NonZeroOid =
merge_base_oid.parse().wrap_err("Parsing merge-base OID")?;
Ok(Some(merge_base_oid))
}
// Cached and NULL.
Some(None) => Ok(None),
// Not cached.
None => {
let merge_base_oid = repo.find_merge_base(lhs_oid, rhs_oid)?;
// Cache computed merge-base OID.
self.conn
.execute(
"
INSERT INTO merge_base_oids VALUES (
:lhs_oid,
:rhs_oid,
:merge_base_oid
)",
rusqlite::named_params! {
":lhs_oid": &lhs_oid.to_string(),
":rhs_oid": &rhs_oid.to_string(),
":merge_base_oid": &merge_base_oid.map(|oid| oid.to_string()),
},
)
.wrap_err("Caching merge-base OID")?;
Ok(merge_base_oid)
}
}
}
}