commit 5c96beea07c6174cd9cc3bcb10d8bfcbe5560114
Author: Puck Meerburg <puck@puckipedia.com>
Date:   Thu May 2 18:01:23 2024 +0000

    Initial commit

diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..ea8c4bf
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1 @@
+/target
diff --git a/Cargo.lock b/Cargo.lock
new file mode 100644
index 0000000..1ac7f76
--- /dev/null
+++ b/Cargo.lock
@@ -0,0 +1,426 @@
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diff --git a/Cargo.toml b/Cargo.toml
new file mode 100644
index 0000000..0244b84
--- /dev/null
+++ b/Cargo.toml
@@ -0,0 +1,10 @@
+[package]
+name = "gerrit-filter-branch"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
+clap = { version = "4.5.4", features = ["derive"] }
+git2 = "0.18.3"
diff --git a/src/main.rs b/src/main.rs
new file mode 100644
index 0000000..9f33336
--- /dev/null
+++ b/src/main.rs
@@ -0,0 +1,251 @@
+use std::collections::{HashMap, HashSet};
+
+use clap::Parser;
+use git2::{Oid, Repository, Signature};
+
+#[derive(Parser, Debug)]
+#[command(version, about, long_about = None)]
+struct Args {
+    #[arg(short, long)]
+    /// The path to the repository to rewrite.
+    repo: String,
+
+    #[arg(short, long)]
+    /// The email address to look for.
+    rewrite_email: String,
+
+    #[arg(short, long)]
+    /// The author/committer name to replace the old one with.
+    new_name: String,
+
+    #[arg(short, long)]
+    /// The author/committer email to replace the old one with.
+    new_email: String,
+}
+
+struct State {
+    repo: git2::Repository,
+    rewritten: HashMap<Oid, Oid>,
+    unrewritten: HashSet<Oid>,
+    args: Args,
+}
+
+impl State {
+    /// Recursively rewrite a non-meta commit.
+    fn rewrite(&mut self, oid: Oid) -> Oid {
+        if self.unrewritten.contains(&oid) {
+            return oid;
+        }
+
+        if let Some(oid) = self.rewritten.get(&oid) {
+            return *oid;
+        }
+
+        let mut commit = self.repo.find_commit(oid).unwrap();
+        // If the email address matches, we will have to rewrite this commit either way, even if parent IDs match.
+        let rewrite_author = commit.author().email().unwrap() == &self.args.rewrite_email;
+
+        let parent_ids: Vec<_> = commit.parent_ids().collect();
+        drop(commit);
+
+        let mut new_parent_ids = Vec::new();
+        let mut same_parents = true;
+        // Rewrite the parents of this commit recursively.
+        for id in parent_ids {
+            let new_id = self.rewrite(id);
+            if new_id != id {
+                // Keep track if the parents are identical cheaply.
+                same_parents = false;
+            }
+            new_parent_ids.push(new_id);
+        }
+
+        // If this commit has nothing to rewrite, skip it now.
+        if same_parents && !rewrite_author {
+            self.unrewritten.insert(oid);
+            return oid;
+        }
+
+        commit = self.repo.find_commit(oid).unwrap();
+
+        // Find the list of new parent commits for this commit.
+        let new_parents: Vec<_> = new_parent_ids
+            .into_iter()
+            .map(|f| self.repo.find_commit(f).unwrap())
+            .collect();
+        let new_parents_list: Vec<_> = new_parents.iter().collect();
+
+        // Rewrite author and committer respectively, where necessary.
+        let mut author = commit.author().to_owned();
+        if author.email().unwrap() == &self.args.rewrite_email {
+            author =
+                Signature::new(&self.args.new_name, &self.args.new_email, &author.when()).unwrap();
+        }
+
+        let mut committer = commit.committer().to_owned();
+        if committer.email().unwrap() == &self.args.rewrite_email {
+            committer =
+                Signature::new(&self.args.new_name, &self.args.new_email, &committer.when())
+                    .unwrap();
+        }
+
+        // Generate a new commit (Commit::amend doesn't work when changing parents.)
+        let new_id = self
+            .repo
+            .commit(
+                None,
+                &author,
+                &committer,
+                commit.message_raw().unwrap(),
+                &commit.tree().unwrap(),
+                &new_parents_list[..],
+            )
+            .unwrap();
+
+        self.rewritten.insert(oid, new_id);
+
+        new_id
+    }
+
+    /// Rewrite a comment/meta commit.
+    fn rewrite_meta(&mut self, oid: Oid) -> Oid {
+        let mut commit = self.repo.find_commit(oid).unwrap();
+
+        // A notedb commit always has one parent; simplify the logic and rewrite the parent if needed.
+        let mut parent = None;
+        if commit.parent_count() > 0 {
+            let parent_oid = commit.parent_id(0).unwrap();
+            drop(commit);
+            let new_meta = self.rewrite_meta(parent_oid);
+            parent = Some(self.repo.find_commit(new_meta).unwrap());
+            commit = self.repo.find_commit(oid).unwrap();
+        }
+
+        let tree = commit.tree().unwrap();
+        let mut builder = self.repo.treebuilder(None).unwrap();
+
+        // Notedb comment files are always in the root of the tree.
+        for entry in &tree {
+            let name = entry.name().unwrap();
+
+            // The name of the meta file is a valid OID of a commit.
+            let oid = Oid::from_str(name).expect("unexpected filename");
+
+            // Find the new OID to use for the filename.
+            let newoid = if self.unrewritten.contains(&oid) {
+                oid
+            } else {
+                *self.rewritten.get(&oid).unwrap()
+            };
+
+            let blob = self.repo.find_blob(entry.id()).unwrap();
+            let content = std::str::from_utf8(blob.content()).unwrap();
+            let mut newcontent = content.to_owned();
+            // Rewrite each commit ID that we've rewritten so far.
+            // This is a hack: the data is JSON and we could parse it as such, but this will be likely as reliable,
+            // and SHA1 hashes are unique enough we don't have to worry about accidental misrewrites causing invalid data.
+            for (from, to) in &self.rewritten {
+                newcontent = newcontent.replace(&from.to_string(), &to.to_string());
+            }
+
+            // Store as a blob, and insert into the tree using the rewritten OID's name.
+            let blob = self.repo.blob(newcontent.as_bytes()).unwrap();
+            builder
+                .insert(newoid.to_string(), blob, entry.filemode_raw())
+                .unwrap();
+        }
+
+        // Write the tree to disk.
+        let newtree = builder.write().unwrap();
+
+        // Replace the rewritten OIDs in the update messages too.
+        // This is a hack: see above for why it's fine.
+        let mut message = commit.message_raw().unwrap().to_string();
+        for (from, to) in &self.rewritten {
+            message = message.replace(&from.to_string(), &to.to_string());
+        }
+
+        // Write the commit back to disk.
+        if let Some(parent) = parent.as_ref() {
+            self.repo
+                .commit(
+                    None,
+                    &commit.author(),
+                    &commit.committer(),
+                    &message,
+                    &self.repo.find_tree(newtree).unwrap(),
+                    &[parent],
+                )
+                .unwrap()
+        } else {
+            self.repo
+                .commit(
+                    None,
+                    &commit.author(),
+                    &commit.committer(),
+                    &message,
+                    &self.repo.find_tree(newtree).unwrap(),
+                    &[],
+                )
+                .unwrap()
+        }
+    }
+}
+
+fn main() {
+    let args = Args::parse();
+    let repo = args.repo.clone();
+
+    let mut state = State {
+        repo: Repository::open_bare(repo).unwrap(),
+        rewritten: HashMap::new(),
+        unrewritten: HashSet::new(),
+        args,
+    };
+
+    let refs: Vec<String> = state
+        .repo
+        .references()
+        .unwrap()
+        .names()
+        .map(|f| f.unwrap().to_owned())
+        .collect();
+
+    // First rewrite the non-meta references, as the meta references will always refer to these.
+    for gref in &refs {
+        if gref.ends_with("/meta") {
+            continue;
+        }
+
+        let refoid = state.repo.find_reference(&gref).unwrap().target().unwrap();
+        let newoid = state.rewrite(refoid);
+        if refoid != newoid {
+            println!("{:?}: {:?} -> {:?}", gref, refoid, newoid);
+            state
+                .repo
+                .reference(&gref, newoid, true, "filter-branch rewrite")
+                .unwrap();
+        } else {
+            println!("{:?}: {:?} (unchanged)", gref, refoid);
+        }
+    }
+
+    for gref in &refs {
+        if !gref.ends_with("/meta") {
+            continue;
+        }
+
+        let refoid = state.repo.find_reference(&gref).unwrap().target().unwrap();
+        let newoid = state.rewrite_meta(refoid);
+
+        if refoid != newoid {
+            println!("{:?}: {:?} -> {:?}", gref, refoid, newoid);
+            state
+                .repo
+                .reference(&gref, newoid, true, "filter-branch rewrite")
+                .unwrap();
+        } else {
+            println!("{:?}: {:?} (unchanged)", gref, refoid);
+        }
+    }
+}