feat: Add Jujutsu diff handling to Helix

helix-term/Cargo.toml

@@ -13,10 +13,14 @@ repository.workspace = true
 homepage.workspace = true
 
 [features]
-default = ["git"]
+default = ["vcs"]
 unicode-lines = ["helix-core/unicode-lines"]
 integration = ["helix-event/integration_test"]
+
+# All VCSes available for diffs in Helix
+vcs = ["git", "jujutsu"]
 git = ["helix-vcs/git"]
+jujutsu = ["helix-vcs/jujutsu"]
 
 [[bin]]
 name = "hx"

helix-vcs/Cargo.toml

@@ -24,9 +24,13 @@ imara-diff = "0.1.5"
 anyhow = "1"
 
 log = "0.4"
+# For `jujutsu`
+tempfile = { version = "3.10", optional = true }
 
 [features]
 git = ["gix"]
+jujutsu = ["tempfile"]
+
 
 [dev-dependencies]
 tempfile = "3.10"

helix-vcs/src/jujutsu.rs

@@ -0,0 +1,117 @@
+//! Jujutsu works with several backends and could add new ones in the future. Private builds of
+//! it could also have private backends. Those make it hard to use `jj-lib` since it won't have
+//! access to newer or private backends and fail to compute the diffs for them.
+//!
+//! Instead in case there *is* a diff to base ourselves on, we copy it to a tempfile or just use the
+//! current file if not.
+
+use std::path::Path;
+use std::process::Command;
+use std::sync::Arc;
+
+use anyhow::{Context, Result};
+use arc_swap::ArcSwap;
+
+use crate::DiffProvider;
+
+pub(super) struct Jujutsu;
+
+impl DiffProvider for Jujutsu {
+    fn get_diff_base(&self, file: &Path) -> Result<Vec<u8>> {
+        let jj_root_dir = find_jj_root(file)?;
+
+        // We extracted the `jj_root_dir` from the file itself, if stripping the prefix fails
+        // something has gone very very wrong
+        let file_rel_to_dot_jj = file
+            .strip_prefix(jj_root_dir)
+            .expect("failed to strip diff path from jj root dir");
+
+        let tmpfile = tempfile::NamedTempFile::with_prefix("helix-jj-diff-")
+            .context("could not create tempfile to save jj diff base")?;
+        let tmppath = tmpfile.path();
+
+        let copy_bin = if cfg!(windows) { "copy.exe" } else { "cp" };
+
+        let status = Command::new("jj")
+            .arg("--repository")
+            .arg(jj_root_dir)
+            .args(["diff", "--revision", "@", "--config-toml"])
+            // Copy the temporary file provided by jujutsu to a temporary path of our own,
+            // because the `$left` directory is deleted when `jj` finishes executing.
+            .arg(format!(
+                "ui.diff.tool = ['{exe}', '$left/{base}', '{target}']",
+                exe = copy_bin,
+                base = file_rel_to_dot_jj.display(),
+                // Where to copy the jujutsu-provided file
+                target = tmppath.display(),
+            ))
+            // Restrict the diff to the current file
+            .arg(file)
+            .stdout(std::process::Stdio::null())
+            .stderr(std::process::Stdio::null())
+            .status()
+            .context("failed to execute jj diff command")?;
+
+        let use_jj_path =
+            status.success() && std::fs::metadata(tmppath).map_or(false, |m| m.len() > 0);
+        // If the copy call inside `jj diff` succeeded, the tempfile is the one containing the base
+        // else it's just the original file (so no diff). We check for size since `jj` can return
+        // 0-sized files when there are no diffs to present for the file.
+        let diff_base_path = if use_jj_path { tmppath } else { file };
+
+        // If the command succeeded, it means we either copied the jujutsu base or the current file,
+        // so there should always be something to read and compare to.
+        std::fs::read(diff_base_path).context("could not read jj diff base from the target")
+    }
+
+    fn get_current_head_name(&self, file: &Path) -> Result<Arc<ArcSwap<Box<str>>>> {
+        let jj_root_dir = find_jj_root(file)?;
+
+        // See <https://github.com/martinvonz/jj/blob/main/docs/templates.md>
+        //
+        // This will produce the following:
+        //
+        // - If there are no branches:    `vyvqwlmsvnlkmqrvqktpuluvuknuxpmm`
+        // - If there is a single branch: `vyvqwlmsvnlkmqrvqktpuluvuknuxpmm (master)`
+        // - If there are 2+ branches:    `vyvqwlmsvnlkmqrvqktpuluvuknuxpmm (master, jj-diffs)`
+        //
+        // Always using the long id makes it easy to share it with others, which would not be the
+        // case for shorter ones: they could have a local change that renders it ambiguous.
+        let template = r#"separate(" ", change_id, surround("(", ")", branches.join(", ")))"#;
+
+        let out = Command::new("jj")
+            .arg("--repository")
+            .arg(jj_root_dir)
+            .args([
+                "log",
+                "--color",
+                "never",
+                "--revisions",
+                "@", // Only display the current revision
+                "--no-graph",
+                "--template",
+                template,
+            ])
+            .output()?;
+
+        if !out.status.success() {
+            anyhow::bail!("jj log command executed but failed");
+        }
+
+        let out = String::from_utf8(out.stdout)?;
+
+        let rev = out
+            .lines()
+            .next()
+            .context("should always find at least one line")?;
+
+        Ok(Arc::new(ArcSwap::from_pointee(rev.into())))
+    }
+}
+
+// Move up until we find the repository's root
+fn find_jj_root(file: &Path) -> Result<&Path> {
+    file.ancestors()
+        .find(|p| p.join(".jj").exists())
+        .context("no .jj dir found in parents")
+}

helix-vcs/src/lib.rs

@@ -10,6 +10,9 @@ pub use Dummy as Git;
 #[cfg(feature = "git")]
 mod git;
 
+#[cfg(feature = "jujutsu")]
+mod jujutsu;
+
 mod diff;
 
 pub use diff::{DiffHandle, Hunk};
@@ -72,7 +75,14 @@ impl Default for DiffProviderRegistry {
         // currently only git is supported
         // TODO make this configurable when more providers are added
         let git: Box<dyn DiffProvider> = Box::new(Git);
-        let providers = vec![git];
+        #[cfg(feature = "jujutsu")]
+        let jj: Box<dyn DiffProvider> = Box::new(jujutsu::Jujutsu);
+
+        let providers = vec![
+            git,
+            #[cfg(feature = "jujutsu")]
+            jj,
+        ];
         DiffProviderRegistry { providers }
     }
 }