external diff editor: move utility functions to a new file

This is preparation for jj-vcs#3292, which will use these functions. The main
goal is to merge the parts of jj-vcs#3292 that are likely to cause merge
conflicts with other PRs while I polish it up.

cli/src/merge_tools/diff_working_copies.rs

@@ -0,0 +1,280 @@
+use std::collections::HashMap;
+use std::fs::File;
+use std::io::{self, Write};
+use std::path::{Path, PathBuf};
+use std::sync::Arc;
+
+use futures::StreamExt;
+use jj_lib::backend::MergedTreeId;
+use jj_lib::fsmonitor::FsmonitorKind;
+use jj_lib::gitignore::GitIgnoreFile;
+use jj_lib::local_working_copy::{TreeState, TreeStateError};
+use jj_lib::matchers::Matcher;
+use jj_lib::merged_tree::MergedTree;
+use jj_lib::repo_path::RepoPathBuf;
+use jj_lib::store::Store;
+use jj_lib::working_copy::{CheckoutError, SnapshotOptions};
+use pollster::FutureExt;
+use tempfile::TempDir;
+use thiserror::Error;
+
+use super::external::ExternalToolError;
+use super::DiffEditError;
+
+#[derive(Debug, Error)]
+pub enum DiffCheckoutError {
+    #[error("Failed to write directories to diff")]
+    Checkout(#[from] CheckoutError),
+    #[error("Error setting up temporary directory")]
+    SetUpDir(#[source] std::io::Error),
+    #[error(transparent)]
+    TreeState(#[from] TreeStateError),
+}
+
+pub(crate) struct DiffWorkingCopies {
+    _temp_dir: TempDir, // Temp dir will be deleted when this is dropped
+    left_tree_state: TreeState,
+    right_tree_state: TreeState,
+    output_tree_state: Option<TreeState>,
+    instructions_path_to_cleanup: Option<PathBuf>,
+}
+
+impl DiffWorkingCopies {
+    pub fn left_working_copy_path(&self) -> &Path {
+        self.left_tree_state.working_copy_path()
+    }
+
+    pub fn right_working_copy_path(&self) -> &Path {
+        self.right_tree_state.working_copy_path()
+    }
+
+    pub fn output_working_copy_path(&self) -> Option<&Path> {
+        self.output_tree_state
+            .as_ref()
+            .map(|state| state.working_copy_path())
+    }
+
+    pub fn to_command_variables(&self) -> HashMap<&'static str, &str> {
+        let left_wc_dir = self.left_working_copy_path();
+        let right_wc_dir = self.right_working_copy_path();
+        let mut result = maplit::hashmap! {
+            "left" => left_wc_dir.to_str().expect("temp_dir should be valid utf-8"),
+            "right" => right_wc_dir.to_str().expect("temp_dir should be valid utf-8"),
+        };
+        if let Some(output_wc_dir) = self.output_working_copy_path() {
+            result.insert(
+                "output",
+                output_wc_dir
+                    .to_str()
+                    .expect("temp_dir should be valid utf-8"),
+            );
+        }
+        result
+    }
+}
+
+fn check_out(
+    store: Arc<Store>,
+    wc_dir: PathBuf,
+    state_dir: PathBuf,
+    tree: &MergedTree,
+    sparse_patterns: Vec<RepoPathBuf>,
+) -> Result<TreeState, DiffCheckoutError> {
+    std::fs::create_dir(&wc_dir).map_err(DiffCheckoutError::SetUpDir)?;
+    std::fs::create_dir(&state_dir).map_err(DiffCheckoutError::SetUpDir)?;
+    let mut tree_state = TreeState::init(store, wc_dir, state_dir)?;
+    tree_state.set_sparse_patterns(sparse_patterns)?;
+    tree_state.check_out(tree)?;
+    Ok(tree_state)
+}
+
+pub(crate) fn new_utf8_temp_dir(prefix: &str) -> io::Result<TempDir> {
+    let temp_dir = tempfile::Builder::new().prefix(prefix).tempdir()?;
+    if temp_dir.path().to_str().is_none() {
+        // Not using .display() as we know the path contains unprintable character
+        let message = format!("path {:?} is not valid UTF-8", temp_dir.path());
+        return Err(io::Error::new(io::ErrorKind::InvalidData, message));
+    }
+    Ok(temp_dir)
+}
+
+pub(crate) fn set_readonly_recursively(path: &Path) -> Result<(), std::io::Error> {
+    // Directory permission is unchanged since files under readonly directory cannot
+    // be removed.
+    let metadata = path.symlink_metadata()?;
+    if metadata.is_dir() {
+        for entry in path.read_dir()? {
+            set_readonly_recursively(&entry?.path())?;
+        }
+        Ok(())
+    } else if metadata.is_file() {
+        let mut perms = metadata.permissions();
+        perms.set_readonly(true);
+        std::fs::set_permissions(path, perms)
+    } else {
+        Ok(())
+    }
+}
+
+#[derive(Debug, Clone, Copy)]
+pub(crate) enum DiffSide {
+    // Left,
+    Right,
+}
+
+/// Check out the two trees in temporary directories. Only include changed files
+/// in the sparse checkout patterns.
+pub(crate) fn check_out_trees(
+    store: &Arc<Store>,
+    left_tree: &MergedTree,
+    right_tree: &MergedTree,
+    matcher: &dyn Matcher,
+    output_is: Option<DiffSide>,
+) -> Result<DiffWorkingCopies, DiffCheckoutError> {
+    let changed_files: Vec<_> = left_tree
+        .diff_stream(right_tree, matcher)
+        .map(|(path, _diff)| path)
+        .collect()
+        .block_on();
+
+    let temp_dir = new_utf8_temp_dir("jj-diff-").map_err(DiffCheckoutError::SetUpDir)?;
+    let left_wc_dir = temp_dir.path().join("left");
+    let left_state_dir = temp_dir.path().join("left_state");
+    let right_wc_dir = temp_dir.path().join("right");
+    let right_state_dir = temp_dir.path().join("right_state");
+    let left_tree_state = check_out(
+        store.clone(),
+        left_wc_dir,
+        left_state_dir,
+        left_tree,
+        changed_files.clone(),
+    )?;
+    let right_tree_state = check_out(
+        store.clone(),
+        right_wc_dir,
+        right_state_dir,
+        right_tree,
+        changed_files.clone(),
+    )?;
+    let output_tree_state = output_is
+        .map(|output_side| {
+            let output_wc_dir = temp_dir.path().join("output");
+            let output_state_dir = temp_dir.path().join("output_state");
+            check_out(
+                store.clone(),
+                output_wc_dir,
+                output_state_dir,
+                match output_side {
+                    // DiffSide::Left => left_tree,
+                    DiffSide::Right => right_tree,
+                },
+                changed_files,
+            )
+        })
+        .transpose()?;
+    Ok(DiffWorkingCopies {
+        _temp_dir: temp_dir,
+        left_tree_state,
+        right_tree_state,
+        output_tree_state,
+        instructions_path_to_cleanup: None,
+    })
+}
+
+/// Checks out the trees, populates JJ_INSTRUCTIONS, and makes appropriate sides
+/// readonly.
+pub fn check_out_trees_for_diffedit(
+    store: &Arc<Store>,
+    left_tree: &MergedTree,
+    right_tree: &MergedTree,
+    matcher: &dyn Matcher,
+    output_is: Option<DiffSide>,
+    instructions: Option<&str>,
+) -> Result<DiffWorkingCopies, DiffEditError> {
+    let mut diff_wc = check_out_trees(store, left_tree, right_tree, matcher, output_is)?;
+    let got_output_field = output_is.is_some();
+
+    set_readonly_recursively(diff_wc.left_working_copy_path())
+        .map_err(ExternalToolError::SetUpDir)?;
+    if got_output_field {
+        set_readonly_recursively(diff_wc.right_working_copy_path())
+            .map_err(ExternalToolError::SetUpDir)?;
+    }
+    let output_wc_path = diff_wc
+        .output_working_copy_path()
+        .unwrap_or_else(|| diff_wc.right_working_copy_path());
+    let instructions_path_to_cleanup = output_wc_path.join("JJ-INSTRUCTIONS");
+    // In the unlikely event that the file already exists, then the user will simply
+    // not get any instructions.
+    let add_instructions = if !instructions_path_to_cleanup.exists() {
+        instructions
+    } else {
+        None
+    };
+    if let Some(instructions) = add_instructions {
+        let mut output_instructions_file =
+            File::create(&instructions_path_to_cleanup).map_err(ExternalToolError::SetUpDir)?;
+        if diff_wc.right_working_copy_path() != output_wc_path {
+            let mut right_instructions_file =
+                File::create(diff_wc.right_working_copy_path().join("JJ-INSTRUCTIONS"))
+                    .map_err(ExternalToolError::SetUpDir)?;
+            right_instructions_file
+                .write_all(
+                    b"\
+The content of this pane should NOT be edited. Any edits will be
+lost.
+
+You are using the experimental 3-pane diff editor config. Some of
+the following instructions may have been written with a 2-pane
+diff editing in mind and be a little inaccurate.
+
+",
+                )
+                .map_err(ExternalToolError::SetUpDir)?;
+            right_instructions_file
+                .write_all(instructions.as_bytes())
+                .map_err(ExternalToolError::SetUpDir)?;
+            // Note that some diff tools might not show this message and delete the contents
+            // of the output dir instead. Meld does show this message.
+            output_instructions_file
+                .write_all(
+                    b"\
+Please make your edits in this pane.
+
+You are using the experimental 3-pane diff editor config. Some of
+the following instructions may have been written with a 2-pane
+diff editing in mind and be a little inaccurate.
+
+",
+                )
+                .map_err(ExternalToolError::SetUpDir)?;
+        }
+        output_instructions_file
+            .write_all(instructions.as_bytes())
+            .map_err(ExternalToolError::SetUpDir)?;
+        diff_wc.instructions_path_to_cleanup = Some(instructions_path_to_cleanup);
+    }
+
+    Ok(diff_wc)
+}
+
+pub fn snapshot_diffedit_results(
+    diff_wc: DiffWorkingCopies,
+    base_ignores: Arc<GitIgnoreFile>,
+) -> Result<MergedTreeId, DiffEditError> {
+    if let Some(path) = diff_wc.instructions_path_to_cleanup {
+        std::fs::remove_file(path).ok();
+    }
+
+    // Snapshot changes in the temporary output directory.
+    let mut output_tree_state = diff_wc
+        .output_tree_state
+        .unwrap_or(diff_wc.right_tree_state);
+    output_tree_state.snapshot(SnapshotOptions {
+        base_ignores,
+        fsmonitor_kind: FsmonitorKind::None,
+        progress: None,
+        max_new_file_size: u64::MAX,
+    })?;
+    Ok(output_tree_state.current_tree_id().clone())
+}