cli: add a command for printing the contents of a file in a revision

I'm adding this mostly because it's useful for testing. That's also
the reason it supports displaying conflicts. I didn't call it `cat`
like `hg cat` because I haven't found `hg cat` on multiple files
useful.

CHANGELOG.md

@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### New features
+
+* The new `jj print` command prints the contents of a file in a revision.
+
 ## [0.4.0] - 2022-04-02
 
 ### Breaking changes

src/commands.rs

@@ -936,6 +936,7 @@ enum Commands {
     Checkout(CheckoutArgs),
     Untrack(UntrackArgs),
     Files(FilesArgs),
+    Print(PrintArgs),
     Diff(DiffArgs),
     Show(ShowArgs),
     Status(StatusArgs),
@@ -1012,6 +1013,15 @@ struct FilesArgs {
     paths: Vec<String>,
 }
 
+/// Print contents of a file in a revision
+#[derive(clap::Args, Clone, Debug)]
+struct PrintArgs {
+    /// The revision to get the file contents from
+    #[clap(long, short, default_value = "@")]
+    revision: String,
+    path: String,
+}
+
 #[derive(clap::Args, Clone, Debug)]
 #[clap(group(ArgGroup::new("format").args(&["summary", "git", "color-words"])))]
 struct DiffFormatArgs {
@@ -1899,6 +1909,34 @@ fn cmd_files(ui: &mut Ui, command: &CommandHelper, args: &FilesArgs) -> Result<(
     Ok(())
 }
 
+fn cmd_print(ui: &mut Ui, command: &CommandHelper, args: &PrintArgs) -> Result<(), CommandError> {
+    let mut workspace_command = command.workspace_helper(ui)?;
+    let commit = workspace_command.resolve_single_rev(ui, &args.revision)?;
+    let path = ui.parse_file_path(workspace_command.workspace_root(), &args.path)?;
+    let repo = workspace_command.repo();
+    match commit.tree().path_value(&path) {
+        None => {
+            return Err(CommandError::UserError("No such path".to_string()));
+        }
+        Some(TreeValue::Normal { id, .. }) => {
+            let mut contents = repo.store().read_file(&path, &id)?;
+            std::io::copy(&mut contents, &mut ui.stdout_formatter().as_mut())?;
+        }
+        Some(TreeValue::Conflict(id)) => {
+            let conflict = repo.store().read_conflict(&path, &id)?;
+            let mut contents = vec![];
+            conflicts::materialize_conflict(repo.store(), &path, &conflict, &mut contents).unwrap();
+            ui.stdout_formatter().write_all(&contents)?;
+        }
+        _ => {
+            return Err(CommandError::UserError(
+                "Path exists but is not a file".to_string(),
+            ));
+        }
+    }
+    Ok(())
+}
+
 fn show_color_words_diff_hunks(
     left: &[u8],
     right: &[u8],
@@ -4556,6 +4594,7 @@ where
         Commands::Checkout(sub_args) => cmd_checkout(&mut ui, &command_helper, sub_args),
         Commands::Untrack(sub_args) => cmd_untrack(&mut ui, &command_helper, sub_args),
         Commands::Files(sub_args) => cmd_files(&mut ui, &command_helper, sub_args),
+        Commands::Print(sub_args) => cmd_print(&mut ui, &command_helper, sub_args),
         Commands::Diff(sub_args) => cmd_diff(&mut ui, &command_helper, sub_args),
         Commands::Show(sub_args) => cmd_show(&mut ui, &command_helper, sub_args),
         Commands::Status(sub_args) => cmd_status(&mut ui, &command_helper, sub_args),

tests/test_print_command.rs

@@ -0,0 +1,67 @@
+// Copyright 2022 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+use crate::common::TestEnvironment;
+
+pub mod common;
+
+#[test]
+fn test_print() {
+    let test_env = TestEnvironment::default();
+    test_env.jj_cmd_success(test_env.env_root(), &["init", "repo", "--git"]);
+    let repo_path = test_env.env_root().join("repo");
+
+    std::fs::write(repo_path.join("file1"), "a\n").unwrap();
+    test_env.jj_cmd_success(&repo_path, &["new"]);
+    std::fs::write(repo_path.join("file1"), "b\n").unwrap();
+    std::fs::create_dir(repo_path.join("dir")).unwrap();
+    std::fs::write(repo_path.join("dir").join("file2"), "c\n").unwrap();
+
+    let stdout = test_env.jj_cmd_success(&repo_path, &["print", "file1", "-r", "@-"]);
+    insta::assert_snapshot!(stdout, @"a
+");
+    let stdout = test_env.jj_cmd_success(&repo_path, &["print", "file1"]);
+    insta::assert_snapshot!(stdout, @"b
+");
+    let subdir_file = if cfg!(unix) {
+        "dir/file2"
+    } else {
+        "dir\\file2"
+    };
+    let stdout = test_env.jj_cmd_success(&repo_path, &["print", subdir_file]);
+    insta::assert_snapshot!(stdout, @"c
+");
+    let stdout = test_env.jj_cmd_failure(&repo_path, &["print", "non-existent"]);
+    insta::assert_snapshot!(stdout, @"Error: No such path
+");
+    let stdout = test_env.jj_cmd_failure(&repo_path, &["print", "dir"]);
+    insta::assert_snapshot!(stdout, @"Error: Path exists but is not a file
+");
+
+    // Can print a conflict
+    test_env.jj_cmd_success(&repo_path, &["new"]);
+    std::fs::write(repo_path.join("file1"), "c\n").unwrap();
+    test_env.jj_cmd_success(&repo_path, &["rebase", "-d", "@--"]);
+    let stdout = test_env.jj_cmd_success(&repo_path, &["print", "file1"]);
+    insta::assert_snapshot!(stdout, @r###"
+    <<<<<<<
+    -------
+    +++++++
+    -b
+    +a
+    +++++++
+    c
+    >>>>>>>
+    "###);
+}