Set the exit code when changes are found

Closes #285

CHANGELOG.md

@@ -15,6 +15,12 @@ directory.
 Fixed a race condition where diffing directories would lead to
 interleaved output from different files.
 
+### Command Line Interface
+
+Difftastic now sets the exit code if it finds changes. See [usage in
+the manual](https://difftastic.wilfred.me.uk/usage.html) for the full
+list of exit codes used.
+
 ## 0.38 (released 14th November 2022)
 
 ### Parsing

manual/src/usage.md

@@ -53,3 +53,14 @@ are also visible in `--help`.
 For example, `DFT_BACKGROUND=light` is equivalent to
 `--background=light`. This is useful when using VCS tools like git,
 where you are not invoking the `difft` binary directly.
+
+## Exit Codes
+
+0: Difftastic found no syntactic changes (in text files) or no byte
+changes (in binary files).
+
+1: Difftastic found syntactic changes (in text files) or byte changes
+(in binary files).
+
+2: Difftastic was given invalid arguments, such as file paths that it
+couldn't read, or the wrong number of arguments.

src/exit_codes.rs

@@ -0,0 +1,12 @@
+/// Successfully ran a diff, found no syntactic changes in text files
+/// or byte changes in binary files.
+pub const EXIT_SUCCESS: i32 = 0;
+
+/// Successfully ran a diff, found syntactic changes in text files or
+/// byte changes in binary files.
+pub const EXIT_FOUND_CHANGES: i32 = 1;
+
+/// Invalid arguments given to difftastic. This could be usage errors
+/// (e.g. invalid numbers of arguments) or invalid paths (e.g. files
+/// we don't have permission to read).
+pub const EXIT_BAD_ARGUMENTS: i32 = 2;

src/files.rs

@@ -10,6 +10,7 @@ use std::{
 use rustc_hash::FxHashSet;
 use walkdir::WalkDir;
 
+use crate::exit_codes::EXIT_BAD_ARGUMENTS;
 use crate::options::FileArgument;
 
 pub fn read_files_or_die(
@@ -39,7 +40,7 @@ pub fn read_files_or_die(
             if let Err(e) = rhs_res {
                 eprint_read_error(rhs_path, &e);
             }
-            std::process::exit(1);
+            std::process::exit(EXIT_BAD_ARGUMENTS);
         }
     }
 }
@@ -96,7 +97,7 @@ pub fn read_or_die(path: &Path) -> Vec<u8> {
         Ok(src) => src,
         Err(e) => {
             eprint_read_error(&FileArgument::NamedPath(path.to_path_buf()), &e);
-            std::process::exit(1);
+            std::process::exit(EXIT_BAD_ARGUMENTS);
         }
     }
 }

src/main.rs

@@ -22,6 +22,7 @@
 mod constants;
 mod diff;
 mod display;
+mod exit_codes;
 mod files;
 mod line_parser;
 mod lines;
@@ -40,6 +41,7 @@ use crate::parse::syntax;
 use diff::changes::ChangeMap;
 use diff::dijkstra::ExceededGraphLimit;
 use display::context::opposite_positions;
+use exit_codes::{EXIT_FOUND_CHANGES, EXIT_SUCCESS};
 use files::{
     guess_content, read_files_or_die, read_or_die, relative_paths_in_either, ProbableFileKind,
 };
@@ -58,6 +60,8 @@ use diff::sliders::fix_all_sliders;
 use options::{DisplayMode, DisplayOptions, FileArgument, Mode, DEFAULT_TAB_WIDTH};
 use owo_colors::OwoColorize;
 use rayon::prelude::*;
+use std::sync::atomic::{AtomicBool, Ordering};
+use std::sync::Arc;
 use std::{env, path::Path};
 use summary::{DiffResult, FileContent};
 use syntax::init_next_prev;
@@ -175,6 +179,7 @@ fn main() {
                 );
             }
 
+            let encountered_changes = Arc::new(AtomicBool::new(false));
             match (&lhs_path, &rhs_path) {
                 (
                     options::FileArgument::NamedPath(lhs_path),
@@ -186,10 +191,16 @@ fn main() {
                     // https://github.com/rayon-rs/rayon/issues/210#issuecomment-551319338
                     let (send, recv) = std::sync::mpsc::sync_channel(1);
 
+                    let encountered_changes = encountered_changes.clone();
                     let print_options = display_options.clone();
+
                     let printing_thread = std::thread::spawn(move || {
                         for diff_result in recv.into_iter() {
                             print_diff_result(&print_options, &diff_result);
+
+                            if diff_result.has_reportable_change() {
+                                encountered_changes.store(true, Ordering::Relaxed);
+                            }
                         }
                     });
 
@@ -221,9 +232,20 @@ fn main() {
                         language_override,
                     );
                     print_diff_result(&display_options, &diff_result);
+
+                    if diff_result.has_reportable_change() {
+                        encountered_changes.store(true, Ordering::Relaxed);
                     }
                 }
             }
+
+            let exit_code = if encountered_changes.load(Ordering::Relaxed) {
+                EXIT_FOUND_CHANGES
+            } else {
+                EXIT_SUCCESS
+            };
+            std::process::exit(exit_code);
+        }
     };
 }
 

src/options.rs

@@ -6,7 +6,9 @@ use atty::Stream;
 use clap::{crate_authors, crate_description, crate_version, Arg, Command};
 use const_format::formatcp;
 
-use crate::{display::style::BackgroundColor, parse::guess_language};
+use crate::{
+    display::style::BackgroundColor, exit_codes::EXIT_BAD_ARGUMENTS, parse::guess_language,
+};
 
 pub const DEFAULT_BYTE_LIMIT: usize = 1_000_000;
 // Chosen experimentally: this is sufficiently many for all the sample
@@ -383,7 +385,7 @@ pub fn parse_args() -> Mode {
             }
             eprintln!("USAGE:\n\n    {}\n", USAGE);
             eprintln!("For more information try --help");
-            std::process::exit(1);
+            std::process::exit(EXIT_BAD_ARGUMENTS);
         }
     };
 

src/summary.rs

@@ -23,3 +23,15 @@ pub struct DiffResult {
     pub rhs_positions: Vec<MatchedPos>,
     pub has_same_bytes: bool,
 }
+
+impl DiffResult {
+    pub fn has_reportable_change(&self) -> bool {
+        if matches!(self.lhs_src, FileContent::Binary)
+            || matches!(self.rhs_src, FileContent::Binary)
+        {
+            return !self.has_same_bytes;
+        }
+
+        !self.hunks.is_empty()
+    }
+}