Print a notice when delta panics

Setting DELTA_DEBUG_LOGFILE=crash.log and repeating the command
writes all information needed to reproduce the crash into that file.

src/delta.rs

@@ -15,6 +15,7 @@ use crate::handlers::{self, merge_conflict};
 use crate::paint::Painter;
 use crate::style::DecorationStyle;
 use crate::utils;
+use crate::utils::RecordDeltaCall;
 
 #[derive(Clone, Debug, PartialEq, Eq)]
 pub enum State {
@@ -147,7 +148,11 @@ impl<'a> StateMachine<'a> {
     where
         I: BufRead,
     {
+        let mut debug_helper = RecordDeltaCall::new(self.config);
+
         while let Some(Ok(raw_line_bytes)) = lines.next() {
+            debug_helper.write(raw_line_bytes);
+
             self.ingest_line(raw_line_bytes);
 
             if self.source == Source::Unknown {

src/env.rs

@@ -10,7 +10,11 @@ const DELTA_EXPERIMENTAL_MAX_LINE_DISTANCE_FOR_NAIVELY_PAIRED_LINES: &str =
     "DELTA_EXPERIMENTAL_MAX_LINE_DISTANCE_FOR_NAIVELY_PAIRED_LINES";
 const DELTA_PAGER: &str = "DELTA_PAGER";
 
-#[derive(Default, Clone)]
+// debug env variables:
+pub const DELTA_DEBUG_LOGFILE: &str = "DELTA_DEBUG_LOGFILE";
+pub const DELTA_DEBUG_LOGFILE_MAX_SIZE: &str = "DELTA_DEBUG_LOGFILE_MAX_SIZE";
+
+#[derive(Default, Clone, Debug)]
 pub struct DeltaEnv {
     pub bat_theme: Option<String>,
     pub colorterm: Option<String>,

src/main.rs

@@ -43,7 +43,7 @@ where
 {
     #[cfg(not(test))]
     {
-        eprintln!("{errmsg}");
+        eprintln!("delta: {errmsg}");
         // As in Config::error_exit_code: use 2 for error
         // because diff uses 0 and 1 for non-error.
         process::exit(2);
@@ -81,6 +81,7 @@ pub fn run_app(
     args: Vec<OsString>,
     capture_output: Option<&mut Cursor<Vec<u8>>>,
 ) -> std::io::Result<i32> {
+    let _panic_printer = utils::PrintNoticeOnPanic::new();
     let env = env::DeltaEnv::init();
     let assets = utils::bat::assets::load_highlighting_assets();
     let (call, opt) = cli::Opt::from_args_and_git_config(args, &env, assets);

src/utils/debug.rs

@@ -0,0 +1,201 @@
+use crate::config::Config;
+use crate::env::{DeltaEnv, DELTA_DEBUG_LOGFILE, DELTA_DEBUG_LOGFILE_MAX_SIZE};
+use crate::fatal;
+use crate::utils::DELTA_ATOMIC_ORDERING;
+
+use console::Term;
+
+use std::ffi::OsString;
+use std::fs::File;
+use std::io::{Seek, SeekFrom, Write};
+use std::sync::atomic::AtomicBool;
+
+const RUST_BACKTRACE: &str = "RUST_BACKTRACE";
+
+// Use a global because the config where this might be stored could
+// itself panic while it is being built.
+static USING_DELTA_DEBUG_LOGFILE: AtomicBool = AtomicBool::new(false);
+
+#[derive(Debug)]
+pub struct PrintNoticeOnPanic;
+
+impl PrintNoticeOnPanic {
+    pub fn new() -> Self {
+        Self {}
+    }
+}
+
+#[cfg(not(test))]
+impl Drop for PrintNoticeOnPanic {
+    fn drop(&mut self) {
+        // Nothing elaborate with std::panic::set_hook or std::panic::catch_unwind to also get the backtrace,
+        // only set RUST_BACKTRACE when recording
+        if std::thread::panicking() {
+            if USING_DELTA_DEBUG_LOGFILE.load(DELTA_ATOMIC_ORDERING) {
+                if let Some(logfile) = std::env::var_os(DELTA_DEBUG_LOGFILE) {
+                    eprintln!("  Wrote {logfile:?} (if you want to share it, ensure no sensitive information is contained). You may also want to include the stack backtrace.");
+                }
+            } else {
+                // Setting GIT_PAGER however does not override interactive.diffFilter!
+                eprintln!("\n\
+            delta panicked and crashed, sorry :(\n\
+            To quickly repeat the previous command without delta, run 'export GIT_PAGER=less' first. If you want\n\
+            to report the crash and it is easy to repeat, do so after 'export DELTA_DEBUG_LOGFILE=crash.log'\n\
+            then submit the logfile to github at <https://github.com/dandavison/delta/issues>. Thank you!\n\n\
+            !! Make sure there is NO sensitive information in the log file, it will contain the entire diff !!\n\
+            ")
+            }
+        }
+    }
+}
+
+#[derive(Debug)]
+struct DeltaDetailInternal {
+    file: File,
+    bytes_written: u64,
+    max_log_size: u64,
+    truncate_back_to: u64,
+}
+
+pub struct RecordDeltaCall(Option<Box<DeltaDetailInternal>>);
+
+impl RecordDeltaCall {
+    pub fn new(config: &Config) -> Self {
+        fn make(logfile: &OsString, config: &Config) -> Result<Box<DeltaDetailInternal>, String> {
+            let mut file = File::create(logfile).map_err(|e| e.to_string())?;
+
+            let mut write = |s: String| file.write_all(s.as_bytes()).map_err(|e| e.to_string());
+
+            write(
+                "<details>\n\
+                <summary>Input which caused delta to crash</summary>\n```\n\n"
+                    .into(),
+            )?;
+
+            if let Some(cfg) = config.git_config.as_ref() {
+                write("git config values:\n".into())?;
+                cfg.for_each(".*", |entry, value: Option<&str>| {
+                    if !(entry.starts_with("user.")
+                        || entry.starts_with("remote.")
+                        || entry.starts_with("branch.")
+                        || entry.starts_with("gui."))
+                    {
+                        let _ = write(format!("{} = {:?}\n", entry, value.unwrap_or("")));
+                    }
+                })
+            } else {
+                write("(NO git config)\n".into())?;
+            };
+
+            write(format!(
+                "command line: {:?}\n",
+                std::env::args_os().collect::<Vec<_>>()
+            ))?;
+            let mut delta_env = DeltaEnv::init();
+            // redact, not interesting:
+            delta_env.current_dir = None;
+            delta_env.hostname = None;
+            write(format!("DeltaEnv: {:?}\n", delta_env))?;
+
+            let term = Term::stdout();
+
+            write(format!(
+                "TERM: {:?}, is_term: {}, size: {:?}\n",
+                std::env::var_os("TERM"),
+                term.is_term(),
+                term.size()
+            ))?;
+
+            write(
+                "raw crash input to delta (usually something git diff etc. generated):\n".into(),
+            )?;
+            write("================================\n".into())?;
+
+            file.flush().map_err(|e| e.to_string())?;
+            let truncate_back_to = file.stream_position().map_err(|e| e.to_string())?;
+
+            let max_log_size = std::env::var_os(DELTA_DEBUG_LOGFILE_MAX_SIZE)
+                .map(|v| {
+                    v.to_string_lossy().parse::<u64>().unwrap_or_else(|_| {
+                        fatal(format!(
+                            "Invalid env var value in {} (got {:?}, expected integer)",
+                            DELTA_DEBUG_LOGFILE_MAX_SIZE, v
+                        ));
+                    })
+                })
+                .unwrap_or(512 * 1024);
+
+            if std::env::var_os(RUST_BACKTRACE).is_none() {
+                // SAFETY:
+                // a) we only get here when `DELTA_DEBUG_LOGFILE` is set, which means a user is expecting a crash anyhow
+                // b) the requirement is "no other threads concurrently writing or reading(!) [env vars],
+                // other than the ones in this [std::env] module.", the rust backtrace handler should use std::env.
+                unsafe {
+                    std::env::set_var(RUST_BACKTRACE, "1");
+                }
+            }
+
+            USING_DELTA_DEBUG_LOGFILE.store(true, DELTA_ATOMIC_ORDERING);
+
+            Ok(Box::new(DeltaDetailInternal {
+                file,
+                bytes_written: 0,
+                max_log_size,
+                truncate_back_to,
+            }))
+        }
+
+        if let Some(logfile) = std::env::var_os(DELTA_DEBUG_LOGFILE) {
+            Self(
+                make(&logfile, config)
+                    .map_err(|e| {
+                        eprintln!(
+                            "\nnotice: failed to write {logfile:?} given by {DELTA_DEBUG_LOGFILE}: {e}"
+                        );
+                    })
+                    .ok(),
+            )
+        } else {
+            Self(None)
+        }
+    }
+
+    #[inline]
+    pub fn write(&mut self, line: &[u8]) {
+        if self.0.is_some() {
+            self._write(line);
+        }
+    }
+
+    fn _write(&mut self, line: &[u8]) {
+        let internal = self.0.as_mut().unwrap();
+        if internal.bytes_written > internal.max_log_size {
+            let _ = internal.file.flush();
+            let _ = internal
+                .file
+                .seek(SeekFrom::Start(internal.truncate_back_to));
+            let _ = internal.file.set_len(internal.truncate_back_to);
+            let _ = internal.file.write_all(
+                format!(
+                    "(truncated [max log size is {},\
+                     set {DELTA_DEBUG_LOGFILE_MAX_SIZE} to override])\n",
+                    internal.max_log_size
+                )
+                .as_bytes(),
+            );
+            internal.bytes_written = 0;
+        }
+        let _ = internal.file.write_all(line);
+        let _ = internal.file.write_all(b"\n");
+        internal.bytes_written += line.len() as u64 + 1;
+        let _ = internal.file.flush();
+    }
+}
+
+impl Drop for RecordDeltaCall {
+    fn drop(&mut self) {
+        if let Some(ref mut internal) = self.0 {
+            let _ = internal.file.write_all(b"\n```\n</details>\n");
+        }
+    }
+}

src/utils/mod.rs

@@ -10,6 +10,10 @@ pub mod syntect;
 pub mod tabs;
 pub mod workarounds;
 
+mod debug;
+pub use debug::PrintNoticeOnPanic;
+pub use debug::RecordDeltaCall;
+
 // Use the most (even overly) strict ordering. Atomics are not used in hot loops so
 // a one-size-fits-all approach which is never incorrect is okay.
 pub const DELTA_ATOMIC_ORDERING: std::sync::atomic::Ordering = std::sync::atomic::Ordering::SeqCst;