Move parent process query into a thread

Start the query even before determining if information about the
parent process is required (which are most invocations anyhow).

src/handlers/git_show_file.rs

@@ -9,9 +9,7 @@ impl<'a> StateMachine<'a> {
         self.painter.emit()?;
         let mut handled_line = false;
         if matches!(self.state, State::Unknown) {
-            if let Some(process::CallingProcess::GitShow(_, extension)) =
-                process::calling_process().as_deref()
-            {
+            if let process::CallingProcess::GitShow(_, extension) = &*process::calling_process() {
                 self.state = State::GitShowFile;
                 self.painter.set_syntax(extension.as_deref());
                 self.painter.set_highlighter();

src/handlers/grep.rs

@@ -246,8 +246,8 @@ fn get_code_style_sections<'b>(
 }
 
 fn make_output_config() -> GrepOutputConfig {
-    match process::calling_process().as_deref() {
-        Some(process::CallingProcess::GitGrep(command_line))
+    match &*process::calling_process() {
+        process::CallingProcess::GitGrep(command_line)
             if command_line.short_options.contains("-W")
                 || command_line.long_options.contains("--function-context") =>
         {
@@ -265,7 +265,7 @@ fn make_output_config() -> GrepOutputConfig {
                 pad_line_number: true,
             }
         }
-        Some(process::CallingProcess::GitGrep(command_line))
+        process::CallingProcess::GitGrep(command_line)
             if command_line.short_options.contains("-p")
                 || command_line.long_options.contains("--show-function") =>
         {
@@ -380,9 +380,8 @@ pub fn parse_grep_line(line: &str) -> Option<GrepLine> {
     if line.starts_with('{') {
         ripgrep_json::parse_line(line)
     } else {
-        match process::calling_process().as_deref() {
-            Some(process::CallingProcess::GitGrep(_))
-            | Some(process::CallingProcess::OtherGrep) => [
+        match &*process::calling_process() {
+            process::CallingProcess::GitGrep(_) | process::CallingProcess::OtherGrep => [
                 &*GREP_LINE_REGEX_ASSUMING_FILE_EXTENSION,
                 &*GREP_LINE_REGEX_ASSUMING_NO_INTERNAL_SEPARATOR_CHARS,
             ]

src/handlers/hunk.rs

@@ -27,13 +27,11 @@ lazy_static! {
 }
 
 fn compute_is_word_diff() -> bool {
-    match process::calling_process().as_deref() {
-        Some(
-            CallingProcess::GitDiff(cmd_line)
-            | CallingProcess::GitShow(cmd_line, _)
-            | CallingProcess::GitLog(cmd_line)
-            | CallingProcess::GitReflog(cmd_line),
-        ) => {
+    match &*process::calling_process() {
+        CallingProcess::GitDiff(cmd_line)
+        | CallingProcess::GitShow(cmd_line, _)
+        | CallingProcess::GitLog(cmd_line)
+        | CallingProcess::GitReflog(cmd_line) => {
             cmd_line.long_options.contains("--word-diff")
                 || cmd_line.long_options.contains("--word-diff-regex")
                 || cmd_line.long_options.contains("--color-words")

src/main.rs

@@ -65,6 +65,12 @@ pub mod errors {
 
 #[cfg(not(tarpaulin_include))]
 fn main() -> std::io::Result<()> {
+    // Do this first because both parsing all the input in `run_app()` and
+    // listing all processes takes about 50ms on Linux.
+    // It also improves the chance that the calling process is still around when
+    // input is piped into delta (e.g. `git show  --word-diff=color | delta`).
+    utils::process::start_determining_calling_process_in_thread();
+
     // Ignore ctrl-c (SIGINT) to avoid leaving an orphaned pager process.
     // See https://github.com/dandavison/delta/issues/681
     ctrlc::set_handler(|| {})

src/utils/process.rs

@@ -1,9 +1,9 @@
-use std::borrow::Cow;
 use std::collections::{HashMap, HashSet};
 use std::path::Path;
-use sysinfo::{Pid, Process, ProcessExt, ProcessRefreshKind, SystemExt};
+use std::sync::{Arc, Condvar, Mutex, MutexGuard};
 
 use lazy_static::lazy_static;
+use sysinfo::{Pid, Process, ProcessExt, ProcessRefreshKind, SystemExt};
 
 #[derive(Clone, Debug, PartialEq)]
 pub enum CallingProcess {
@@ -13,6 +13,8 @@ pub enum CallingProcess {
     GitReflog(CommandLine),
     GitGrep(CommandLine),
     OtherGrep, // rg, grep, ag, ack, etc
+    None,      // no matching process could be found
+    Pending,   // calling process is currently being determined
 }
 // TODO: Git blame is currently handled differently
 
@@ -23,23 +25,49 @@ pub struct CommandLine {
     last_arg: Option<String>,
 }
 
-pub fn calling_process() -> Option<Cow<'static, CallingProcess>> {
-    #[cfg(not(test))]
-    {
-        CACHED_CALLING_PROCESS.as_ref().map(Cow::Borrowed)
-    }
-    #[cfg(test)]
-    {
-        determine_calling_process().map(Cow::Owned)
-    }
-}
-
 lazy_static! {
-    static ref CACHED_CALLING_PROCESS: Option<CallingProcess> = determine_calling_process();
+    static ref CALLER: Arc<(Mutex<CallingProcess>, Condvar)> =
+        Arc::new((Mutex::new(CallingProcess::Pending), Condvar::new()));
 }
 
-fn determine_calling_process() -> Option<CallingProcess> {
+pub fn start_determining_calling_process_in_thread() {
+    // The handle is neither kept nor returned nor joined but dropped, so the main
+    // thread can exit early if it does not need to know its parent process.
+    std::thread::Builder::new()
+        .name("find_calling_process".into())
+        .spawn(move || {
+            let calling_process = determine_calling_process();
+
+            let (caller_mutex, determine_done) = &**CALLER;
+
+            let mut caller = caller_mutex.lock().unwrap();
+            *caller = calling_process;
+            determine_done.notify_all();
+        })
+        .unwrap();
+}
+
+#[cfg(not(test))]
+pub fn calling_process() -> MutexGuard<'static, CallingProcess> {
+    let (caller_mutex, determine_done) = &**CALLER;
+
+    determine_done
+        .wait_while(caller_mutex.lock().unwrap(), |caller| {
+            *caller == CallingProcess::Pending
+        })
+        .unwrap()
+}
+
+// The return value is duck-typed to work in place of a MutexGuard when testing.
+#[cfg(test)]
+pub fn calling_process() -> Box<CallingProcess> {
+    type _UnusedImport = MutexGuard<'static, i8>;
+    Box::new(determine_calling_process())
+}
+
+fn determine_calling_process() -> CallingProcess {
     calling_process_cmdline(ProcInfo::new(), describe_calling_process)
+        .unwrap_or(CallingProcess::None)
 }
 
 // Return value of `extract_args(args: &[String]) -> ProcessArgs<T>` function which is