cli: global --show-heap-stats argument for EOL heap stats

Summary: One of the more useful features of mimalloc. In the future this
interface could be expanded to support printing to a callback, so that it
could be triggered and dumped e.g. on debugging endpoints for longer running
processes, when we get there.

However, it's not enough to just add it to the `main` module; not only would it
be ugly and overspecific, it should be reusable so other custom `jj` commands
can opt-in to statistics trivially as well. Users should be strongly encouraged
to do this -- enable mimalloc and stat tracking -- since that's what upstream
will now work and test with; but, of course, all things are optional.

Thus, we expose an API from the `cli_utils` module for use with
`cli_util::CliRunner::add_global_args`, but ultimately it's up to the client to
call it.

Signed-off-by: Austin Seipp <[email protected]>
Change-Id: I6abe4b962bbe7c62ebca97be630b750c

cli/src/main.rs

@@ -12,14 +12,70 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+use std::sync::OnceLock;
+
 #[cfg(feature = "mimalloc")]
 use jj_cbits::mimalloc::MiMalloc;
 use jj_cli::cli_util::CliRunner;
+use jj_cli::command_error::CommandError;
+
+/// Lazy global static. Used only to defer printing mimalloc stats until the
+/// program exits, if set to `true`.
+static PRINT_HEAP_STATS: OnceLock<bool> = OnceLock::new();
+
+#[derive(clap::Args, Clone, Debug)]
+pub struct ShowAllocStats {
+    /// Show memory allocation statistics from the internal heap allocator
+    /// on `stderr`, when the program exits.
+    #[arg(long, global = true)]
+    show_heap_stats: bool,
+}
+
+/// Enable heap statistics for the user interface; should be used with
+/// [`CliRunner::add_global_args`]. Does nothing if the memory allocator is
+/// unused, i.e. `#[global_allocator]` is not set to mimalloc in your program.
+pub fn heap_stats_enable(
+    _ui: &mut jj_cli::ui::Ui,
+    opts: ShowAllocStats,
+) -> Result<(), CommandError> {
+    if opts.show_heap_stats {
+        PRINT_HEAP_STATS.set(true).unwrap();
+    }
+    Ok(())
+}
 
 #[cfg(feature = "mimalloc")]
 #[global_allocator]
 static ALLOC: MiMalloc = MiMalloc;
 
 fn main() -> std::process::ExitCode {
-    CliRunner::init().version(env!("JJ_VERSION")).run()
+    let result = CliRunner::init()
+        // NOTE (aseipp): always attach heap_stats_enable here, even if compiled
+        // without mimalloc; we don't want the test suite or other users to have
+        // to worry about if the command exists or not based on the build
+        // configuration
+        .add_global_args(heap_stats_enable)
+        .version(env!("JJ_VERSION"))
+        .run();
+
+    if PRINT_HEAP_STATS.get() == Some(&true) {
+        #[cfg(feature = "mimalloc")]
+        {
+            // NOTE (aseipp): can we do our own custom printing here? it's kind of ugly
+            eprintln!("========================================");
+            eprintln!("mimalloc memory allocation statistics:\n");
+            jj_cbits::mimalloc::stats_print(&|l| {
+                eprint!("{}", l.to_string_lossy());
+            });
+        }
+
+        #[cfg(not(feature = "mimalloc"))]
+        {
+            eprintln!(
+                "Note: heap statistics requested, but custom memory allocator (mimalloc) is not \
+                 enabled."
+            );
+        }
+    }
+    result
 }

cli/tests/cli-reference@.md.snap

@@ -163,6 +163,10 @@ repository.
   Possible values: `true`, `false`
 
 * `--config-toml <TOML>` — Additional configuration options (can be repeated)
+* `--show-heap-stats` — Show memory allocation statistics from the internal heap allocator on `stderr`, when the program exits
+
+  Possible values: `true`, `false`
+
 
 
 

cli/tests/test_global_opts.rs

@@ -481,6 +481,8 @@ fn test_help() {
           --color <WHEN>                 When to colorize output (always, never, auto)
           --no-pager                     Disable the pager
           --config-toml <TOML>           Additional configuration options (can be repeated)
+          --show-heap-stats              Show memory allocation statistics from the internal heap
+                                         allocator on `stderr`, when the program exits
     "###);
 }