New jj debug watchman status command

This command checks not only whether Watchman works, but also whether
it's enabled in the config. Also, the output is easier to understand
than that of the other `jj debug watchman` commands.

It would be nice if `jj debug watchman` called `jj debug watchman
status`, but it's not trivial in `clap` to have a default subcommand.

CHANGELOG.md

@@ -40,6 +40,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   evaluates to true if the operation was a snapshot created by a non-mutating
   command (e.g. `jj log`).
 
+* You can check whether Watchman fsmonitor is enabled or installed with the new
+  `jj debug watchman status` command.
+
 ### Fixed bugs
 
 * Revsets now support `\`-escapes in string literal.

cli/src/commands/debug.rs

@@ -118,6 +118,8 @@ pub struct DebugTreeArgs {
 
 #[derive(Subcommand, Clone, Debug)]
 pub enum DebugWatchmanSubcommand {
+    /// Check whether `watchman` is enabled and whether it's correctly installed
+    Status,
     QueryClock,
     QueryChangedFiles,
     ResetClock,
@@ -367,6 +369,31 @@ fn cmd_debug_watchman(
     let mut workspace_command = command.workspace_helper(ui)?;
     let repo = workspace_command.repo().clone();
     match subcommand {
+        DebugWatchmanSubcommand::Status => {
+            // TODO(ilyagr): It would be nice to add colors here
+            match command.settings().fsmonitor_kind()? {
+                jj_lib::fsmonitor::FsmonitorKind::Watchman => {
+                    writeln!(ui.stdout(), "Watchman is enabled via `core.fsmonitor`.")?
+                }
+                jj_lib::fsmonitor::FsmonitorKind::None => writeln!(
+                    ui.stdout(),
+                    "Watchman is disabled. Set `core.fsmonitor=\"watchman\"` to \
+                     enable.\nAttempting to contact the `watchman` CLI regardless..."
+                )?,
+                other_fsmonitor => {
+                    return Err(user_error(format!(
+                        "This command does not support the currently enabled filesystem monitor: \
+                         {other_fsmonitor:?}."
+                    )))
+                }
+            };
+            let wc = check_local_disk_wc(workspace_command.working_copy().as_any())?;
+            let _ = wc.query_watchman()?;
+            writeln!(
+                ui.stdout(),
+                "The watchman server seems to be installed and working correctly."
+            )?;
+        }
         DebugWatchmanSubcommand::QueryClock => {
             let wc = check_local_disk_wc(workspace_command.working_copy().as_any())?;
             let (clock, _changed_files) = wc.query_watchman()?;

docs/config.md

@@ -727,7 +727,8 @@ To configure the Watchman filesystem monitor, set
 `core.fsmonitor = "watchman"`. Ensure that you have [installed the Watchman
 executable on your system](https://facebook.github.io/watchman/docs/install).
 
-Debugging commands are available under `jj debug watchman`.
+You can check whether Watchman is enabled and whether it is installed correctly
+using `jj debug watchman status`.
 
 ## Ways to specify `jj` config: details
 

lib/src/fsmonitor.rs

@@ -24,7 +24,7 @@ use std::path::PathBuf;
 use std::str::FromStr;
 
 /// The recognized kinds of filesystem monitors.
-#[derive(Eq, PartialEq)]
+#[derive(Eq, PartialEq, Clone, Debug)]
 pub enum FsmonitorKind {
     /// The Watchman filesystem monitor (<https://facebook.github.io/watchman/>).
     Watchman,