Document Watchman config better, new jj debug watchman status command

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()?;

cli/src/config-schema.json

@@ -155,6 +155,16 @@
                 }
             }
         },
+        "core": {
+            "type": "object",
+            "properties": {
+                "fsmonitor": {
+                    "type": "string",
+                    "enum": ["none", "watchman"],
+                    "description": "Whether to use an external filesystem monitor, useful for large repos"
+                }
+            }
+        },
         "colors": {
             "type": "object",
             "description": "Mapping from jj formatter labels to colors",

docs/config.md

@@ -718,13 +718,17 @@ In large repositories, it may be beneficial to use a "filesystem monitor" to
 track changes to the working copy. This allows `jj` to take working copy
 snapshots without having to rescan the entire working copy.
 
+This is governed by the `core.fsmonitor` option. Currently, the valid values are
+`"none"` or `"watchman"`.
+
 ### Watchman
 
 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,