From 9359852ee762d46de15dbf0761b7c609911a7b5c Mon Sep 17 00:00:00 2001
From: Gabe Levi <gabe@fb.com>
Date: Wed, 21 Nov 2018 17:16:45 -0800
Subject: [PATCH] Watchman.get_mergebase

Summary:
This adds an API to `Watchman` and `Watchman_lwt` which uses a one-off scm-aware watchman query to figure out the mergebase.

I am adding this because I want to know when the mergebase changes. Unfortunately, I can't use an scm-aware subscription, because it would filter out files which change during a rebase. So instead, I plan to call `Watchman_lwt.get_mergebase` whenever an `hg.update` finishes, to detect when the mergebase changes.

It's important to note, this will NOT use the existing connection for this one-off query. The existing connection might be used for a subscription, which means watchman might push a message to us and we may mis-interpret that message as a response.

Reviewed By: jbrown215

Differential Revision: D13060161

fbshipit-source-id: db179da6dd49d38da02fc6b66c9d632a27acb445
---
 hphp/hack/src/watchman/watchman.ml     | 44 +++++++++++++++++++++++++---------
 hphp/hack/src/watchman/watchman_sig.ml |  2 ++
 2 files changed, 35 insertions(+), 11 deletions(-)

diff --git a/hphp/hack/src/watchman/watchman.ml b/hphp/hack/src/watchman/watchman.ml
index 44397cb24c2..629d7d1ac68 100644
--- a/hphp/hack/src/watchman/watchman.ml
+++ b/hphp/hack/src/watchman/watchman.ml
@@ -665,19 +665,30 @@ struct
     | `Leave ->
       State_leave (name, metadata)
 
-  let make_mergebase_changed_response env data =
+  let extract_mergebase data =
     let open Hh_json.Access in
     let accessor = return data in
-    accessor >>=
+    let ret =
+      accessor >>=
       get_obj "clock" >>=
-      get_string "clock" >>= fun (clock, _) ->
-    accessor >>= get_obj "clock" >>=
-      get_obj "scm" >>=
-      get_string "mergebase" >>= fun (mergebase, keytrace) ->
-    let files = set_of_list @@ extract_file_names env data in
-    env.clockspec <- clock;
-    let response = Changed_merge_base (mergebase, files, clock) in
-    Ok ((env, response), keytrace)
+      get_string "clock" >>=
+      fun (clock, _) ->
+        accessor >>= get_obj "clock" >>=
+        get_obj "scm" >>=
+        get_string "mergebase" >>=
+        fun (mergebase, _) ->
+          return (clock, mergebase)
+    in
+    to_option ret
+
+  let make_mergebase_changed_response env data =
+    match extract_mergebase data with
+    | None -> Error "Failed to extract mergebase"
+    | Some (clock, mergebase) ->
+      let files = set_of_list @@ extract_file_names env data in
+      env.clockspec <- clock;
+      let response = Changed_merge_base (mergebase, files, clock) in
+      Ok (env, response)
 
   let transform_asynchronous_get_changes_response env data = match data with
     | None ->
@@ -685,7 +696,7 @@ struct
     | Some data -> begin
 
       match make_mergebase_changed_response env data with
-      | Ok ((env, response), _) -> env, response
+      | Ok (env, response) -> env, response
       | Error _ ->
         env.clockspec <- J.get_string_val "clock" data;
         assert_no_fresh_instance data;
@@ -724,6 +735,16 @@ struct
       (get_changes_since_mergebase_query env)
     >|= extract_file_names env
 
+  let get_mergebase env =
+    Watchman_process.request
+      ~timeout:(float_of_int env.settings.init_timeout)
+      ~debug_logging:env.settings.debug_logging
+      (get_changes_since_mergebase_query env)
+    >|= fun response ->
+      match extract_mergebase response with
+      | Some (_clock, mergebase) -> mergebase
+      | None -> raise (Watchman_error "Failed to extract mergebase from response")
+
   let flush_request ~(timeout:int) watch_root =
     let open Hh_json in
     let directive = JSON_Object [
@@ -900,6 +921,7 @@ module Watchman_mock = struct
 
   let get_changes_since_mergebase _  = []
 
+  let get_mergebase _ = "mergebase"
 end
 
 module type S = sig
diff --git a/hphp/hack/src/watchman/watchman_sig.ml b/hphp/hack/src/watchman/watchman_sig.ml
index def50e81e50..5da18879c15 100644
--- a/hphp/hack/src/watchman/watchman_sig.ml
+++ b/hphp/hack/src/watchman/watchman_sig.ml
@@ -123,6 +123,8 @@ module type S = sig
 
   val get_changes_since_mergebase: env -> string list result
 
+  val get_mergebase: env -> string result
+
   val get_changes: ?deadline:float ->
     watchman_instance -> (watchman_instance * changes) result
   val get_changes_synchronously: timeout:int ->
-- 
2.11.4.GIT