watchman: cache Mercurial::getFilesChangedSinceMergeBaseWith results

Summary:
This commit uses the same technique as we use for the mergeBase
cache in the prior commit to produce a cache key, so that we'll effectively
invalidate the results when the dirstate is changed.

This provides both caching and thundering herd protection when multiple
clients are performing SCM aware queries concurrently.  We've had some
issues in the past where we'd OOM a cgroup slice under the combined
memory pressure of the the concurrent `hg` processes.

Reviewed By: chadaustin

Differential Revision: D22582273

fbshipit-source-id: 19f6a81b1c18831878f4a7aab4ec4a1ef82033da

scm/Mercurial.cpp

@@ -115,7 +115,12 @@ ChildProcess::Options Mercurial::makeHgOptions(w_string requestId) const {
 Mercurial::Mercurial(w_string_piece rootPath, w_string_piece scmRoot)
     : SCM(rootPath, scmRoot),
       dirStatePath_(to<std::string>(getSCMRoot(), "/.hg/dirstate")),
-      mergeBases_(Configuration(), "scm_hg_mergebase", 32, 10) {}
+      mergeBases_(Configuration(), "scm_hg_mergebase", 32, 10),
+      filesChangedSinceMergeBaseWith_(
+          Configuration(),
+          "scm_hg_files_since_mergebase",
+          32,
+          10) {}
 
 struct timespec Mercurial::getDirStateMtime() const {
   try {
@@ -165,22 +170,35 @@ w_string Mercurial::mergeBaseWith(w_string_piece commitId, w_string requestId)
 std::vector<w_string> Mercurial::getFilesChangedSinceMergeBaseWith(
     w_string_piece commitId,
     w_string requestId) const {
-  // The "" argument at the end causes paths to be printed out relative to the
-  // cwd (set to root path above).
-  auto result = runMercurial(
-      {hgExecutablePath(),
-       "--traceback",
-       "status",
-       "-n",
-       "--rev",
-       commitId,
-       ""},
-      makeHgOptions(requestId),
-      "query for files changed since merge base");
+  auto mtime = getDirStateMtime();
+  auto key =
+      folly::to<std::string>(commitId, ":", mtime.tv_sec, ":", mtime.tv_nsec);
+  auto commitCopy = folly::to<std::string>(commitId);
 
-  std::vector<w_string> lines;
-  w_string_piece(result.output).split(lines, '\n');
-  return lines;
+  return filesChangedSinceMergeBaseWith_
+      .get(
+          key,
+          [this, commit = std::move(commitCopy), requestId](
+              const std::string&) {
+            auto result = runMercurial(
+                {hgExecutablePath(),
+                 "--traceback",
+                 "status",
+                 "-n",
+                 "--rev",
+                 commit,
+                 // The "" argument at the end causes paths to be printed out
+                 // relative to the cwd (set to root path above).
+                 ""},
+                makeHgOptions(requestId),
+                "query for files changed since merge base");
+
+            std::vector<w_string> lines;
+            w_string_piece(result.output).split(lines, '\n');
+            return folly::makeFuture(lines);
+          })
+      .get()
+      ->value();
 }
 
 SCM::StatusResult Mercurial::getFilesChangedBetweenCommits(

scm/Mercurial.h

@@ -39,6 +39,8 @@ class Mercurial : public SCM {
  private:
   std::string dirStatePath_;
   mutable LRUCache<std::string, w_string> mergeBases_;
+  mutable LRUCache<std::string, std::vector<w_string>>
+      filesChangedSinceMergeBaseWith_;
 
   // Returns options for invoking hg
   ChildProcess::Options makeHgOptions(w_string requestId) const;