diff --git a/scm/Mercurial.cpp b/scm/Mercurial.cpp
index b911b5336198..8e37d80814a5 100644
--- a/scm/Mercurial.cpp
+++ b/scm/Mercurial.cpp
@@ -293,4 +293,29 @@ time_point<system_clock> Mercurial::convertCommitDate(const char* commitDate) {
   }
   return system_clock::from_time_t(date);
 }
+
+std::vector<w_string> Mercurial::getCommitsPriorToAndIncluding(
+    w_string_piece commitId,
+    int numCommits,
+    w_string requestId) const {
+  auto revset = to<std::string>(
+      "reverse(last(_firstancestors(", commitId, "), ", numCommits, "))\n");
+  ChildProcess proc(
+      {hgExecutablePath(), "log", "-r", revset, "-T", "{node}\n"},
+      makeHgOptions(requestId));
+  auto outputs = proc.communicate();
+  auto status = proc.wait();
+  if (status) {
+    throw std::runtime_error(to<std::string>(
+        "failed query for hg log; command returned with status ",
+        status,
+        " out=",
+        outputs.first,
+        " err=",
+        outputs.second));
+  }
+  std::vector<w_string> lines;
+  w_string_piece(outputs.first).split(lines, '\n');
+  return lines;
+}
 } // namespace watchman
diff --git a/scm/Mercurial.h b/scm/Mercurial.h
index 7b228da1d997..c31a1b40cd54 100644
--- a/scm/Mercurial.h
+++ b/scm/Mercurial.h
@@ -30,6 +30,10 @@ class Mercurial : public SCM {
   // public for testing
   static std::chrono::time_point<std::chrono::system_clock> convertCommitDate(
       const char* commitDate);
+  std::vector<w_string> getCommitsPriorToAndIncluding(
+      w_string_piece commitId,
+      int numCommits,
+      w_string requestId = nullptr) const override;
 
  private:
   // Returns options for invoking hg
diff --git a/scm/SCM.h b/scm/SCM.h
index 1b7b83a4bd29..d1f7e0b0cb37 100644
--- a/scm/SCM.h
+++ b/scm/SCM.h
@@ -69,6 +69,14 @@ class SCM {
       w_string_piece commitId,
       w_string requestId = nullptr) const = 0;
 
+  // Compute the numCommits commits prior to and including the specified commit
+  // in source control history. Returns an ordered list with the most recent
+  // commit (the one specified) first.
+  virtual std::vector<w_string> getCommitsPriorToAndIncluding(
+      w_string_piece commitId,
+      int numCommits,
+      w_string requestId = nullptr) const = 0;
+
  private:
   w_string rootPath_;
   w_string scmRoot_;
diff --git a/watcher/eden.cpp b/watcher/eden.cpp
index 20cd988891b6..5d62b19a4bfc 100644
--- a/watcher/eden.cpp
+++ b/watcher/eden.cpp
@@ -567,6 +567,14 @@ class EdenWrappedSCM : public SCM {
     return inner_->getCommitDate(commitId, requestId);
   }
 
+  std::vector<w_string> getCommitsPriorToAndIncluding(
+      w_string_piece commitId,
+      int numCommits,
+      w_string requestId = nullptr) const override {
+    return inner_->getCommitsPriorToAndIncluding(
+        commitId, numCommits, requestId);
+  }
+
   static std::unique_ptr<EdenWrappedSCM> wrap(std::unique_ptr<SCM> inner) {
     if (!inner) {
       return nullptr;