From ba1ac210ef087cdc4c2caf8d3c5feb9d31447d31 Mon Sep 17 00:00:00 2001
From: Brian Daniels <briandaniels@google.com>
Date: Wed, 29 Apr 2026 10:43:20 -0400
Subject: [PATCH] [assemble_cvd] Replace slow lsof for file cleanup

CleanPriorFiles called lsof to ensure files weren't in use before
deletion, but this was taking ~5 seconds, significantly slowing down
startup. lsof was slow because it scanned all processes on the
system and performed name resolutions by default.

Replacing it with a targeted C++ scan of /proc/*/fd for the current
user's processes avoids these overheads and reduces the execution time
to ~132 ms.

Bug: 401259067
Test: cvd create
      check launcher.log for ~5s speed up to UBOOT start
---
 .../host/commands/assemble_cvd/BUILD.bazel    |  1 +
 .../host/commands/assemble_cvd/clean.cc       | 83 ++++++++++++++-----
 2 files changed, 65 insertions(+), 19 deletions(-)

diff --git a/base/cvd/cuttlefish/host/commands/assemble_cvd/BUILD.bazel b/base/cvd/cuttlefish/host/commands/assemble_cvd/BUILD.bazel
index 7389abac3fa..0ccfd1dee08 100644
--- a/base/cvd/cuttlefish/host/commands/assemble_cvd/BUILD.bazel
+++ b/base/cvd/cuttlefish/host/commands/assemble_cvd/BUILD.bazel
@@ -163,6 +163,7 @@ cf_cc_library(
     hdrs = ["clean.h"],
     deps = [
         "//cuttlefish/common/libs/utils:in_sandbox",
+        "//cuttlefish/common/libs/utils:proc_file_utils",
         "//cuttlefish/common/libs/utils:subprocess",
         "//cuttlefish/common/libs/utils:subprocess_managed_stdio",
         "//cuttlefish/host/libs/config:config_utils",
diff --git a/base/cvd/cuttlefish/host/commands/assemble_cvd/clean.cc b/base/cvd/cuttlefish/host/commands/assemble_cvd/clean.cc
index f3b8e971579..7b97c208397 100644
--- a/base/cvd/cuttlefish/host/commands/assemble_cvd/clean.cc
+++ b/base/cvd/cuttlefish/host/commands/assemble_cvd/clean.cc
@@ -28,6 +28,7 @@
 #include "absl/log/log.h"
 
 #include "cuttlefish/common/libs/utils/in_sandbox.h"
+#include "cuttlefish/common/libs/utils/proc_file_utils.h"
 #include "cuttlefish/common/libs/utils/subprocess.h"
 #include "cuttlefish/common/libs/utils/subprocess_managed_stdio.h"
 #include "cuttlefish/host/libs/config/config_utils.h"
@@ -85,6 +86,64 @@ Result<void> CleanPriorFiles(const std::string& path,
   return {};
 }
 
+Result<std::vector<pid_t>> GetPidsUsingFiles(
+    const std::vector<std::string>& prior_dirs,
+    const std::vector<std::string>& prior_files) {
+  std::vector<pid_t> pids_in_use;
+
+  VLOG(0) << "Checking if prior dirs or files are in use: ";
+  for (const auto& prior_dir : prior_dirs) {
+    VLOG(0) << prior_dir;
+  }
+  for (const auto& prior_file : prior_files) {
+    VLOG(0) << prior_file;
+  }
+  auto pids = CF_EXPECT(CollectPids(getuid()));
+  for (const auto pid : pids) {
+    std::string fd_dir_path = fmt::format("/proc/{}/fd", pid);
+    std::unique_ptr<DIR, int (*)(DIR*)> dir(opendir(fd_dir_path.c_str()),
+                                            closedir);
+    if (!dir) {
+      continue;
+    }
+    for (auto entity = readdir(dir.get()); entity != nullptr;
+         entity = readdir(dir.get())) {
+      std::string entity_name(entity->d_name);
+      if (entity_name == "." || entity_name == "..") {
+        continue;
+      }
+      std::string fd_path = fd_dir_path + "/" + entity_name;
+      std::string target;
+      if (!android::base::Readlink(fd_path, &target)) {
+        continue;
+      }
+
+      bool match = false;
+      for (const auto& prior_dir : prior_dirs) {
+        if (target.starts_with(prior_dir)) {
+          match = true;
+          break;
+        }
+      }
+      if (!match) {
+        for (const auto& prior_file : prior_files) {
+          if (target == prior_file) {
+            match = true;
+            break;
+          }
+        }
+      }
+
+      if (match) {
+        pids_in_use.push_back(pid);
+        break;
+      }
+    }
+  }
+
+  return pids_in_use;
+}
+
 Result<void> CleanPriorFiles(const std::vector<std::string>& paths,
                              const std::set<std::string>& preserving) {
   std::vector<std::string> prior_dirs;
@@ -105,25 +164,11 @@ Result<void> CleanPriorFiles(const std::vector<std::string>& paths,
 
   // TODO(schuffelen): Fix logic for host-sandboxing mode.
   if (!InSandbox() && (!prior_dirs.empty() || !prior_files.empty())) {
-    Command lsof("lsof");
-    lsof.AddParameter("-t");
-    lsof.AddParameter("-Q");  // ignore failed search terms
-    for (const auto& prior_dir : prior_dirs) {
-      lsof.AddParameter("+D").AddParameter(prior_dir);
-    }
-    for (const auto& prior_file : prior_files) {
-      lsof.AddParameter(prior_file);
-    }
-
-    Result<std::string> lsof_out = RunAndCaptureStdout(std::move(lsof));
-    if (lsof_out.ok()) {
-      CF_EXPECTF(
-          lsof_out->empty(),
-          "Instance directory files in use. Try `cvd reset`? Observed PIDs: {}",
-          fmt::join(absl::StrSplit(*lsof_out, "\n"), ", "));
-    } else {
-      LOG(ERROR) << "Failed to run `lsof`: " << lsof_out.error();
-    }
+    auto pids = CF_EXPECT(GetPidsUsingFiles(prior_dirs, prior_files));
+    CF_EXPECTF(
+        pids.empty(),
+        "Instance directory files in use. Try `cvd reset`? Observed PIDs: {}",
+        fmt::join(pids, ", "));
   }
 
   for (const auto& path : paths) {