From 374975a73c1b930fad168e9b4ad873912080661d Mon Sep 17 00:00:00 2001
From: Scott Lamb <slamb@slamb.org>
Date: Sat, 30 Apr 2016 08:38:29 -0700
Subject: [PATCH] On startup, ensure --sample_file_dir is writable.

---
 src/filesystem.cc        |  4 ++++
 src/filesystem.h         |  4 ++++
 src/moonfire-nvr-main.cc | 16 ++++++++++++++--
 3 files changed, 22 insertions(+), 2 deletions(-)

diff --git a/src/filesystem.cc b/src/filesystem.cc
index b1d18b5..4a05944 100644
--- a/src/filesystem.cc
+++ b/src/filesystem.cc
@@ -65,6 +65,10 @@ class RealFile : public File {
 
   ~RealFile() final { Close(); }
 
+  int Access(const char *path, int mode, int flags) final {
+    return faccessat(fd_, path, mode, flags) < 0 ? errno : 0;
+  }
+
   int Close() final {
     if (fd_ < 0) {
       return 0;
diff --git a/src/filesystem.h b/src/filesystem.h
index a5adaad..bc55ffa 100644
--- a/src/filesystem.h
+++ b/src/filesystem.h
@@ -58,6 +58,9 @@ class File {
   // Close the file, ignoring the result.
   virtual ~File() {}
 
+  // faccessat(), returning 0 on success or errno>0 on failure.
+  virtual int Access(const char *path, int mode, int flags) = 0;
+
   // Close the file, returning 0 on success or errno>0 on failure.
   // Already closed is considered a success.
   virtual int Close() = 0;
@@ -92,6 +95,7 @@ class File {
 
 class MockFile : public File {
  public:
+  MOCK_METHOD3(Access, int(const char *, int, int));
   MOCK_METHOD0(Close, int());
 
   // The std::unique_ptr<File> variants of Open are wrapped here because gmock's
diff --git a/src/moonfire-nvr-main.cc b/src/moonfire-nvr-main.cc
index 00c2451..4f0d9c2 100644
--- a/src/moonfire-nvr-main.cc
+++ b/src/moonfire-nvr-main.cc
@@ -131,13 +131,25 @@ int main(int argc, char** argv) {
   env.video_source = moonfire_nvr::GetRealVideoSource();
 
   std::unique_ptr<moonfire_nvr::File> sample_file_dir;
+  std::string sample_file_dirname = FLAGS_sample_file_dir;
   int ret = moonfire_nvr::GetRealFilesystem()->Open(
-      FLAGS_sample_file_dir.c_str(), O_DIRECTORY | O_RDONLY, &sample_file_dir);
+      sample_file_dirname.c_str(), O_DIRECTORY | O_RDONLY, &sample_file_dir);
   if (ret != 0) {
-    LOG(ERROR) << "Unable to open --sample_file_dir=" << FLAGS_sample_file_dir
+    LOG(ERROR) << "Unable to open --sample_file_dir=" << sample_file_dirname
                << ": " << strerror(ret) << "; exiting.";
     exit(1);
   }
+
+  // Separately, ensure the sample file directory is writable.
+  // (Opening the directory above with O_DIRECTORY|O_RDWR doesn't work even
+  // when the directory is writable; it fails with EISDIR.)
+  ret = sample_file_dir->Access(".", W_OK, 0);
+  if (ret != 0) {
+    LOG(ERROR) << "--sample_file_dir=" << sample_file_dirname
+               << " is not writable: " << strerror(ret) << "; exiting.";
+    exit(1);
+  }
+
   env.sample_file_dir = sample_file_dir.release();
 
   moonfire_nvr::Database db;