Improve warning message on open failure.

Before:

W0430 08:26:53.958887 41576 moonfire-nvr.cc:123]
driveway: Output error; sleeping before retrying: open
031e423c-2a0c-4450-b6cc-8af629606a90: Permission denied

After:

W0430 08:50:06.315666 43514 moonfire-nvr.cc:123]
driveway: Output error; sleeping before retrying: open
98592dfa-4ab4-427a-8ad0-033325f0f0b3 (within dir /home/slamb/moonfire/sample):
Permission denied

src/filesystem.cc

@@ -59,12 +59,15 @@ namespace {
 
 class RealFile : public File {
  public:
-  explicit RealFile(int fd) : fd_(fd) {}
+  RealFile(re2::StringPiece name, int fd)
+      : name_(name.data(), name.size()), fd_(fd) {}
   RealFile(const RealFile &) = delete;
   void operator=(const RealFile &) = delete;
 
   ~RealFile() final { Close(); }
 
+  const std::string &name() const { return name_; }
+
   int Access(const char *path, int mode, int flags) final {
     return faccessat(fd_, path, mode, flags) < 0 ? errno : 0;
   }
@@ -106,7 +109,7 @@ class RealFile : public File {
     if (ret < 0) {
       return errno;
     }
-    f->reset(new RealFile(ret));
+    f->reset(new RealFile(StrCat(name_, "/", path), ret));
     return 0;
   }
 
@@ -145,6 +148,7 @@ class RealFile : public File {
   }
 
  private:
+  std::string name_;
   int fd_ = -1;
 };
 
@@ -186,7 +190,7 @@ class RealFilesystem : public Filesystem {
     if (ret < 0) {
       return errno;
     }
-    f->reset(new RealFile(ret));
+    f->reset(new RealFile(path, ret));
     return 0;
   }
 

src/filesystem.h

@@ -58,6 +58,9 @@ class File {
   // Close the file, ignoring the result.
   virtual ~File() {}
 
+  // A name for the file (typically assigned at open time).
+  virtual const std::string &name() const = 0;
+
   // faccessat(), returning 0 on success or errno>0 on failure.
   virtual int Access(const char *path, int mode, int flags) = 0;
 
@@ -95,6 +98,7 @@ class File {
 
 class MockFile : public File {
  public:
+  MOCK_CONST_METHOD0(name, const std::string &());
   MOCK_METHOD3(Access, int(const char *, int, int));
   MOCK_METHOD0(Close, int());
 

src/recording.cc

@@ -143,7 +143,8 @@ bool SampleFileWriter::Open(const char *filename, std::string *error_message) {
   int ret =
       parent_dir_->Open(filename, O_WRONLY | O_CREAT | O_EXCL, 0600, &file_);
   if (ret != 0) {
-    *error_message = StrCat("open ", filename, ": ", strerror(ret));
+    *error_message = StrCat("open ", filename, " (within dir ",
+                            parent_dir_->name(), "): ", strerror(ret));
     return false;
   }
   return true;