Change file open for posix-win equivalence (random and _SH_DENYWR).

Author: evoskuil

src/file/utilities.cpp

@@ -244,20 +244,51 @@ code copy_directory_ex(const path& from, const path& to) NOEXCEPT
 
 int open(const path& filename) NOEXCEPT
 {
+    // TODO: provide as parameter for random vs. sequential accesss.
+    auto random{ true };
+
     const auto path = system::to_extended_path(filename);
     int file_descriptor{};
 
-    // _wsopen_s and open set errno on failure.
-    // _wsopen_s and wstring do not throw (but are unannotated).
 #if defined(HAVE_MSC)
-    // sets file_descriptor = -1 on error.
-    _wsopen_s(&file_descriptor, path.c_str(),
-        O_RDWR | _O_BINARY | _O_RANDOM, _SH_DENYWR, _S_IREAD | _S_IWRITE);
+    // _wsopen_s and wstring do not throw (but are unannotated).
+    // sets file_descriptor = -1 and errno on error.
+    ::_wsopen_s(&file_descriptor, path.c_str(),
+        O_RDWR | _O_BINARY | (random ? _O_RANDOM : _O_SEQUENTIAL),
+        _SH_DENYWR, _S_IREAD | _S_IWRITE);
 #else
-    // returns -1 on failure.
-    file_descriptor = ::open(path.c_str(),
-        O_RDWR, S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH);
+    // open sets errno on failure.
+    file_descriptor = ::open(path.c_str(), O_RDWR, S_IRUSR | S_IWUSR);
+    if (file_descriptor != -1)
+    {
+        // _O_RANDOM equivalent, posix_fadvise returns error on failure.
+        const auto advice = random ? POSIX_FADV_RANDOM : POSIX_FADV_SEQUENTIAL;
+        const auto result = ::posix_fadvise(file_descriptor, 0, 0, advice);
+        if (!is_zero(result))
+        {
+            close(file_descriptor);
+            file_descriptor = -1;
+            errno = result;
+        }
+        else
+        {
+            // _SH_DENYWR equivalent.
+            const struct flock lock
+            {
+                .l_type = F_WRLCK, .l_whence = SEEK_SET, .l_start = 0, .l_len = 0
+            };
+
+            if (::fcntl(file_descriptor, F_SETLK, &lock) == -1)
+            {
+                const auto last = errno;
+                close(file_descriptor);
+                file_descriptor = -1;
+                errno = last;
+            }
+        }
+    }
 #endif
+
     return file_descriptor;
 }