compat/mingw: allow deletion of most opened files

On Windows, we emulate open(3p) via `mingw_open()`. This function
implements handling of some platform-specific quirks that are required
to make it behave as closely as possible like open(3p) would, but for
most cases we just call the Windows-specific `_wopen()` function.

This function has a major downside though: it does not allow us to
specify the sharing mode. While there is `_wsopen()` that allows us to
pass sharing flags, those sharing flags are not the same `FILE_SHARE_*`
flags as `CreateFileW()` accepts. Instead, `_wsopen()` only allows
concurrent read- and write-access, but does not allow for concurrent
deletions. Unfortunately though, we have to allow concurrent deletions
if we want to have POSIX-style atomic renames on top of an existing file
that has open file handles.

Implement a new function that emulates open(3p) for existing files via
`CreateFileW()` such that we can set the required sharing flags.

While we have the same issue when calling open(3p) with `O_CREAT`,
implementing that mode would be more complex due to the required
permission handling. Furthermore, atomic updates via renames typically
write to exclusive lockfile and then perform the rename, and thus we
don't have to handle the case where the locked path has been created
with `O_CREATE`. So while it would be nice to have proper POSIX
semantics in all paths, we instead aim for a minimum viable fix here.

Signed-off-by: Patrick Steinhardt <[email protected]>
Signed-off-by: Taylor Blau <[email protected]>

Author: pks-t

compat/mingw.c

@@ -532,6 +532,70 @@ static int mingw_open_append(wchar_t const *wfilename, int oflags, ...)
 	return fd;
 }
 
+/*
+ * Ideally, we'd use `_wopen()` to implement this functionality so that we
+ * don't have to reimplement it, but unfortunately we do not have tight control
+ * over the share mode there. And while `_wsopen()` and friends exist that give
+ * us _some_ control over the share mode, this family of functions doesn't give
+ * us the ability to enable FILE_SHARE_DELETE, either. But this is a strict
+ * requirement for us though so that we can unlink or rename over files that
+ * are held open by another process.
+ *
+ * We are thus forced to implement our own emulation of `open()`. To make our
+ * life simpler we only implement basic support for this, namely opening
+ * existing files for reading and/or writing. This means that newly created
+ * files won't have their sharing mode set up correctly, but for now I couldn't
+ * find any case where this matters. We may have to revisit that in the future
+ * though based on our needs.
+ */
+static int mingw_open_existing(const wchar_t *filename, int oflags, ...)
+{
+	SECURITY_ATTRIBUTES security_attributes = {
+		.nLength = sizeof(security_attributes),
+		.bInheritHandle = !(oflags & O_NOINHERIT),
+	};
+	HANDLE handle;
+	DWORD access;
+	int fd;
+
+	/* We only support basic flags. */
+	if (oflags & ~(O_ACCMODE | O_NOINHERIT)) {
+		errno = ENOSYS;
+		return -1;
+	}
+
+	switch (oflags & O_ACCMODE) {
+	case O_RDWR:
+		access = GENERIC_READ | GENERIC_WRITE;
+		break;
+	case O_WRONLY:
+		access = GENERIC_WRITE;
+		break;
+	default:
+		access = GENERIC_READ;
+		break;
+	}
+
+	handle = CreateFileW(filename, access,
+			     FILE_SHARE_WRITE | FILE_SHARE_READ | FILE_SHARE_DELETE,
+			     &security_attributes, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, NULL);
+	if (handle == INVALID_HANDLE_VALUE) {
+		DWORD err = GetLastError();
+
+		/* See `mingw_open_append()` for why we have this conversion. */
+		if (err == ERROR_INVALID_PARAMETER)
+			err = ERROR_PATH_NOT_FOUND;
+
+		errno = err_win_to_posix(err);
+		return -1;
+	}
+
+	fd = _open_osfhandle((intptr_t)handle, oflags | O_BINARY);
+	if (fd < 0)
+		CloseHandle(handle);
+	return fd;
+}
+
 /*
  * Does the pathname map to the local named pipe filesystem?
  * That is, does it have a "//./pipe/" prefix?
@@ -567,6 +631,8 @@ int mingw_open (const char *filename, int oflags, ...)
 
 	if ((oflags & O_APPEND) && !is_local_named_pipe_path(filename))
 		open_fn = mingw_open_append;
+	else if (!(oflags & ~(O_ACCMODE | O_NOINHERIT)))
+		open_fn = mingw_open_existing;
 	else
 		open_fn = _wopen;