Add pgreadlink() on Windows to read junction points

Add support for reading back information about the symbolic
links we've created with pgsymlink(), which are actually
Junction Points. Just like pgsymlink() can only create directory
symlinks, pgreadlink() can only read directory symlinks.

src/include/port.h

@@ -284,8 +284,11 @@ extern int	pgunlink(const char *path);
  */
 #if defined(WIN32) && !defined(__CYGWIN__)
 extern int	pgsymlink(const char *oldpath, const char *newpath);
+extern int	pgreadlink(const char *path, char *buf, size_t size);
+extern bool pgwin32_is_junction(char *path);
 
 #define symlink(oldpath, newpath)	pgsymlink(oldpath, newpath)
+#define readlink(path, buf, size)	pgreadlink(path, buf, size)
 #endif
 
 extern bool rmtree(const char *path, bool rmtopdir);

src/port/dirmod.c

@@ -297,6 +297,124 @@ pgsymlink(const char *oldpath, const char *newpath)
 
 	return 0;
 }
+
+/*
+ *	pgreadlink - uses Win32 junction points
+ */
+int
+pgreadlink(const char *path, char *buf, size_t size)
+{
+	DWORD		attr;
+	HANDLE		h;
+	char		buffer[MAX_PATH * sizeof(WCHAR) + sizeof(REPARSE_JUNCTION_DATA_BUFFER)];
+	REPARSE_JUNCTION_DATA_BUFFER *reparseBuf = (REPARSE_JUNCTION_DATA_BUFFER *) buffer;
+	DWORD		len;
+	int			r;
+
+	attr = GetFileAttributes(path);
+	if (attr == INVALID_FILE_ATTRIBUTES)
+	{
+		_dosmaperr(GetLastError());
+		return -1;
+	}
+	if ((attr & FILE_ATTRIBUTE_REPARSE_POINT) == 0)
+	{
+		errno = EINVAL;
+		return -1;
+	}
+
+	h = CreateFile(path,
+				   GENERIC_READ,
+				   FILE_SHARE_READ | FILE_SHARE_WRITE,
+				   NULL,
+				   OPEN_EXISTING,
+				   FILE_FLAG_OPEN_REPARSE_POINT | FILE_FLAG_BACKUP_SEMANTICS,
+				   0);
+	if (h == INVALID_HANDLE_VALUE)
+	{
+		_dosmaperr(GetLastError());
+		return -1;
+	}
+
+	if (!DeviceIoControl(h,
+						 FSCTL_GET_REPARSE_POINT,
+						 NULL,
+						 0,
+						 (LPVOID) reparseBuf,
+						 sizeof(buffer),
+						 &len,
+						 NULL))
+	{
+		LPSTR		msg;
+
+		errno = 0;
+		FormatMessage(FORMAT_MESSAGE_ALLOCATE_BUFFER | FORMAT_MESSAGE_FROM_SYSTEM,
+					  NULL, GetLastError(),
+					  MAKELANGID(LANG_ENGLISH, SUBLANG_DEFAULT),
+					  (LPSTR) &msg, 0, NULL);
+#ifndef FRONTEND
+		ereport(ERROR,
+				(errcode_for_file_access(),
+				 errmsg("could not get junction for \"%s\": %s",
+						path, msg)));
+#else
+		fprintf(stderr, _("could not get junction for \"%s\": %s\n"),
+				path, msg);
+#endif
+		LocalFree(msg);
+		CloseHandle(h);
+		errno = EINVAL;
+		return -1;
+	}
+	CloseHandle(h);
+
+	/* Got it, let's get some results from this */
+	if (reparseBuf->ReparseTag != IO_REPARSE_TAG_MOUNT_POINT)
+	{
+		errno = EINVAL;
+		return -1;
+	}
+
+	r = WideCharToMultiByte(CP_ACP, 0,
+							reparseBuf->PathBuffer, -1,
+							buf,
+							size,
+							NULL, NULL);
+
+	if (r <= 0)
+	{
+		errno = EINVAL;
+		return -1;
+	}
+
+	/*
+	 * If the path starts with "\??\", which it will do in most (all?) cases,
+	 * strip those out.
+	 */
+	if (r > 4 && strncmp(buf, "\\??\\", 4) == 0)
+	{
+		memmove(buf, buf + 4, strlen(buf + 4) + 1);
+		r -= 4;
+	}
+	return r;
+}
+
+/*
+ * Assumes the file exists, so will return false if it doesn't
+ * (since a nonexistant file is not a junction)
+ */
+bool
+pgwin32_is_junction(char *path)
+{
+	DWORD		attr = GetFileAttributes(path);
+
+	if (attr == INVALID_FILE_ATTRIBUTES)
+	{
+		_dosmaperr(GetLastError());
+		return false;
+	}
+	return ((attr & FILE_ATTRIBUTE_REPARSE_POINT) == FILE_ATTRIBUTE_REPARSE_POINT);
+}
 #endif   /* defined(WIN32) && !defined(__CYGWIN__) */