Remove hard dependency on GetFileAttributesEx.

This fixes a load-time failure on versions of Windows too old to have
that function in kernel32.dll.

We use it to determine whether a file was safe to overwrite in the
context of PuTTY session logging: if it's safe, we skip the 'do you
want to overwrite or append?' dialog box.

On earlier Windows you can use FindFirstFile to get a similar effect,
so that's what we fall back to. It's not quite the same, though - if
you pass a wildcard then it will succeed when you'd rather it had
failed. But it's good enough to at least work in normal cases.

windows/utils/open_for_write_would_lose_data.c

@@ -4,20 +4,19 @@
 
 #include "putty.h"
 
-bool open_for_write_would_lose_data(const Filename *fn)
+/*
+ * This is slightly fiddly because we want to be backwards-compatible
+ * with systems too old to have GetFileAttributesEx. The next best
+ * thing is FindFirstFile, which will return a different data
+ * structure, but one that also contains the fields we want. (But it
+ * will behave more unhelpfully - for this application - in the
+ * presence of wildcards, so we'd prefer to use GFAE if we can.)
+ */
+
+static inline bool open_for_write_would_lose_data_impl(
+    DWORD dwFileAttributes, DWORD nFileSizeHigh, DWORD nFileSizeLow)
 {
-    WIN32_FILE_ATTRIBUTE_DATA attrs;
-    if (!GetFileAttributesEx(fn->path, GetFileExInfoStandard, &attrs)) {
-        /*
-         * Generally, if we don't identify a specific reason why we
-         * should return true from this function, we return false, and
-         * let the subsequent attempt to open the file for real give a
-         * more useful error message.
-         */
-        return false;
-    }
-    if (attrs.dwFileAttributes & (FILE_ATTRIBUTE_DEVICE |
-                                  FILE_ATTRIBUTE_DIRECTORY)) {
+    if (dwFileAttributes & (FILE_ATTRIBUTE_DEVICE|FILE_ATTRIBUTE_DIRECTORY)) {
         /*
          * File is something other than an ordinary disk file, so
          * opening it for writing will not cause truncation. (It may
@@ -25,7 +24,7 @@ bool open_for_write_would_lose_data(const Filename *fn)
          */
         return false;
     }
-    if (attrs.nFileSizeHigh == 0 && attrs.nFileSizeLow == 0) {
+    if (nFileSizeHigh == 0 && nFileSizeLow == 0) {
         /*
          * File is zero-length (or may be a named pipe, which
          * dwFileAttributes can't tell apart from a regular file), so
@@ -36,3 +35,42 @@ bool open_for_write_would_lose_data(const Filename *fn)
     }
     return true;
 }
+
+bool open_for_write_would_lose_data(const Filename *fn)
+{
+    static HMODULE kernel32_module;
+    DECL_WINDOWS_FUNCTION(static, BOOL, GetFileAttributesExA,
+                          (LPCSTR, GET_FILEEX_INFO_LEVELS, LPVOID));
+
+    if (!kernel32_module) {
+        kernel32_module = load_system32_dll("kernel32.dll");
+        GET_WINDOWS_FUNCTION(kernel32_module, GetFileAttributesExA);
+    }
+
+    if (p_GetFileAttributesExA) {
+        WIN32_FILE_ATTRIBUTE_DATA attrs;
+        if (!p_GetFileAttributesExA(fn->path, GetFileExInfoStandard, &attrs)) {
+            /*
+             * Generally, if we don't identify a specific reason why we
+             * should return true from this function, we return false, and
+             * let the subsequent attempt to open the file for real give a
+             * more useful error message.
+             */
+            return false;
+        }
+        return open_for_write_would_lose_data_impl(
+            attrs.dwFileAttributes, attrs.nFileSizeHigh, attrs.nFileSizeLow);
+    } else {
+        WIN32_FIND_DATA fd;
+        HANDLE h = FindFirstFile(fn->path, &fd);
+        if (h == INVALID_HANDLE_VALUE) {
+            /*
+             * As above, if we can't find the file at all, return false.
+             */
+            return false;
+        }
+        CloseHandle(h);
+        return open_for_write_would_lose_data_impl(
+            fd.dwFileAttributes, fd.nFileSizeHigh, fd.nFileSizeLow);
+    }
+}