MDEV-13941 followup.

Try to fix fragmentation (unsparse files), for pre-existing
installations.

Unsparse the innodb file, when it needs to be extended, unless compression
is used. For Win7/2008R2 unsparse  does not work (as documented in MSDN),
therefore for sparse files in older Windows, file extension will be done
via writing zeroes at the end of file.

storage/innobase/include/os0file.h

@@ -1583,8 +1583,10 @@ os_file_set_umask(ulint umask);
 Make file sparse, on Windows.
 
 @param[in]	file  file handle
+@param[in]	is_sparse if true, make file sparse,
+			otherwise "unsparse" the file
 @return true on success, false on error */
-bool os_file_set_sparse_win32(os_file_t file);
+bool os_file_set_sparse_win32(os_file_t file, bool is_sparse = true);
 
 /**
 Changes file size on Windows

storage/innobase/os/os0file.cc

@@ -4743,11 +4743,20 @@ Sets a sparse flag on Windows file.
 @param[in]	file  file handle
 @return true on success, false on error
 */
-bool os_file_set_sparse_win32(os_file_t file)
+#include <versionhelpers.h>
+bool os_file_set_sparse_win32(os_file_t file, bool is_sparse)
 {
-
+	if (!is_sparse && !IsWindows8OrGreater()) {
+		/* Cannot  unset sparse flag on older Windows.
+		Until Windows8 it is documented to produce unpredictable results,
+		if there are unallocated ranges in file.*/
+		return false;
+	}
 	DWORD temp;
-	return os_win32_device_io_control(file, FSCTL_SET_SPARSE, 0, 0, 0, 0,&temp);
+	FILE_SET_SPARSE_BUFFER sparse_buffer;
+	sparse_buffer.SetSparse = is_sparse;
+	return os_win32_device_io_control(file,
+		FSCTL_SET_SPARSE, &sparse_buffer, sizeof(sparse_buffer), 0, 0,&temp);
 }
 
 
@@ -5342,7 +5351,23 @@ os_file_set_size(
 	bool	is_sparse)
 {
 #ifdef _WIN32
+	/* On Windows, changing file size works well and as expected for both
+	sparse and normal files.
+
+	However, 10.2 up until 10.2.9 made every file sparse in innodb,
+	causing NTFS fragmentation issues(MDEV-13941). We try to undo
+	the damage, and unsparse the file.*/
+
+	if (!is_sparse && os_is_sparse_file_supported(file)) {
+		if (!os_file_set_sparse_win32(file, false))
+			/* Unsparsing file failed. Fallback to writing binary
+			zeros, to avoid even higher fragmentation.*/
+			goto fallback;
+	}
+
 	return os_file_change_size_win32(name, file, size);
+
+fallback:
 #else
 	if (is_sparse) {
 		bool success = !ftruncate(file, size);
@@ -5368,6 +5393,7 @@ os_file_set_size(
 	errno = err;
 	return(!err);
 # endif /* HAVE_POSIX_ALLOCATE */
+#endif /* _WIN32*/
 
 	/* Write up to 1 megabyte at a time. */
 	ulint	buf_size = ut_min(
@@ -5386,13 +5412,14 @@ os_file_set_size(
 	/* Write buffer full of zeros */
 	memset(buf, 0, buf_size);
 
-	if (size >= (os_offset_t) 100 << 20) {
+	os_offset_t	current_size = os_file_get_size(file);
+	bool write_progress_info =
+		(size - current_size >= (os_offset_t) 100 << 20);
 
+	if (write_progress_info) {
 		ib::info() << "Progress in MB:";
 	}
 
-	os_offset_t	current_size = os_file_get_size(file);
-
 	while (current_size < size) {
 		ulint	n_bytes;
 
@@ -5415,8 +5442,9 @@ os_file_set_size(
 		}
 
 		/* Print about progress for each 100 MB written */
-		if ((current_size + n_bytes) / (100 << 20)
-		    != current_size / (100 << 20)) {
+		if (write_progress_info &&
+			((current_size + n_bytes) / (100 << 20)
+				!= current_size / (100 << 20))) {
 
 			fprintf(stderr, " %lu00",
 				(ulong) ((current_size + n_bytes)
@@ -5426,7 +5454,7 @@ os_file_set_size(
 		current_size += n_bytes;
 	}
 
-	if (size >= (os_offset_t) 100 << 20) {
+	if (write_progress_info) {
 
 		fprintf(stderr, "\n");
 	}
@@ -5434,7 +5462,6 @@ os_file_set_size(
 	ut_free(buf2);
 
 	return(os_file_flush(file));
-#endif /* !WIN32 */
 }
 
 /** Truncates a file to a specified size in bytes.