diff_file: fix crash if size of diffed file changes in workdir

"diff_file_content_load_workdir_file()" maps a file from the workdir
into memory. It uses git_diff_file.size to determine the size of the
memory mapping.

If this value goes stale, the mmaped area would be sized incorrectly.
This could occur if an external program changes the contents of the
file after libgit2 had cached its size. This used to segfault if the
file becomes smaller (mmaped area too large).

This patch causes diff_file_content_load_workdir_file to fail without
crashing if it detects that the file size has changed.

src/diff_file.c

@@ -328,16 +328,25 @@ static int diff_file_content_load_workdir_file(
 	git_filter_list *fl = NULL;
 	git_file fd = git_futils_open_ro(git_str_cstr(path));
 	git_str raw = GIT_STR_INIT;
+	git_object_size_t new_file_size = 0;
 
 	if (fd < 0)
 		return fd;
 
-	if (!fc->file->size)
-	    error = git_futils_filesize(&fc->file->size, fd);
+	error = git_futils_filesize(&new_file_size, fd);
 
-	if (error < 0 || !fc->file->size)
+	if (error < 0 || !new_file_size)
 		goto cleanup;
 
+	/* if file size doesn't match cached value, abort */
+	if (fc->file->size && fc->file->size != new_file_size)
+	{
+		error = -1;
+		goto cleanup;
+	}
+
+	fc->file->size = new_file_size;
+
 	if ((diff_opts->flags & GIT_DIFF_SHOW_BINARY) == 0 &&
 		diff_file_content_binary_by_size(fc))
 		goto cleanup;

tests/diff/externalmodifications.c

+#include "clar_libgit2.h"
+#include "../checkout/checkout_helpers.h"
+
+#include "index.h"
+#include "repository.h"
+
+static git_repository *g_repo;
+
+void test_diff_externalmodifications__initialize(void)
+{
+	g_repo = cl_git_sandbox_init("testrepo2");
+}
+
+void test_diff_externalmodifications__cleanup(void)
+{
+	cl_git_sandbox_cleanup();
+	g_repo = NULL;
+}
+
+void test_diff_externalmodifications__file_becomes_smaller(void)
+{
+	git_index *index;
+	git_diff *diff;
+	git_patch* patch;
+	git_str path = GIT_STR_INIT;
+	char big_string[500001];
+
+	cl_git_pass(git_str_joinpath(&path, git_repository_workdir(g_repo), "README"));
+
+	/* Modify the file with a large string */
+	memset(big_string, '\n', sizeof(big_string) - 1);
+	big_string[sizeof(big_string) - 1] = '\0';
+	cl_git_mkfile(path.ptr, big_string);
+
+	/* Get a diff */
+	cl_git_pass(git_repository_index(&index, g_repo));
+	cl_git_pass(git_diff_index_to_workdir(&diff, g_repo, index, NULL));
+	cl_assert_equal_i(1, git_diff_num_deltas(diff));
+	cl_assert_equal_i(500000, git_diff_get_delta(diff, 0)->new_file.size);
+
+	/* Simulate file modification after we've gotten the diff.
+	 * Write a shorter string to ensure that we don't mmap 500KB from
+	 * the previous revision, which would most likely crash. */
+	cl_git_mkfile(path.ptr, "hello");
+
+	/* Attempt to get a patch */
+	cl_git_fail(git_patch_from_diff(&patch, diff, 0));
+
+	git_index_free(index);
+	git_diff_free(diff);
+	git_str_dispose(&path);
+}
+
+void test_diff_externalmodifications__file_deleted(void)
+{
+	git_index *index;
+	git_diff *diff;
+	git_patch* patch;
+	git_str path = GIT_STR_INIT;
+
+	cl_git_pass(git_str_joinpath(&path, git_repository_workdir(g_repo), "README"));
+
+	/* Get a diff */
+	cl_git_pass(git_repository_index(&index, g_repo));
+	cl_git_pass(git_diff_index_to_workdir(&diff, g_repo, index, NULL));
+	cl_assert_equal_i(0, git_diff_num_deltas(diff));
+
+	/* Delete the file */
+	cl_git_rmfile(path.ptr);
+	
+	/* Attempt to get a patch */
+	cl_git_fail(git_patch_from_diff(&patch, diff, 0));
+
+	git_index_free(index);
+	git_diff_free(diff);
+	git_str_dispose(&path);
+}