index::nsec: don't expect shit filesystems to not suck

If the underlying filesystem doesn't support better than one
second resolution, then don't expect that turning on `GIT_USE_NSEC`
does anything magical to change that.

tests/index/nsec.c

@@ -24,6 +24,43 @@ void test_index_nsec__cleanup(void)
 	cl_git_sandbox_cleanup();
 }
 
+static bool try_create_file_with_nsec_timestamp(const char *path)
+{
+	struct stat st;
+	int try;
+
+	/* retry a few times to avoid nanos *actually* equal 0 race condition */
+	for (try = 0; try < 3; try++) {
+		cl_git_mkfile(path, "This is hopefully a file with nanoseconds!");
+
+		cl_must_pass(p_stat(path, &st));
+
+		if (st.st_ctime_nsec && st.st_mtime_nsec)
+			return true;
+	}
+
+	return false;
+}
+
+/* try to determine if the underlying filesystem supports a resolution
+ * higher than a single second.  (i'm looking at you, hfs+)
+ */
+static bool should_expect_nsecs(void)
+{
+	git_buf nsec_path = GIT_BUF_INIT;
+	bool expect;
+
+	git_buf_joinpath(&nsec_path, clar_sandbox_path(), "nsec_test");
+
+	expect = try_create_file_with_nsec_timestamp(nsec_path.ptr);
+
+	p_unlink(nsec_path.ptr);
+
+	git_buf_clear(&nsec_path);
+
+	return expect;
+}
+
 static bool has_nsecs(void)
 {
 	const git_index_entry *entry;
@@ -50,8 +87,13 @@ void test_index_nsec__has_nanos(void)
 void test_index_nsec__staging_maintains_other_nanos(void)
 {
 	const git_index_entry *entry;
+	bool expect_nsec, test_file_has_nsec;
+
+    expect_nsec = should_expect_nsecs();
+	test_file_has_nsec = try_create_file_with_nsec_timestamp("nsecs/a.txt");
+
+	cl_assert_equal_b(expect_nsec, test_file_has_nsec);
 
-	cl_git_rewritefile("nsecs/a.txt", "This is file A");
 	cl_git_pass(git_index_add_bypath(repo_index, "a.txt"));
 	cl_git_pass(git_index_write(repo_index));
 
@@ -63,15 +105,15 @@ void test_index_nsec__staging_maintains_other_nanos(void)
 	cl_assert((entry = git_index_get_bypath(repo_index, "a.txt", 0)));
 
 	/* if we are writing nanoseconds to the index, expect them to be
-	 * nonzero.  if we are *not*, expect that we truncated the entry.
+	 * nonzero.
 	 */
-#ifdef GIT_USE_NSEC
-	cl_assert(entry->ctime.nanoseconds != 0);
-	cl_assert(entry->mtime.nanoseconds != 0);
-#else
-	cl_assert_equal_i(0, entry->ctime.nanoseconds);
-	cl_assert_equal_i(0, entry->mtime.nanoseconds);
-#endif
+	if (expect_nsec) {
+		cl_assert(entry->ctime.nanoseconds != 0);
+		cl_assert(entry->mtime.nanoseconds != 0);
+	} else {
+		cl_assert_equal_i(0, entry->ctime.nanoseconds);
+		cl_assert_equal_i(0, entry->mtime.nanoseconds);
+	}
 }
 
 void test_index_nsec__status_doesnt_clear_nsecs(void)