Allow git_buf_joinpath to accept self-joins

It was not safe for git_buf_joinpath to be used with a pointer
into the buf itself because a reallocation could invalidate
the input parameter that pointed into the buffer.  This patch
makes it safe to self join, at least for the leading input to
the join, which is the common "append" case for self joins.

Also added unit tests to explicitly cover this case.

This should actually fix #511

src/buffer.c

@@ -328,6 +328,10 @@ int git_buf_join(
 	size_t strlen_a = strlen(str_a);
 	size_t strlen_b = strlen(str_b);
 	int need_sep = 0;
+	ssize_t offset_a = -1;
+
+	/* not safe to have str_b point internally to the buffer */
+	assert(str_b < buf->ptr || str_b > buf->ptr + buf->size);
 
 	/* figure out if we need to insert a separator */
 	if (separator && strlen_a) {
@@ -336,14 +340,24 @@ int git_buf_join(
 			need_sep = 1;
 	}
 
+	/* str_a could be part of the buffer */
+	if (str_a >= buf->ptr && str_a < buf->ptr + buf->size)
+		offset_a = str_a - buf->ptr;
+
 	error = git_buf_grow(buf, strlen_a + strlen_b + need_sep + 1);
 	if (error < GIT_SUCCESS)
 		return error;
 
-	memmove(buf->ptr, str_a, strlen_a);
+	/* fix up internal pointers */
+	if (offset_a >= 0)
+		str_a = buf->ptr + offset_a;
+
+	/* do the actual copying */
+	if (offset_a != 0)
+		memmove(buf->ptr, str_a, strlen_a);
 	if (need_sep)
 		buf->ptr[strlen_a] = separator;
-	memmove(buf->ptr + strlen_a + need_sep, str_b, strlen_b);
+	memcpy(buf->ptr + strlen_a + need_sep, str_b, strlen_b);
 
 	buf->size = strlen_a + strlen_b + need_sep;
 	buf->ptr[buf->size] = '\0';

tests-clay/clay.h

@@ -106,12 +106,13 @@ extern void test_core_filebuf__4(void);
 extern void test_core_filebuf__5(void);
 extern void test_core_oid__initialize(void);
 extern void test_core_oid__streq(void);
-extern void test_core_path__0(void);
-extern void test_core_path__1(void);
-extern void test_core_path__2(void);
-extern void test_core_path__5(void);
-extern void test_core_path__6(void);
-extern void test_core_path__7(void);
+extern void test_core_path__0_dirname(void);
+extern void test_core_path__1_basename(void);
+extern void test_core_path__2_topdir(void);
+extern void test_core_path__5_joins(void);
+extern void test_core_path__6_long_joins(void);
+extern void test_core_path__7_path_to_dir(void);
+extern void test_core_path__8_self_join(void);
 extern void test_core_rmdir__delete_recursive(void);
 extern void test_core_rmdir__fail_to_delete_non_empty_dir(void);
 extern void test_core_rmdir__initialize(void);

tests-clay/clay_main.c

@@ -167,12 +167,13 @@ static const struct clay_func _clay_cb_core_oid[] = {
     {"streq", &test_core_oid__streq}
 };
 static const struct clay_func _clay_cb_core_path[] = {
-    {"0", &test_core_path__0},
-	{"1", &test_core_path__1},
-	{"2", &test_core_path__2},
-	{"5", &test_core_path__5},
-	{"6", &test_core_path__6},
-	{"7", &test_core_path__7}
+    {"0_dirname", &test_core_path__0_dirname},
+	{"1_basename", &test_core_path__1_basename},
+	{"2_topdir", &test_core_path__2_topdir},
+	{"5_joins", &test_core_path__5_joins},
+	{"6_long_joins", &test_core_path__6_long_joins},
+	{"7_path_to_dir", &test_core_path__7_path_to_dir},
+	{"8_self_join", &test_core_path__8_self_join}
 };
 static const struct clay_func _clay_cb_core_rmdir[] = {
     {"delete_recursive", &test_core_rmdir__delete_recursive},
@@ -361,7 +362,7 @@ static const struct clay_suite _clay_suites[] = {
         "core::path",
         {NULL, NULL},
         {NULL, NULL},
-        _clay_cb_core_path, 6
+        _clay_cb_core_path, 7
     },
 	{
         "core::rmdir",
@@ -516,7 +517,7 @@ static const struct clay_suite _clay_suites[] = {
 };
 
 static size_t _clay_suite_count = 36;
-static size_t _clay_callback_count = 119;
+static size_t _clay_callback_count = 120;
 
 /* Core test functions */
 static void

tests-clay/core/path.c

@@ -70,7 +70,7 @@ check_joinpath_n(
 
 
 /* get the dirname of a path */
-void test_core_path__0(void)
+void test_core_path__0_dirname(void)
 {
 	check_dirname(NULL, ".");
 	check_dirname("", ".");
@@ -90,7 +90,7 @@ void test_core_path__0(void)
 }
 
 /* get the base name of a path */
-void test_core_path__1(void)
+void test_core_path__1_basename(void)
 {
 	check_basename(NULL, ".");
 	check_basename("", ".");
@@ -107,7 +107,7 @@ void test_core_path__1(void)
 }
 
 /* get the latest component in a path */
-void test_core_path__2(void)
+void test_core_path__2_topdir(void)
 {
 	check_topdir(".git/", ".git/");
 	check_topdir("/.git/", ".git/");
@@ -124,7 +124,7 @@ void test_core_path__2(void)
 }
 
 /* properly join path components */
-void test_core_path__5(void)
+void test_core_path__5_joins(void)
 {
 	check_joinpath("", "", "");
 	check_joinpath("", "a", "a");
@@ -148,6 +148,10 @@ void test_core_path__5(void)
 	check_joinpath("/abcdefgh", "/12345678/", "/abcdefgh/12345678/");
 	check_joinpath("/abcdefgh/", "12345678/", "/abcdefgh/12345678/");
 
+	check_joinpath(REP1024("aaaa"), "", REP1024("aaaa") "/");
+	check_joinpath(REP1024("aaaa/"), "", REP1024("aaaa/"));
+	check_joinpath(REP1024("/aaaa"), "", REP1024("/aaaa") "/");
+
 	check_joinpath(REP1024("aaaa"), REP1024("bbbb"),
 				   REP1024("aaaa") "/" REP1024("bbbb"));
 	check_joinpath(REP1024("/aaaa"), REP1024("/bbbb"),
@@ -155,7 +159,7 @@ void test_core_path__5(void)
 }
 
 /* properly join path components for more than one path */
-void test_core_path__6(void)
+void test_core_path__6_long_joins(void)
 {
 	check_joinpath_n("", "", "", "", "");
 	check_joinpath_n("", "a", "", "", "a/");
@@ -208,7 +212,7 @@ check_string_to_dir(
 }
 
 /* convert paths to dirs */
-void test_core_path__7(void)
+void test_core_path__7_path_to_dir(void)
 {
 	check_path_to_dir("", "");
 	check_path_to_dir(".", "./");
@@ -234,3 +238,38 @@ void test_core_path__7(void)
 	check_string_to_dir("abcd", 5, "abcd/");
 	check_string_to_dir("abcd", 6, "abcd/");
 }
+
+/* join path to itself */
+void test_core_path__8_self_join(void)
+{
+	git_buf path = GIT_BUF_INIT;
+	ssize_t asize = 0;
+
+	asize = path.asize;
+	cl_git_pass(git_buf_sets(&path, "/foo"));
+	cl_assert_strequal(path.ptr, "/foo");
+	cl_assert(asize < path.asize);
+
+	asize = path.asize;
+	cl_git_pass(git_buf_joinpath(&path, path.ptr, "this is a new string"));
+	cl_assert_strequal(path.ptr, "/foo/this is a new string");
+	cl_assert(asize < path.asize);
+
+	asize = path.asize;
+	cl_git_pass(git_buf_joinpath(&path, path.ptr, "/grow the buffer, grow the buffer, grow the buffer"));
+	cl_assert_strequal(path.ptr, "/foo/this is a new string/grow the buffer, grow the buffer, grow the buffer");
+	cl_assert(asize < path.asize);
+
+	git_buf_free(&path);
+	cl_git_pass(git_buf_sets(&path, "/foo/bar"));
+
+	cl_git_pass(git_buf_joinpath(&path, path.ptr + 4, "baz"));
+	cl_assert_strequal(path.ptr, "/bar/baz");
+
+	asize = path.asize;
+	cl_git_pass(git_buf_joinpath(&path, path.ptr + 4, "somethinglongenoughtorealloc"));
+	cl_assert_strequal(path.ptr, "/baz/somethinglongenoughtorealloc");
+	cl_assert(asize < path.asize);
+	
+	git_buf_free(&path);
+}

tests/test_helpers.c

@@ -236,7 +236,7 @@ static int copy_filesystem_element_recurs(void *_data, git_buf *source)
 	copydir_data *data = (copydir_data *)_data;
 
 	git_buf_truncate(&data->dst, data->dst_baselen);
-	git_buf_joinpath(&data->dst, data->dst.ptr, source->ptr + data->src_baselen);
+	git_buf_puts(&data->dst, source->ptr + data->src_baselen);
 
 	if (git_futils_isdir(source->ptr) == GIT_SUCCESS)
 		return git_futils_direach(source, copy_filesystem_element_recurs, _data);