strbuf: add strbuf_add_real_path()

Add a function for appending the canonized absolute pathname of a given
path to a strbuf.  It keeps the existing contents intact, as expected of
a function of the strbuf_add() family, while avoiding copying the result
if the given strbuf is empty.  It's more consistent with the rest of the
strbuf API than strbuf_realpath(), which it's wrapping.

Also add a semantic patch demonstrating its intended usage and apply it
to the current tree.  Using strbuf_add_real_path() instead of calling
strbuf_addstr() and real_path() avoids an extra copy to a static buffer.

Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/coccinelle/strbuf.cocci

@@ -38,3 +38,9 @@ expression E1, E2, E3;
 @@
 - strbuf_addstr(E1, find_unique_abbrev(E2, E3));
 + strbuf_add_unique_abbrev(E1, E2, E3);
+
+@@
+expression E1, E2;
+@@
+- strbuf_addstr(E1, real_path(E2));
++ strbuf_add_real_path(E1, E2);

setup.c

@@ -254,7 +254,7 @@ int get_common_dir_noenv(struct strbuf *sb, const char *gitdir)
 		if (!is_absolute_path(data.buf))
 			strbuf_addf(&path, "%s/", gitdir);
 		strbuf_addbuf(&path, &data);
-		strbuf_addstr(sb, real_path(path.buf));
+		strbuf_add_real_path(sb, path.buf);
 		ret = 1;
 	} else
 		strbuf_addstr(sb, gitdir);

strbuf.c

@@ -707,6 +707,17 @@ void strbuf_add_absolute_path(struct strbuf *sb, const char *path)
 	strbuf_addstr(sb, path);
 }
 
+void strbuf_add_real_path(struct strbuf *sb, const char *path)
+{
+	if (sb->len) {
+		struct strbuf resolved = STRBUF_INIT;
+		strbuf_realpath(&resolved, path, 1);
+		strbuf_addbuf(sb, &resolved);
+		strbuf_release(&resolved);
+	} else
+		strbuf_realpath(sb, path, 1);
+}
+
 int printf_ln(const char *fmt, ...)
 {
 	int ret;

strbuf.h

@@ -443,6 +443,20 @@ extern int strbuf_getcwd(struct strbuf *sb);
  */
 extern void strbuf_add_absolute_path(struct strbuf *sb, const char *path);
 
+/**
+ * Canonize `path` (make it absolute, resolve symlinks, remove extra
+ * slashes) and append it to `sb`.  Die with an informative error
+ * message if there is a problem.
+ *
+ * The directory part of `path` (i.e., everything up to the last
+ * dir_sep) must denote a valid, existing directory, but the last
+ * component need not exist.
+ *
+ * Callers that don't mind links should use the more lightweight
+ * strbuf_add_absolute_path() instead.
+ */
+extern void strbuf_add_real_path(struct strbuf *sb, const char *path);
+
 
 /**
  * Normalize in-place the path contained in the strbuf. See