[PATCH v4 3/4] setup: Add 'abspath_part_inside_repo' function

[Martin Erik Werner <martinerikwerner@gmail.com>]

In order to extract the part of an absolute path which lies inside the
repo, it is not possible to directly use real_path, since that would
dereference symlinks both outside and inside the work tree.

Add an 'abspath_part_inside_repo' function which first checks if the
work tree is already the prefix, then incrementally checks each path
level by temporarily NUL-terminating at each '/' and comparing against
the work tree path. If a match is found, it overwrites the input path
with the remainder past the work tree (which will be the in-repo part).

The path being exactly equal to the work tree is handled separately,
since then there is no directory separator between the work tree and
in-repo part.

This function is currently only intended for use in
'prefix_path_gently'.

Signed-off-by: Martin Erik Werner <martinerikwerner@gmail.com>
---
 setup.c | 57 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 57 insertions(+)

diff --git a/setup.c b/setup.c
index 5432a31..e938785 100644
--- a/setup.c
+++ b/setup.c
@@ -6,6 +6,63 @@ static int inside_git_dir = -1;
 static int inside_work_tree = -1;
 
 /*
+ * No checking if the path is in fact an absolute path is done, and it must
+ * already be normalized.
+ *
+ * Find the part of an absolute path that lies inside the work tree by
+ * dereferencing symlinks outside the work tree, for example:
+ * /dir1/repo/dir2/file   (work tree is /dir1/repo)      -> dir2/file
+ * /dir/file              (work tree is /)               -> dir/file
+ * /dir/symlink1/symlink2 (symlink1 points to work tree) -> symlink2
+ * /dir/repo              (exactly equal to work tree)   -> (empty string)
+ */
+static inline int abspath_part_inside_repo(char *path)
+{
+	size_t len;
+	size_t wtlen;
+	char *path0;
+
+	const char* work_tree = get_git_work_tree();
+	if (!work_tree)
+		return -1;
+	wtlen = strlen(work_tree);
+	len = strlen(path);
+
+	/* check if work tree is already the prefix */
+	if (strncmp(path, work_tree, wtlen) == 0) {
+		if (path[wtlen] == '/')
+			memmove(path, path + wtlen + 1, len - wtlen);
+		else
+			/* work tree is the root, or the whole path */
+			memmove(path, path + wtlen, len - wtlen + 1);
+		return 0;
+	}
+	path0 = path;
+	path += offset_1st_component(path);
+
+	/* check each level */
+	while (*path != '\0') {
+		path++;
+		if (*path == '/') {
+			*path = '\0';
+			if (strcmp(real_path(path0), work_tree) == 0) {
+				memmove(path0, path + 1, len - (path - path0));
+				return 0;
+			}
+			*path = '/';
+		}
+	}
+
+	/* check whole path */
+	if (strcmp(real_path(path0), work_tree) == 0) {
+		*path0 = '\0';
+		return 0;
+	}
+
+	return -1;
+}
+
+/*
  * Normalize "path", prepending the "prefix" for relative paths. If
  * remaining_prefix is not NULL, return the actual prefix still
  * remains in the path. For example, prefix = sub1/sub2/ and path is
-- 
1.8.5.2