Add path utilities to resolve relative paths

This makes it easy to take a buffer containing a path with relative
references (i.e. .. or . path segments) and resolve all of those
into a clean path.  This can be applied to URLs as well as file
paths which can be useful.

As part of this, I made the drive-letter detection apply on all
platforms, not just windows.  If you give a path that looks like
"c:/..." on any platform, it seems like we might as well detect
that as a rooted path.  I suppose if you create a directory named
"x:" on another platform and want to use that as the beginning
of a relative path under the root directory of your repo, this
could cause a problem, but then it seems like you're asking for
trouble.

src/path.c

@@ -17,9 +17,7 @@
 #include <stdio.h>
 #include <ctype.h>
 
-#ifdef GIT_WIN32
 #define LOOKS_LIKE_DRIVE_PREFIX(S) (git__isalpha((S)[0]) && (S)[1] == ':')
-#endif
 
 /*
  * Based on the Android implementation, BSD licensed.
@@ -172,11 +170,11 @@ int git_path_root(const char *path)
 {
 	int offset = 0;
 
-#ifdef GIT_WIN32
 	/* Does the root of the path look like a windows drive ? */
 	if (LOOKS_LIKE_DRIVE_PREFIX(path))
 		offset += 2;
 
+#ifdef GIT_WIN32
 	/* Are we dealing with a windows network path? */
 	else if ((path[0] == '/' && path[1] == '/') ||
 		(path[0] == '\\' && path[1] == '\\'))
@@ -464,6 +462,71 @@ int git_path_find_dir(git_buf *dir, const char *path, const char *base)
 	return error;
 }
 
+int git_path_resolve_relative(git_buf *path, size_t ceiling)
+{
+	char *base, *to, *from, *next;
+	size_t len;
+
+	if (!path || git_buf_oom(path))
+		return -1;
+
+	if (ceiling > path->size)
+		ceiling = path->size;
+
+	/* recognize drive prefixes, etc. that should not be backed over */
+	if (ceiling == 0)
+		ceiling = git_path_root(path->ptr) + 1;
+
+	/* recognize URL prefixes that should not be backed over */
+	if (ceiling == 0) {
+		for (next = path->ptr; *next && git__isalpha(*next); ++next);
+		if (next[0] == ':' && next[1] == '/' && next[2] == '/')
+			ceiling = (next + 3) - path->ptr;
+	}
+
+	base = to = from = path->ptr + ceiling;
+
+	while (*from) {
+		for (next = from; *next && *next != '/'; ++next);
+
+		len = next - from;
+
+		if (len == 1 && from[0] == '.')
+			/* do nothing with singleton dot */;
+
+		else if (len == 2 && from[0] == '.' && from[1] == '.') {
+			while (to > base && to[-1] == '/') to--;
+			while (to > base && to[-1] != '/') to--;
+		}
+
+		else {
+			if (*next == '/')
+				len++;
+
+			if (to != from)
+				memmove(to, from, len);
+
+			to += len;
+		}
+
+		from += len;
+
+		while (*from == '/') from++;
+	}
+
+	*to = '\0';
+
+	path->size = to - path->ptr;
+
+	return 0;
+}
+
+int git_path_apply_relative(git_buf *target, const char *relpath)
+{
+	git_buf_joinpath(target, git_buf_cstr(target), relpath);
+	return git_path_resolve_relative(target, 0);
+}
+
 int git_path_cmp(
 	const char *name1, size_t len1, int isdir1,
 	const char *name2, size_t len2, int isdir2)

src/path.h

@@ -185,6 +185,29 @@ extern int git_path_prettify_dir(git_buf *path_out, const char *path, const char
  */
 extern int git_path_find_dir(git_buf *dir, const char *path, const char *base);
 
+/**
+ * Resolve relative references within a path.
+ *
+ * This eliminates "./" and "../" relative references inside a path,
+ * as well as condensing multiple slashes into single ones.  It will
+ * not touch the path before the "ceiling" length.
+ *
+ * Additionally, this will recognize an "c:/" drive prefix or a "xyz://" URL
+ * prefix and not touch that part of the path.
+ */
+extern int git_path_resolve_relative(git_buf *path, size_t ceiling);
+
+/**
+ * Apply a relative path to base path.
+ *
+ * Note that the base path could be a filename or a URL and this
+ * should still work.  The relative path is walked segment by segment
+ * with three rules: series of slashes will be condensed to a single
+ * slash, "." will be eaten with no change, and ".." will remove a
+ * segment from the base path.
+ */
+extern int git_path_apply_relative(git_buf *target, const char *relpath);
+
 /**
  * Walk each directory entry, except '.' and '..', calling fn(state).
  *

tests-clar/core/path.c

@@ -418,3 +418,54 @@ void test_core_path__13_cannot_prettify_a_non_existing_file(void)
 
 	git_buf_free(&p);
 }
+
+void test_core_path__14_apply_relative(void)
+{
+	git_buf p = GIT_BUF_INIT;
+
+	cl_git_pass(git_buf_sets(&p, "/this/is/a/base"));
+
+	cl_git_pass(git_path_apply_relative(&p, "../test"));
+	cl_assert_equal_s("/this/is/a/test", p.ptr);
+
+	cl_git_pass(git_path_apply_relative(&p, "../../the/./end"));
+	cl_assert_equal_s("/this/is/the/end", p.ptr);
+
+	cl_git_pass(git_path_apply_relative(&p, "./of/this/../the/string"));
+	cl_assert_equal_s("/this/is/the/end/of/the/string", p.ptr);
+
+	cl_git_pass(git_path_apply_relative(&p, "../../../../../.."));
+	cl_assert_equal_s("/this/", p.ptr);
+
+	cl_git_pass(git_path_apply_relative(&p, "../../../../../"));
+	cl_assert_equal_s("/", p.ptr);
+
+	cl_git_pass(git_path_apply_relative(&p, "../../../../.."));
+	cl_assert_equal_s("/", p.ptr);
+
+
+	cl_git_pass(git_buf_sets(&p, "d:/another/test"));
+
+	cl_git_pass(git_path_apply_relative(&p, "../../../../.."));
+	cl_assert_equal_s("d:/", p.ptr);
+
+	cl_git_pass(git_path_apply_relative(&p, "from/here/to/../and/./back/."));
+	cl_assert_equal_s("d:/from/here/and/back/", p.ptr);
+
+
+	cl_git_pass(git_buf_sets(&p, "https://my.url.com/test.git"));
+
+	cl_git_pass(git_path_apply_relative(&p, "../another.git"));
+	cl_assert_equal_s("https://my.url.com/another.git", p.ptr);
+
+	cl_git_pass(git_path_apply_relative(&p, "../full/path/url.patch"));
+	cl_assert_equal_s("https://my.url.com/full/path/url.patch", p.ptr);
+
+	cl_git_pass(git_path_apply_relative(&p, ".."));
+	cl_assert_equal_s("https://my.url.com/full/path/", p.ptr);
+
+	cl_git_pass(git_path_apply_relative(&p, "../../../../../"));
+	cl_assert_equal_s("https://", p.ptr);
+
+	git_buf_free(&p);
+}