read-tree: explicitly disallow prefixes with a leading '/'

Exit with an error if a prefix provided to `git read-tree --prefix` begins
with '/'. In most cases, prefixes like this result in an "invalid path"
error; however, the repository root would be interpreted as valid when
specified as '--prefix=/'. This is due to leniency around trailing directory
separators on prefixes (e.g., allowing both '--prefix=my-dir' and
'--prefix=my-dir/') - the '/' in the prefix is actually the *trailing*
slash, although it could be misinterpreted as a *leading* slash.

To remove the confusing repo root-as-'/' case and make it clear that
prefixes should not begin with '/', exit with an error if the first
character of the provided prefix is '/'.

Helped-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Victoria Dye <vdye@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 4 insertions, 0 deletions

diff --git a/builtin/read-tree.c b/builtin/read-tree.c
index 2109c4c9e5..c1a401971c 100644
--- a/builtin/read-tree.c
+++ b/builtin/read-tree.c
@@ -166,6 +166,10 @@ int cmd_read_tree(int argc, const char **argv, const char *cmd_prefix)
 	if (1 < opts.merge + opts.reset + prefix_set)
 		die("Which one? -m, --reset, or --prefix?");
 
+	/* Prefix should not start with a directory separator */
+	if (opts.prefix && opts.prefix[0] == '/')
+		die("Invalid prefix, prefix cannot start with '/'");
+
 	if (opts.reset)
 		opts.reset = UNPACK_RESET_OVERWRITE_UNTRACKED;