[PATCH v3 3/8] read-tree: explicitly disallow prefixes with a leading '/'

["Victoria Dye via GitGitGadget" <gitgitgadget@gmail.com>]

From: Victoria Dye <vdye@github.com>

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>
---
 builtin/read-tree.c         |  4 ++++
 t/t1003-read-tree-prefix.sh | 10 ++++++++++
 2 files changed, 14 insertions(+)

diff --git a/builtin/read-tree.c b/builtin/read-tree.c
index 2109c4c9e5c..c1a401971c2 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;
 
diff --git a/t/t1003-read-tree-prefix.sh b/t/t1003-read-tree-prefix.sh
index e0db2066f31..c860c08ecb4 100755
--- a/t/t1003-read-tree-prefix.sh
+++ b/t/t1003-read-tree-prefix.sh
@@ -25,4 +25,14 @@ test_expect_success 'read-tree --prefix' '
 	cmp expect actual
 '
 
+test_expect_success 'read-tree --prefix with leading slash exits with error' '
+	git rm -rf . &&
+	test_must_fail git read-tree --prefix=/two/ $tree &&
+	git read-tree --prefix=two/ $tree &&
+
+	git rm -rf . &&
+	test_must_fail git read-tree --prefix=/ $tree &&
+	git read-tree --prefix= $tree
+'
+
 test_done
-- 
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