Teach read-tree the -n|--dry-run option

The option can be used to check if read-tree with the same set of other
options like "-m" and "-u" would succeed without actually changing either
the index or the working tree.

The relevant tests in the t10?? range were extended to do a read-tree -n
before the real read-tree to make sure neither the index nor any local
files were changed with -n and the same exit code as without -n is
returned. The helper functions added for that purpose reside in the new
t/lib-read-tree.sh file.

The only exception is #13 in t1004 ("unlinking an un-unlink-able
symlink"). As this is an issue of wrong directory permissions it is not
detected with -n.

Signed-off-by: Jens Lehmann <Jens.Lehmann@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-read-tree.txt

@@ -53,6 +53,11 @@ OPTIONS
 	trees that are not directly related to the current
 	working tree status into a temporary index file.
 
+-n::
+--dry-run::
+	Check if the command would error out, without updating the index
+	nor the files in the working tree for real.
+
 -v::
 	Show the progress of checking files out.
 

builtin/read-tree.c

@@ -130,6 +130,7 @@ int cmd_read_tree(int argc, const char **argv, const char *unused_prefix)
 		  PARSE_OPT_NONEG, exclude_per_directory_cb },
 		OPT_SET_INT('i', NULL, &opts.index_only,
 			    "don't check the working tree after merging", 1),
+		OPT__DRY_RUN(&opts.dry_run, "don't update the index or the work tree"),
 		OPT_SET_INT(0, "no-sparse-checkout", &opts.skip_sparse_checkout,
 			    "skip applying sparse checkout filter", 1),
 		OPT_SET_INT(0, "debug-unpack", &opts.debug_unpack,
@@ -219,7 +220,7 @@ int cmd_read_tree(int argc, const char **argv, const char *unused_prefix)
 	if (unpack_trees(nr_trees, t, &opts))
 		return 128;
 
-	if (opts.debug_unpack)
+	if (opts.debug_unpack || opts.dry_run)
 		return 0; /* do not write the index out */
 
 	/*

t/lib-read-tree.sh

@@ -0,0 +1,43 @@
+#!/bin/sh
+#
+# Helper functions to check if read-tree would succeed/fail as expected with
+# and without the dry-run option. They also test that the dry-run does not
+# write the index and that together with -u it doesn't touch the work tree.
+#
+read_tree_must_succeed () {
+    git ls-files -s >pre-dry-run &&
+    git read-tree -n "$@" &&
+    git ls-files -s >post-dry-run &&
+    test_cmp pre-dry-run post-dry-run &&
+    git read-tree "$@"
+}
+
+read_tree_must_fail () {
+    git ls-files -s >pre-dry-run &&
+    test_must_fail git read-tree -n "$@" &&
+    git ls-files -s >post-dry-run &&
+    test_cmp pre-dry-run post-dry-run &&
+    test_must_fail git read-tree "$@"
+}
+
+read_tree_u_must_succeed () {
+    git ls-files -s >pre-dry-run &&
+    git diff-files -p >pre-dry-run-wt &&
+    git read-tree -n "$@" &&
+    git ls-files -s >post-dry-run &&
+    git diff-files -p >post-dry-run-wt &&
+    test_cmp pre-dry-run post-dry-run &&
+    test_cmp pre-dry-run-wt post-dry-run-wt &&
+    git read-tree "$@"
+}
+
+read_tree_u_must_fail () {
+    git ls-files -s >pre-dry-run &&
+    git diff-files -p >pre-dry-run-wt &&
+    test_must_fail git read-tree -n "$@" &&
+    git ls-files -s >post-dry-run &&
+    git diff-files -p >post-dry-run-wt &&
+    test_cmp pre-dry-run post-dry-run &&
+    test_cmp pre-dry-run-wt post-dry-run-wt &&
+    test_must_fail git read-tree "$@"
+}