checkout: Don't crash when switching away from an invalid branch.

When using alternates, it is possible for HEAD to end up pointing to
an invalid commit. git checkout should be able to recover from that
situation without crashing.

Signed-off-by: Alexandre Julliard <julliard@winehq.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/builtin-checkout.c b/builtin-checkout.c
index 1deda927cdfcbcdcb751aca3338ac77ef0aad836..05eee4ecc71d7a20d9faf59c849d3c693ae4cc8d 100644 (file)
--- a/builtin-checkout.c
+++ b/builtin-checkout.c
@@ -32,7 +32,7 @@ static int post_checkout_hook(struct commit *old, struct commit *new,
 
        memset(&proc, 0, sizeof(proc));
        argv[0] = name;
-       argv[1] = xstrdup(sha1_to_hex(old->object.sha1));
+       argv[1] = xstrdup(sha1_to_hex(old ? old->object.sha1 : null_sha1));
        argv[2] = xstrdup(sha1_to_hex(new->object.sha1));
        argv[3] = changed ? "1" : "0";
        argv[4] = NULL;
@@ -360,10 +360,10 @@ static void update_refs_for_switch(struct checkout_opts *opts,
 
        strbuf_init(&msg, 0);
        old_desc = old->name;
-       if (!old_desc)
+       if (!old_desc && old->commit)
                old_desc = sha1_to_hex(old->commit->object.sha1);
        strbuf_addf(&msg, "checkout: moving from %s to %s",
-                   old_desc, new->name);
+                   old_desc ? old_desc : "(invalid)", new->name);
 
        if (new->path) {
                create_symref("HEAD", new->path, msg.buf);
@@ -419,7 +419,7 @@ static int switch_branches(struct checkout_opts *opts, struct branch_info *new)
         * a new commit, we want to mention the old commit once more
         * to remind the user that it might be lost.
         */
-       if (!opts->quiet && !old.path && new->commit != old.commit)
+       if (!opts->quiet && !old.path && old.commit && new->commit != old.commit)
                describe_detached_head("Previous HEAD position was", old.commit);
 
        if (!old.commit) {

diff --git a/t/t2011-checkout-invalid-head.sh b/t/t2011-checkout-invalid-head.sh
new file mode 100755 (executable)
index 0000000..764bb0a
--- /dev/null
+++ b/t/t2011-checkout-invalid-head.sh
@@ -0,0 +1,18 @@
+#!/bin/sh
+
+test_description='checkout switching away from an invalid branch'
+
+. ./test-lib.sh
+
+test_expect_success 'setup' '
+       echo hello >world &&
+       git add world &&
+       git commit -m initial
+'
+
+test_expect_success 'checkout master from invalid HEAD' '
+       echo 0000000000000000000000000000000000000000 >.git/HEAD &&
+       git checkout master --
+'
+
+test_done