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checkout: do not corrupt HEAD on empty repo

In abe1998 ("git checkout -b: allow switching out of an unborn
branch"), a code-path overly-optimisticly assumed that a
branch-name was specified. This is not always the case, and as
a result a NULL-pointer was attempted printed to .git/HEAD.

This could lead to at least two different failure modes:
 1) vsnprintf formated the NULL-string as something useful (e.g
    "(null)")
 2) vsnprintf crashed

Neither were very convenient for formatting a new HEAD-reference.

To fix this, reintroduce some strictness so we only take this
new codepath if a banch-name was specified.

Signed-off-by: Erik Faye-Lund <kusmabite@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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1 parent abe1998 commit 8338f771fe9720766dfcb25b6dca58832e24a0fd @kusma kusma committed with gitster May 8, 2012
Showing with 12 additions and 1 deletion.
  1. +1 −1 builtin/checkout.c
  2. +11 −0 t/t2015-checkout-unborn.sh
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@@ -1100,7 +1100,7 @@ int cmd_checkout(int argc, const char **argv, const char *prefix)
if (opts.writeout_stage)
die(_("--ours/--theirs is incompatible with switching branches."));
- if (!new.commit) {
+ if (!new.commit && opts.new_branch) {
unsigned char rev[20];
int flag;
@@ -46,4 +46,15 @@ test_expect_success 'checking out another branch from unborn state' '
test_cmp expect actual
'
+test_expect_success 'checking out in a newly created repo' '
+ test_create_repo empty &&
+ (
+ cd empty &&
+ git symbolic-ref HEAD >expect &&
+ test_must_fail git checkout &&
+ git symbolic-ref HEAD >actual &&
+ test_cmp expect actual
+ )
+'
+
test_done

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