branch_get() can return NULL (so far on detached HEAD only) but some
code paths in builtin/branch.c cannot deal with that and cause
segfaults.

While at there, make sure to bail out when the user gives 2 or more
branches with --set-upstream-to or --unset-upstream, where only the
first branch is processed and the rest silently dropped.

Reported-by: Per Cederqvist <ced...@opera.com>
Signed-off-by: Nguyễn Thái Ngọc Duy <pclo...@gmail.com>
---
 On Sat, Feb 23, 2013 at 3:27 AM, Junio C Hamano <gits...@pobox.com> wrote:
 > Instead of asking "Is it detached?", perhaps we can say something
 > like "You told me to set the upstream of HEAD to branch X, but " in
 > front?  At least, that will be a better explanation for the reason
 > why the operation is failing.

 Fixed.

 > What you can do is to have a single helper function that can explain
 > why branch_get() returned NULL (or extend branch_get() to serve that
 > purpose as well); then you do not have to duplicate the logic twice
 > on the caller's side (and there may be other callers that want to do
 > the same).

 The explanation mentions about the failed operation, which makes a
 helper less useful. We could still do the helper, but it may lead to
 i18n legos. So no helper in this version.

 > The existing test might be wrong, by the way.  Your HEAD may point
 > at a branch Y but you may not have any commit on it yet, and you may
 > want to allow setting the upstream of that to-be-born branch to
 > another branch X with "branch --set-upstream-to=X [Y|HEAD]".

 It sounds complicated. I think we can revisit it when a user actually
 complains about it.

 builtin/branch.c  | 27 +++++++++++++++++++++++++++
 t/t3200-branch.sh | 21 +++++++++++++++++++++
 2 files changed, 48 insertions(+)

diff --git a/builtin/branch.c b/builtin/branch.c
index 6371bf9..00d17d2 100644
--- a/builtin/branch.c
+++ b/builtin/branch.c
@@ -889,6 +889,17 @@ int cmd_branch(int argc, const char **argv, const char 
*prefix)
        } else if (new_upstream) {
                struct branch *branch = branch_get(argv[0]);
 
+               if (argc > 1)
+                       die(_("too many branches to set new upstream"));
+
+               if (!branch) {
+                       if (!argc || !strcmp(argv[0], "HEAD"))
+                               die(_("could not set upstream of HEAD to %s 
when "
+                                     "it does not point to any branch."),
+                                   new_upstream);
+                       die(_("no such branch '%s'"), argv[0]);
+               }
+
                if (!ref_exists(branch->refname))
                        die(_("branch '%s' does not exist"), branch->name);
 
@@ -901,6 +912,16 @@ int cmd_branch(int argc, const char **argv, const char 
*prefix)
                struct branch *branch = branch_get(argv[0]);
                struct strbuf buf = STRBUF_INIT;
 
+               if (argc > 1)
+                       die(_("too many branches to unset upstream"));
+
+               if (!branch) {
+                       if (!argc || !strcmp(argv[0], "HEAD"))
+                               die(_("could not unset upstream of HEAD when "
+                                     "it does not point to any branch."));
+                       die(_("no such branch '%s'"), argv[0]);
+               }
+
                if (!branch_has_merge_config(branch)) {
                        die(_("Branch '%s' has no upstream information"), 
branch->name);
                }
@@ -916,6 +937,12 @@ int cmd_branch(int argc, const char **argv, const char 
*prefix)
                int branch_existed = 0, remote_tracking = 0;
                struct strbuf buf = STRBUF_INIT;
 
+               if (!strcmp(argv[0], "HEAD"))
+                       die(_("it does not make sense to create 'HEAD' 
manually"));
+
+               if (!branch)
+                       die(_("no such branch '%s'"), argv[0]);
+
                if (kinds != REF_LOCAL_BRANCH)
                        die(_("-a and -r options to 'git branch' do not make 
sense with a branch name"));
 
diff --git a/t/t3200-branch.sh b/t/t3200-branch.sh
index f3e0e4a..12f1e4a 100755
--- a/t/t3200-branch.sh
+++ b/t/t3200-branch.sh
@@ -42,6 +42,10 @@ test_expect_success \
     'git branch a/b/c should create a branch' \
     'git branch a/b/c && test_path_is_file .git/refs/heads/a/b/c'
 
+test_expect_success \
+    'git branch HEAD should fail' \
+    'test_must_fail git branch HEAD'
+
 cat >expect <<EOF
 $_z40 $HEAD $GIT_COMMITTER_NAME <$GIT_COMMITTER_EMAIL> 1117150200 +0000        
branch: Created from master
 EOF
@@ -388,6 +392,14 @@ test_expect_success \
     'git tag foobar &&
      test_must_fail git branch --track my11 foobar'
 
+test_expect_success '--set-upstream-to fails on multiple branches' \
+    'test_must_fail git branch --set-upstream-to master a b c'
+
+test_expect_success '--set-upstream-to fails on detached HEAD' \
+    'git checkout HEAD^{} &&
+     test_must_fail git branch --set-upstream-to master &&
+     git checkout -'
+
 test_expect_success 'use --set-upstream-to modify HEAD' \
     'test_config branch.master.remote foo &&
      test_config branch.master.merge foo &&
@@ -417,6 +429,15 @@ test_expect_success 'test --unset-upstream on HEAD' \
      test_must_fail git branch --unset-upstream
 '
 
+test_expect_success '--unset-upstream should fail on multiple branches' \
+    'test_must_fail git branch --unset-upstream a b c'
+
+test_expect_success '--unset-upstream should fail on detached HEAD' \
+    'git checkout HEAD^{} &&
+     test_must_fail git branch --unset-upstream &&
+     git checkout -
+'
+
 test_expect_success 'test --unset-upstream on a particular branch' \
     'git branch my15
      git branch --set-upstream-to master my14 &&
-- 
1.8.1.2.536.gf441e6d

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