log: diagnose empty HEAD more clearly

If you init or clone an empty repository, the initial
message from running "git log" is not very friendly:

  $ git init
  Initialized empty Git repository in /home/peff/foo/.git/
  $ git log
  fatal: bad default revision 'HEAD'

Let's detect this situation and write a more friendly
message:

  $ git log
  fatal: your current branch 'master' does not have any commits yet

We also detect the case that 'HEAD' points to a broken ref;
this should be even less common, but is easy to see. Note
that we do not diagnose all possible cases. We rely on
resolve_ref, which means we do not get information about
complex cases. E.g., "--default master" would use dwim_ref
to find "refs/heads/master", but we notice only that
"master" does not exist. Similarly, a complex sha1
expression like "--default HEAD^2" will not resolve as a
ref.

But that's OK. We fall back to a generic error message in
those cases, and they are unlikely to be used anyway.
Catching an empty or broken "HEAD" improves the common case,
and the other cases are not regressed.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 14 insertions, 0 deletions

diff --git a/t/t4202-log.sh b/t/t4202-log.sh
index 1b2e981..19277dd 100755
--- a/t/t4202-log.sh
+++ b/t/t4202-log.sh
@@ -871,4 +871,18 @@ test_expect_success 'log --graph --no-walk is forbidden' '
 	test_must_fail git log --graph --no-walk
 '
 
+test_expect_success 'log diagnoses bogus HEAD' '
+	git init empty &&
+	test_must_fail git -C empty log 2>stderr &&
+	test_i18ngrep does.not.have.any.commits stderr &&
+	echo 1234abcd >empty/.git/refs/heads/master &&
+	test_must_fail git -C empty log 2>stderr &&
+	test_i18ngrep broken stderr &&
+	echo "ref: refs/heads/invalid.lock" >empty/.git/HEAD &&
+	test_must_fail git -C empty log 2>stderr &&
+	test_i18ngrep broken stderr &&
+	test_must_fail git -C empty log --default totally-bogus 2>stderr &&
+	test_i18ngrep broken stderr
+'
+
 test_done