[PATCH v4] for-each-ref: Always check stat_tracking_info()'s return value.

The code handling %(upstream:track) and %(upstream:trackshort) assumed
it always had a valid branch that had been sanitized earlier in
populate_value(), and thus did not check the return value of the call to
stat_tracking_info().

While there is indeed some sanitization code that basically corresponds
to stat_tracking_info() returning 0 (no base branch set), the function
can also return -1 when the base branch did exist but has since then
been deleted.

In this case, num_ours and num_theirs had undefined values and a call to
`git for-each-ref --format="%(upstream:track)"` could print spurious
values such as

  [behind -111794512]
  [ahead 38881640, behind 5103867]

even for repositories with one single commit.

We now properly verify stat_tracking_info()'s return value and do not
print anything if it returns -1. This behavior also matches the
documentation ("has no effect if the ref does not have tracking
information associated with it").

Helped-by: Eric Sunshine [off-list ref]
Helped-by: Jeff King [off-list ref]
Signed-off-by: Raphael Kubo da Costa <redacted>
---
v4: Use Jeff's suggestion and simplify the test case and making it
    easier to understand what is actually being tested.

 builtin/for-each-ref.c  | 11 +++++++++--
 t/t6300-for-each-ref.sh | 14 ++++++++++++++
 2 files changed, 23 insertions(+), 2 deletions(-)

diff --git a/builtin/for-each-ref.c b/builtin/for-each-ref.c
index 603a90e..52e6323 100644
--- a/builtin/for-each-ref.c
+++ b/builtin/for-each-ref.c

@@ -717,7 +717,10 @@ static void populate_value(struct refinfo *ref)
 				 starts_with(name, "upstream")) {
 				char buf[40];
 
-				stat_tracking_info(branch, &num_ours, &num_theirs);
+				if (stat_tracking_info(branch, &num_ours,
+						       &num_theirs) != 1)
+					continue;
+
 				if (!num_ours && !num_theirs)
 					v->s = "";
 				else if (!num_ours) {

@@ -735,7 +738,11 @@ static void populate_value(struct refinfo *ref)
 			} else if (!strcmp(formatp, "trackshort") &&
 				   starts_with(name, "upstream")) {
 				assert(branch);
-				stat_tracking_info(branch, &num_ours, &num_theirs);
+
+				if (stat_tracking_info(branch, &num_ours,
+							&num_theirs) != 1)
+					continue;
+
 				if (!num_ours && !num_theirs)
 					v->s = "=";
 				else if (!num_ours)

diff --git a/t/t6300-for-each-ref.sh b/t/t6300-for-each-ref.sh
index bda354c..dcee7a0 100755
--- a/t/t6300-for-each-ref.sh
+++ b/t/t6300-for-each-ref.sh

@@ -335,6 +335,20 @@ test_expect_success 'Check that :track[short] cannot be used with other atoms' '
 '
 
 cat >expected <<EOF
+
+
+EOF
+
+test_expect_success 'Check that :track[short] works when upstream is invalid' '
+	test_when_finished "git config branch.master.merge refs/heads/master" &&
+	git config branch.master.merge refs/heads/does-not-exist &&
+	git for-each-ref \
+		--format="%(upstream:track)$LF%(upstream:trackshort)" \
+		refs/heads > actual &&
+	test_cmp expected actual
+'
+
+cat >expected <<EOF
 $(git rev-parse --short HEAD)
 EOF
 

-- 
2.1.4