strbuf_getwholeline: NUL-terminate getdelim buffer on error

Commit 0cc30e0e (strbuf_getwholeline: use getdelim if it is
available, 2015-04-16) tries to clean up after getdelim()
returns EOF, but gets one case wrong, which can lead in some
obscure cases to us reading uninitialized memory.

After getdelim() returns -1, we re-initialize the strbuf
only if sb->buf is NULL. The thinking was that either:

  1. We fed an existing allocated buffer to getdelim(), and
     at most it would have realloc'd, leaving our NUL in
     place.

  2. We didn't have a buffer to feed, so we gave getdelim()
     NULL; sb->buf will remain NULL, and we just want to
     restore the empty slopbuf.

But that second case isn't quite right. getdelim() may
allocate a buffer, write nothing into it, and then return
EOF. The resulting strbuf rightfully has sb->len set to "0",
but is missing the NUL terminator in the first byte.

Most call-sites are fine with this. They see the EOF and
don't bother looking at the strbuf. Or they notice that
sb->len is empty, and don't look at the contents. But
there's at least one case that does neither, and relies on
parsing the resulting (possibly zero-length) string:
fast-import. You can see this in action with the new test
(though we probably only notice failure there when run with
--valgrind or ASAN).

We can fix this by unconditionally resetting the strbuf when
we have a buffer after getdelim(). That fixes case 2 above.
Case 1 is probably already fine in practice, but it does not
hurt for us to re-assert our invariants (especially because
we are relying on whatever getdelim() happens to do, which
may vary from platform to platform). Our fix covers that
case, too.
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

strbuf.c

@@ -470,9 +470,15 @@ int strbuf_getwholeline(struct strbuf *sb, FILE *fp, int term)
 	if (errno == ENOMEM)
 		die("Out of memory, getdelim failed");
 
-	/* Restore slopbuf that we moved out of the way before */
+	/*
+	 * Restore strbuf invariants; if getdelim left us with a NULL pointer,
+	 * we can just re-init, but otherwise we should make sure that our
+	 * length is empty, and that the result is NUL-terminated.
+	 */
 	if (!sb->buf)
 		strbuf_init(sb, 0);
+	else
+		strbuf_reset(sb);
 	return EOF;
 }
 #else

t/t9300-fast-import.sh

@@ -55,6 +55,10 @@ test_expect_success 'empty stream succeeds' '
 	git fast-import </dev/null
 '
 
+test_expect_success 'truncated stream complains' '
+	echo "tag foo" | test_must_fail git fast-import
+'
+
 test_expect_success 'A: create pack from stdin' '
 	test_tick &&
 	cat >input <<-INPUT_END &&