makefile: fix parallel make check failing on the first run (#177)

After the changes in #141, parallel `make check` started failing with the
following GNU Parallel error:

> Option tmpdir requires an argument

This is caused by the fact that `test_config.mk` uses `?=` for setting
`TEST_TMPDIR` and on make restarts this doesn't work for some reason
(perhaps having to do with empty `TEST_TMPDIR` being somehow exported).

Replace it with a check for empty value and unconditional assignment
instead. While at it, include `test_config.mk` only on make restarts for
check targets instead of forcing everything that makes use of `TEST_TMPDIR`
to reuse the unit tests configuration.

Also use 6 Xs for the mktemp template in order to support BusyBox mktemp
and maybe some others as well, that require the template to end with 6 Xs
exactly.

common.mk

@@ -14,10 +14,11 @@ endif
 ifeq ($(TEST_TMPDIR),)
 TEST_TMPDIR := $(TMPD)
 endif
-# Avoid setting up the tmp directory on Makefile restarts or when the target
-# isn't a check target
-ifeq ($(MAKE_RESTARTS),)
+
+# Avoid setting up the tmp directory when the target isn't a check target or
+# on Makefile restarts
 ifneq ($(filter %check,$(MAKECMDGOALS)),)
+ifeq ($(MAKE_RESTARTS),)
 
 ifeq ($(TEST_TMPDIR),)
 ifeq ($(BASE_TMPDIR),)
@@ -27,11 +28,13 @@ ifeq ($(BASE_TMPDIR),)
 BASE_TMPDIR :=/tmp
 endif
 # Use /dev/shm on Linux if it has the sticky bit set (otherwise, /tmp or other
-# base dir), and create a randomly-named rocksdb.XXXX directory therein.
+# base dir), and create a randomly-named rocksdb.XXXXXX directory therein.
 ifneq ($(shell [ "$$(uname -s)" = "Linux" ] && [ -k /dev/shm ] && echo 1),)
 BASE_TMPDIR :=/dev/shm
 endif
-TEST_TMPDIR := $(shell mktemp -d "$(BASE_TMPDIR)/rocksdb.XXXX")
+# Use 6 Xs in the template in order to appease the BusyBox mktemp command,
+# which requires the template to end with exactly 6 Xs.
+TEST_TMPDIR := $(shell mktemp -d "$(BASE_TMPDIR)/rocksdb.XXXXXX")
 endif
 
 # The `export` line below doesn't work in case Make restarts (due to included
@@ -41,11 +44,15 @@ endif
 # to avoid having the TEST_TMPDIR implicitly set for test that are run through
 # makefiles that include make_config.mk, and because we don't want to change
 # make_config.mk on every run)
-$(shell echo 'TEST_TMPDIR?=$(TEST_TMPDIR)' > test_config.mk)
+$(shell printf 'ifeq ($$(TEST_TMPDIR),)\nTEST_TMPDIR:=$(TEST_TMPDIR)\nendif\n' > test_config.mk)
+
+else
+
+# If neither TEST_TMPDIR nor TMPD were specified, try to load TEST_TMPDIR from
+# a previous run as saved in test_config.mk (generated by the shell call above)
+include test_config.mk
 
 endif
 endif
 
--include test_config.mk
-
 export TEST_TMPDIR