From 23a9b93e773daaa0546b08e5d339399b23027360 Mon Sep 17 00:00:00 2001
From: Max Horn <max@quendi.de>
Date: Wed, 13 May 2020 11:57:20 +0200
Subject: [PATCH 1/2] buildsys: only include .d files that exist

Fixes #4025
---
 Makefile.rules | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Makefile.rules b/Makefile.rules
index f991bd89ac..0abff033c3 100644
--- a/Makefile.rules
+++ b/Makefile.rules
@@ -290,7 +290,7 @@ endif
 DEPFILES = $(patsubst %,gen/deps/%.d,$(SOURCES))
 
 # Include the dependency tracking files, skip any missing ones
--include $(DEPFILES)
+-include $(wildcard $(DEPFILES))
 
 # the name of the .d and .lo file generated by one of our compiler
 # rules; you may wonder why we don't just use $@ here: this is needed

From 2417390ed142f89bd0e0437a303fcb2bf151325e Mon Sep 17 00:00:00 2001
From: Max Horn <max@quendi.de>
Date: Thu, 14 May 2020 00:52:12 +0200
Subject: [PATCH 2/2] Add some buildsystem tests to Travis

---
 .travis.yml |  4 ++--
 etc/ci.sh   | 30 ++++++++++++++++++++++++++++++
 2 files changed, 32 insertions(+), 2 deletions(-)

diff --git a/.travis.yml b/.travis.yml
index b140878269..c9b482af51 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -130,7 +130,7 @@ matrix:
     #
     # Also change the compiler to GCC 4.7, to ensure we stay compatible
     # with that older version.
-    - env: TEST_SUITES="docomp testinstall" NO_COVERAGE=1 ABI=64 BUILDDIR=build CONFIGFLAGS="--enable-valgrind CC=gcc-4.7 CXX=g++-4.7"
+    - env: TEST_SUITES="docomp testbuildsys testinstall" NO_COVERAGE=1 ABI=64 BUILDDIR=build CONFIGFLAGS="--enable-valgrind CC=gcc-4.7 CXX=g++-4.7"
       addons:
         apt_packages:
           - gcc-4.7
@@ -139,7 +139,7 @@ matrix:
 
     # same as above, but in 32 bit mode, also turn off debugging (see elsewhere in this file for
     # an explanation).
-    - env: TEST_SUITES="docomp testinstall" NO_COVERAGE=1 ABI=32 BUILDDIR=build CONFIGFLAGS=""
+    - env: TEST_SUITES="docomp testbuildsys testinstall" NO_COVERAGE=1 ABI=32 BUILDDIR=build CONFIGFLAGS=""
       addons:
         apt_packages:
           - gcc-multilib
diff --git a/etc/ci.sh b/etc/ci.sh
index b6479fc2cb..afd05df713 100644
--- a/etc/ci.sh
+++ b/etc/ci.sh
@@ -125,6 +125,36 @@ GAPInput
     git diff --exit-code -- src
     ;;
 
+  testbuildsys)
+    # this test assumes we are doing an out-of-tree build
+    test $BUILDDIR != $SRCDIR
+
+    # test: create garbage *.d and *.lo files in the source dir; these should not
+    # affect the out of tree build, nor should they be removed by `make clean`
+    mkdir -p $SRCDIR/gen/deps/src
+    mkdir -p $SRCDIR/gen/obj/src
+    echo "garbage content !!!" > $SRCDIR/gen/deps/src/bool.c.d
+    echo "garbage content !!!" > $SRCDIR/gen/obj/src/bool.c.lo
+
+    # test: `make clean` works and afterwards we can still `make`
+    make clean
+    make
+
+    # verify our "garbage" files are still there
+    test -f $SRCDIR/gen/deps/src/bool.c.d
+    test -f $SRCDIR/gen/obj/src/bool.c.lo
+
+    # test: `make` should regenerate removed *.d files (and then also regenerate the
+    # corresponding *.lo file, which we verify by overwriting it with garbage)
+    rm gen/deps/src/bool.c.d
+    echo "garbage content !!!" > gen/obj/src/bool.c.lo
+    make
+    test -f gen/deps/src/bool.c.d
+
+    # test: running `make` a second time should produce no output
+    test -z "$(make)"
+    ;;
+
   makemanuals)
     make doc
     make check-manuals