Fix output directory race in parallel CI builds

.github/workflows/c-cpp.yml

@@ -17,14 +17,19 @@ jobs:
 
     steps:
     - uses: actions/checkout@v2
-    - name: Run tests
+    - name: Build KenOS from a clean checkout
       run: |
         echo $DOCKER_PASS | docker login --username $DOCKER_USER --password-stdin
         docker pull memegauste/kenos-build:alphadocker
-        docker run -w $GITHUB_DIR -v $GITHUB_DIR:$GITHUB_DIR memegauste/kenos-build:alphadocker /bin/bash -c "make -j$(nproc) -k RPT_FLAGS='-ffreestanding -O2 -Wall -Wno-error'"
+        docker run --rm -w "$GITHUB_DIR" -v "$GITHUB_DIR:$GITHUB_DIR" memegauste/kenos-build:alphadocker /bin/bash -ec '
+          make clean
+          make -j"$(nproc)"
+          grub-file --is-x86-multiboot kenos.bin
+        '
 
     - uses: actions/upload-artifact@v4
       name: Upload current KenOS build
       with:
         name: kenos.iso
         path: ./kenos.iso
+        if-no-files-found: error

makefile

@@ -16,19 +16,19 @@ kenos.iso: kenos.bin
 kenos.bin: build/kernel.o build/boot.o $(OBJECTS)
 	${CC} -T linker.ld -o $@ -ffreestanding -O2 -nostdlib $^
 
-build/kernel.o: kernel.c
-	mkdir build
+build:
+	mkdir -p $@
+
+build/kernel.o: kernel.c | build
 	${CC} -c $< -o $@ -std=gnu99 ${RPT_FLAGS} -Iinclude -Wextra
 
-build/%.o: lib/%.c
+build/%.o: lib/%.c | build
 	${CC} -c $< -o $@ -std=gnu99 ${RPT_FLAGS} -Iinclude -Wextra
 
-build/boot.o: boot.asm
-	${ASM} -felf32 $? -o $@
+build/boot.o: boot.asm | build
+	${ASM} -felf32 $< -o $@
 
+.PHONY: clean
 clean:
 	rm -rf build
-	rm kenos.bin
-	rm build/*
-	rm isodir/boot/kenos.bin
-	rm kenos.iso
+	rm -f kenos.bin isodir/boot/kenos.bin kenos.iso