Bug#967240: makes sense

[Hans-Christoph Steiner]

Yeah, this downloader package is quite simple, and they can also be a
time sink to really get working right.  I'm not keen on spending so much
time on them, since I'd rather spend time improving 100% free software.

Your idea makes sense, I'd take it a bit further (untested):

diff --git a/for-postinst/Makefile b/for-postinst/Makefile
index 5f6df96..53171d7 100644
--- a/for-postinst/Makefile
+++ b/for-postinst/Makefile
@@ -29,13 +29,14 @@ install: all
                exit 1 ; \
        fi

-$(UNPACK_DIR)/source.properties: $(DL_DIR)/$(PKG_SOURCE)
+$(UNPACK_DIR)/source.properties: $(DL_DIR)/$(PKG_SOURCE)-stamp
        cd $(DL_DIR) && unzip -ou $(PKG_SOURCE)

-$(DL_DIR)/$(PKG_SOURCE):
+$(DL_DIR)/$(PKG_SOURCE)-stamp: $(DL_DIR)/$(PKG_SOURCE)
        cd $(DL_DIR) && \
                su nobody -s /bin/sh -c "wget --continue $(PKG_SOURCE_URL)"
-       sha1sum -c $(PKG_SOURCE).sha1
+       sha1sum -c $(PKG_SOURCE).sha1 || rm $(PKG_SOURCE)
+       touch $(PKG_SOURCE).checked

 clean:
        -rm -rf -- $(UNPACK_DIR)

diff --git a/for-postinst/Makefile b/for-postinst/Makefile
index 5f6df96..53171d7 100644
--- a/for-postinst/Makefile
+++ b/for-postinst/Makefile
@@ -29,13 +29,14 @@ install: all
 		exit 1 ; \
 	fi
 
-$(UNPACK_DIR)/source.properties: $(DL_DIR)/$(PKG_SOURCE)
+$(UNPACK_DIR)/source.properties: $(DL_DIR)/$(PKG_SOURCE)-stamp
 	cd $(DL_DIR) && unzip -ou $(PKG_SOURCE)
 
-$(DL_DIR)/$(PKG_SOURCE):
+$(DL_DIR)/$(PKG_SOURCE)-stamp: $(DL_DIR)/$(PKG_SOURCE)
 	cd $(DL_DIR) && \
 		su nobody -s /bin/sh -c "wget --continue $(PKG_SOURCE_URL)"
-	sha1sum -c $(PKG_SOURCE).sha1
+	sha1sum -c $(PKG_SOURCE).sha1 || rm $(PKG_SOURCE)
+	touch $(PKG_SOURCE).checked
 
 clean:
 	-rm -rf -- $(UNPACK_DIR)