Re-try Minisat download (up to two times) in case it fails

Running in CI we do see transient network access errors from time to time, which
should not result in task failures (as long as they are genuinely transient).

Author: tautschnig

src/Makefile

@@ -89,7 +89,13 @@ TAR = tar
 
 minisat2-download:
 	@echo "Downloading Minisat 2.2.1"
-	@$(DOWNLOADER) http://ftp.debian.org/debian/pool/main/m/minisat2/minisat2_2.2.1.orig.tar.gz
+	@for i in `seq 1 3` ; do \
+	  $(DOWNLOADER) \
+	    http://ftp.debian.org/debian/pool/main/m/minisat2/minisat2_2.2.1.orig.tar.gz && \
+	    exit 0 ; \
+		$(RM) minisat2_2.2.1.orig.tar.gz ; \
+	  if [ $$i -lt 3 ] ; then echo "Re-trying in 10 seconds" 1>&2 ; sleep 10 ; fi ; \
+	done ; exit 1
 	@$(TAR) xfz minisat2_2.2.1.orig.tar.gz
 	@rm -Rf ../minisat-2.2.1
 	@mv minisat2-2.2.1 ../minisat-2.2.1

src/solvers/CMakeLists.txt

@@ -60,6 +60,9 @@ include("${CBMC_SOURCE_DIR}/../cmake/DownloadProject.cmake")
 if("${sat_impl}" STREQUAL "minisat2")
     message(STATUS "Building solvers with minisat2")
 
+    # once we upgrade to CMake 3.7 or higher we can specify multiple URLs as a
+    # fall-back in case the first URL fails (the Makefile-based build retries up
+    # to 2 times)
     download_project(PROJ minisat2
         URL http://ftp.debian.org/debian/pool/main/m/minisat2/minisat2_2.2.1.orig.tar.gz
         PATCH_COMMAND patch -p1 -i ${CBMC_SOURCE_DIR}/../scripts/minisat-2.2.1-patch