test: qualify partial host image writes and recovery

tests/fixtures/updates/file-write-limit.cppadded+28-0

Changes to tests/fixtures/updates/file-write-limit.cpp

		@@ -0,0 +1,28 @@
	1	+// Test-only inherited POSIX file-size limit. No application or board linkage.
	2	+#include <cerrno>
	3	+#include <csignal>
	4	+#include <cstdio>
	5	+#include <cstdlib>
	6	+#include <cstring>
	7	+#include <sys/resource.h>
	8	+#include <unistd.h>
	9	+
	10	+int main(int argc, char** argv) {
	11	+ if (argc < 3 || !argv[1][0] || std::strlen(argv[1]) > 8
	12	+ || std::strspn(argv[1], "0123456789") != std::strlen(argv[1])) return 125;
	13	+ errno = 0;
	14	+ char* end = nullptr;
	15	+ const auto requested = std::strtoull(argv[1], &end, 10);
	16	+ if (errno || !end || *end || requested < 1 || requested > 16777216) return 125;
	17	+ const rlim_t bytes = static_cast<rlim_t>(requested);
	18	+ const rlimit limit{bytes, bytes};
	19	+ if (setrlimit(RLIMIT_FSIZE, &limit) != 0 || std::signal(SIGXFSZ, SIG_IGN) == SIG_ERR) return 125;
	20	+ rlimit observed{};
	21	+ if (getrlimit(RLIMIT_FSIZE, &observed) != 0 || observed.rlim_cur != bytes || observed.rlim_max != bytes) return 125;
	22	+ if (setenv("DRONE_TEST_FILE_LIMIT", argv[1], 1) != 0) return 125;
	23	+ std::fprintf(stderr, "FILE_LIMIT_ACTIVE %llu\n", requested);
	24	+ std::fflush(stderr);
	25	+ execvp(argv[2], argv + 2);
	26	+ std::perror("File-limit test exec");
	27	+ return 127;
	28	+}