handle file size vs. upload_offset during load

src/tus_file.c

@@ -590,6 +590,7 @@ tus_file_add(struct VPFX(tus_server) *srv, int basefd, const char *filename)
 	struct stat st;
 	int fd = -1;
 	const char *err;
+	ssize_t sz;
 
 	if (fstatat(basefd, filename, &st, AT_SYMLINK_NOFOLLOW)) {
 		fprintf(stderr, "tus add stat %s: %d\n", filename, errno);
@@ -634,6 +635,25 @@ tus_file_add(struct VPFX(tus_server) *srv, int basefd, const char *filename)
 		goto err;
 	}
 
+	sz = (uintmax_t)st.st_size - header_size;
+	assert(sz >= 0);
+	/*
+	 * if data was written before we updated the offset,
+	 * we truncate to avoid potentially corrupted data
+	 */
+	if (fdisk->location_length > 0) {
+		// .done() called & truncated
+	}
+	else if (fdisk->upload_offset < sz) {
+		AZ(ftruncate(fd, header_size + fdisk->upload_offset));
+		fprintf(stderr, "tus add %s: truncated to %zd\n", filename, sz);
+	}
+	else if (fdisk->upload_offset > sz) {
+		fprintf(stderr, "tus add %s: updated offset from %zd to %zd\n",
+			filename, fdisk->upload_offset, sz);
+		fdisk->upload_offset = sz;
+	}
+
 	/* note: the close can cause a SIGSEGV if a file is unlinked
 	 * concurrently. But as we should be the only user of the base
 	 * directory, that should not happen