make fdatasync() explicit

src/tus_file.c

@@ -949,11 +949,9 @@ tus_body_to_file(VRT_CTX, struct tus_file_core *fcore)
 	if (written < 0 && errno == EFBIG) {
 		fdisk->upload_offset = fdisk->upload_length = suck.max;
 		AZ(ftruncate(fcore->fd, header_size + suck.max));
-		AZ(fdatasync(fcore->fd));
 		return (413);
 	}
 
-	AZ(fdatasync(fcore->fd));
 	if (suck.chksum == NULL) {
 		if (written >= 0)
 			return (204);
@@ -966,7 +964,6 @@ tus_body_to_file(VRT_CTX, struct tus_file_core *fcore)
 
 	fcore->disk->upload_offset = offset_saved;
 	AZ(ftruncate(fcore->fd, header_size + offset_saved));
-	AZ(fdatasync(fcore->fd));
 	return (460);
 }
 
@@ -998,8 +995,20 @@ tus_file_done(struct tus_file_core *fcore, struct tus_file_disk *fdisk, const ch
 	fd = tus_file_open(fcore);
 	if (fd >= 0) {
 		AZ(ftruncate(fd, header_size));
-		AZ(fdatasync(fd));
 		tus_file_close(fcore);
 	}
 	return (1);
 }
+
+void
+tus_file_sync(struct tus_file_core *fcore)
+{
+	if (fcore == NULL)
+		return;
+
+	CHECK_OBJ_NOTNULL(fcore, VMOD_TUS_FILE_CORE_MAGIC);
+
+	if (fcore->fd < 0)
+		return;
+	AZ(fdatasync(fcore->fd));
+}

src/tus_file.h

@@ -175,3 +175,4 @@ void
 	tus_file_final_abort(struct concat_embryo *embryo);
 VCL_BOOL
 tus_file_done(struct tus_file_core *, struct tus_file_disk *, const char *);
+void tus_file_sync(struct tus_file_core *);

src/vmod_tus.c

@@ -532,3 +532,18 @@ tus_server_metadata(VRT_CTX, struct VPFX(tus_server) *tussrv,
 
 	return (b);
 }
+
+VCL_VOID
+tus_server_sync(VRT_CTX, struct VPFX(tus_server) *tussrv)
+{
+	struct tus_response *r;
+
+	CHECK_OBJ_NOTNULL(ctx, VRT_CTX_MAGIC);
+	CHECK_OBJ_NOTNULL(tussrv, VMOD_TUS_SERVER_MAGIC);
+
+	r = tus_task_use(ctx, tussrv);
+	if (r == NULL)
+		return;
+
+	tus_file_sync(r->fcore);
+}

src/vmod_tus.vcc

@@ -216,5 +216,19 @@ Extract *key* from metadata and return the corresponding value decoded.
 Only available if vmod_blob is available (see `Hashes` _) and on the
 client side after `xserver.recv()`_ was called from ``vcl_recv {}``.
 
+$Method VOID .sync()
+
+For lowest latencies, vmod_tus stores data in files opened without any
+additional options affecting caching such that the operating system is
+free to write data delayed. This could, in turn, lead to upload data
+and/or state to be lost or reverted to an earlier state when the
+system crashes.
+
+When the `xserver.sync()`_ method is called, an `fdatasync(2)` call is
+issued on the file backing the current upload, such that its current
+state is flushed to stable storage (given that the underlying file
+does provide stable storage).
+
+
 SEE ALSO
 ========vcl\(7),varnishd\(1)

src/vtc/plain.vtc

@@ -73,6 +73,7 @@ varnish v1 -vcl+backend {
 	    if (resp.status == 4200) {
 		call meta;
 		tmp.synth();
+		tmp.sync();
 		return (deliver);
 	    }
 	}