Schcom v0: a compositor that actually composites

Clients connect over a Unix socket, are handed a shared buffer, draw into
it and commit; Schcom composites and presents. Both of the hard blockers
its README listed -- unix domain sockets and raw input events -- have been
done for a while, so nothing was in the way.

The model is Wayland's, on purpose. A surface is not a window: it is a
rectangle of pixels with no policy attached, because placement belongs to
SchWM and the moment this program starts deciding where windows go the
split has failed. Nothing is shown until COMMIT, so a client halfway
through drawing is never presented. The compositor tells clients when to
draw rather than letting them render frames nobody sees.

Wayland is MIT-licensed and supporting it later is intended, so the
divergences are the ones that are cheap to undo: a buffer handle is a
System V shm id rather than an fd over SCM_RIGHTS, and the message
carries a handle_kind field so switching is a new value rather than a
protocol break. Every message carries its own length, so an unknown
message is skipped instead of desynchronising the stream -- which is what
lets a v0 compositor survive a v1 client.

The compositor owns the shm segment, not the client: it has to survive a
client crashing mid-frame, and it must not be something a client can
resize underneath it. A dropped connection takes its surfaces and their
segments with it.

Composites into a back buffer and copies once, because compositing
straight into the framebuffer shows every intermediate state -- the
flicker would be the whole desktop being repainted under the user's eyes.

projects/ gets build glue for the first time: `make projects` builds
against the sysroot's libc and stages into sysroot/bin. It is not a
dependency of anything above it, and `make` never descends there.

Verified by `make schcom-test`, which boots SchOS, runs the compositor and
a client, screendumps the framebuffer and asserts the pixels on the host.
That last part is the point -- the compositor reporting that it
composited something proves nothing, and a screenshot nobody inspects
proves less. It checks the client's colour mid-surface, the compositor's
background, and the white border exactly at the requested origin, so
"arrived" and "arrived at the wrong offset" cannot pass for each other.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Makefile

@@ -449,6 +449,31 @@ cmd: $(CMDBINS)
 ports:
 	$(MAKE) -C ports
 
+# projects/ -- things that run *on* SchOS rather than things that *are* SchOS.
+#
+# Deliberately not a dependency of anything above: `make` never descends here,
+# and the kernel build does not know these exist. They are ordinary programs
+# built against the sysroot's libc, exactly as toybox is, and the boundary
+# between them and the OS is syscalls and protocols rather than a shared header
+# nobody meant to share. See projects/README.md.
+#
+# Staged into $(SYSROOT) so the next image build picks them up; run
+# `make projects && make` to get them onto a disk.
+PROJECTS ?= schcom
+
+projects:
+	@test -d $(MUSLPREFIX)/include || { \
+	  echo "projects: no musl in $(MUSLPREFIX) -- run 'make ports' first" >&2; \
+	  exit 1; }
+	@for p in $(PROJECTS); do \
+	  echo "==> projects/$$p"; \
+	  $(MAKE) -C projects/$$p \
+	      CC='$(PORTCC)' \
+	      CFLAGS='-O2 -static -D__SCHOS__ -I$(MUSLPREFIX)/include -fno-stack-protector' \
+	      LDFLAGS='-static -L$(MUSLPREFIX)/lib' \
+	      DESTDIR='$(abspath $(SYSROOT))' install || exit 1; \
+	done
+
 # -----------------------------------------------------------------------
 # Bootable ISO
 #
@@ -885,6 +910,15 @@ $(BUILD)/fs.img: $(MKFS) $(BUILD)/sysroot.stamp
 # it gets worse -- because stage2 loads the kernel by PIO one sector at a time,
 # and under KVM every one of those port accesses is a VM exit, where under TCG
 # it is cheap.
+# Schcom, end to end, checked from outside the machine.
+#
+# Boots, starts the compositor and a client, and then screendumps the
+# framebuffer and asserts the pixels on the host. That last part is the whole
+# value: the compositor saying "I composited it" proves nothing, and a
+# screenshot nobody inspects proves less. Needs `make projects` first.
+schcom-test: schos.img $(BUILD)/schfs.img
+	sh tools/posix/schcom_test.sh
+
 qemu-kvm: schos.img $(BUILD)/schfs.img
 	$(QEMU) -enable-kvm -serial mon:stdio $(QEMU_DISPLAY) -cpu host \
 	    -hda schos.img -hdb $(BUILD)/schfs.img \
@@ -989,7 +1023,7 @@ qrtest: $(QRTEST)
 qrcheck: $(QRTEST)
 	$(PYTHON) tools/python/qrcheck.py
 
-.PHONY: all kernel ramroot cmd ports sysroot-install qemu qemu-kvm \
+.PHONY: all kernel ramroot cmd ports projects sysroot-install qemu qemu-kvm schcom-test \
         schfs-test schfs-objtest schfs-image-test schfs-check mkschfs \
         schfs-fsck crashshot qemu-schfs iso qemu-iso clean distclean \
         logo qrtest qrcheck crashqr usb-ids