Refuse build steps under a free-space floor and release workspaces, so the build modules stop filling a build node's disk (issue 1839)

mesh-build-go, mesh-build-node and mesh-build-image refuse every fetch, test
and build, and say unhealthy, while their work or cache filesystem has less
than 20 GiB free (build-agent's floor), so the builder chooses another node
instead of filling this one. Each serves build_<x>_release, removing a
workspace once its caller's last step answered; a failed fetch removes its
own; a workspace never released goes two hours after its last use (ADR 0509)
in place of a day, never while a step holds it. Pins mesh-sdk's
fix/1839-build-floor (buildstep.Floor, buildstep.Workspaces).

modules/mesh-build-node/cmd/mesh-build-node/room.go

@@ -0,0 +1,67 @@
+package main
+
+// A step's room to work and its workspace's end (novox/hq issue 1839, ADR 0509): every step is refused while the work
+// or toolchain cache filesystem is under the floor, a workspace is held while a step runs on it, removed when its caller releases
+// it, and swept unheld past buildstep.UnheldBound.
+
+import (
+	stdio "git.novox.be/novox/mesh-sdk/go"
+	"git.novox.be/novox/mesh-sdk/go/buildstep"
+)
+
+// ReleaseTool removes a workspace whose caller's steps have ended.
+const ReleaseTool = "build_node_release"
+
+// floorOf is the floor the module keeps on node: build-agent's.
+func floorOf(node string) buildstep.Floor {
+	return buildstep.Floor{Node: node, Bytes: buildstep.FreeSpaceFloor}
+}
+
+// room refuses while the work or toolchain cache filesystem is under the floor.
+func (m *Module) room() error {
+	paths := []string{m.Work}
+	if m.Toolchain != nil {
+		paths = append(paths, m.Toolchain.Paths.Cache)
+	}
+	return m.Floor.Check(paths...)
+}
+
+// spaces are the module's workspaces under Work.
+func (m *Module) spaces() *buildstep.Workspaces {
+	m.spacesOnce.Do(func() { m.workspaces = &buildstep.Workspaces{Dir: m.Work} })
+	return m.workspaces
+}
+
+// holdArg holds the workspace a step's arguments name while the step runs; nothing when they name none.
+func (m *Module) holdArg(args map[string]any) func() {
+	ws, _ := args["workspace"].(string)
+	if ws == "" {
+		return func() {}
+	}
+	return m.spaces().Hold(ws)
+}
+
+// releaseTool is build_node_release: no guard and no floor, since it only frees room.
+func (m *Module) releaseTool() stdio.Tool {
+	return stdio.Tool{Name: ReleaseTool,
+		Description: "Remove a workspace whose steps have ended (fetch, test, build): its caller says so once its last step " +
+			"answered. A workspace already gone is no error; one a running step holds is refused. A workspace never released " +
+			"is removed two hours after its last use.",
+		Input: map[string]any{"type": "object", "properties": map[string]any{
+			"workspace": map[string]any{"type": "string", "description": "the workspace id a fetch answered"},
+		}, "required": []string{"workspace"}},
+		Run: func(args map[string]any) (any, error) {
+			ws, _ := args["workspace"].(string)
+			if err := m.spaces().Release(ws); err != nil {
+				return nil, err
+			}
+			return map[string]any{"released": ws}, nil
+		}}
+}
+
+// sweepSaid is CleanWorkspaces' saying of a workspace a caller never released: a fault of the caller (ADR 0509).
+func (m *Module) sweepSaid(removed []string) {
+	if len(removed) > 0 && m.Say != nil {
+		m.Say("removed %d workspaces no caller released within %s: %v", len(removed), buildstep.UnheldBound, removed)
+	}
+}