internal/system: relocate from system

These packages are highly specific to hakurei and are difficult to use safely from other pieces of code.

Their exported symbols are made available until v0.4.0 where they will be removed for #24.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/system/link.go

@@ -0,0 +1,50 @@
+package system
+
+import (
+	"fmt"
+
+	"hakurei.app/container/check"
+	"hakurei.app/hst"
+)
+
+// Link calls LinkFileType with the [Process] criteria.
+func (sys *I) Link(oldname, newname *check.Absolute) *I {
+	return sys.LinkFileType(Process, oldname, newname)
+}
+
+// LinkFileType maintains a hardlink until its [Enablement] is no longer satisfied.
+func (sys *I) LinkFileType(et hst.Enablement, oldname, newname *check.Absolute) *I {
+	sys.ops = append(sys.ops, &hardlinkOp{et, newname.String(), oldname.String()})
+	return sys
+}
+
+// hardlinkOp implements [I.LinkFileType].
+type hardlinkOp struct {
+	et       hst.Enablement
+	dst, src string
+}
+
+func (l *hardlinkOp) Type() hst.Enablement { return l.et }
+
+func (l *hardlinkOp) apply(sys *I) error {
+	sys.msg.Verbose("linking", l)
+	return newOpError("hardlink", sys.link(l.src, l.dst), false)
+}
+
+func (l *hardlinkOp) revert(sys *I, ec *Criteria) error {
+	if ec.hasType(l.Type()) {
+		sys.msg.Verbosef("removing hard link %q", l.dst)
+		return newOpError("hardlink", sys.remove(l.dst), true)
+	} else {
+		sys.msg.Verbosef("skipping hard link %q", l.dst)
+		return nil
+	}
+}
+
+func (l *hardlinkOp) Is(o Op) bool {
+	target, ok := o.(*hardlinkOp)
+	return ok && l != nil && target != nil && *l == *target
+}
+
+func (l *hardlinkOp) Path() string   { return l.src }
+func (l *hardlinkOp) String() string { return fmt.Sprintf("%q from %q", l.dst, l.src) }