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+From iris.proofmode Require Import base tactics classes.
+From Perennial.base_logic Require Export invariants.
+From Perennial.program_logic Require Import invariants_mutable staged_invariant_alt.
+From Perennial.base_logic.lib Require Import wsat.
+Set Default Proof Using "Type".
+Import uPred.
+
+(*
+
+  A simplified version of "crash borrows" that can essentially only opened
+  over an atomic step. Essentially, we allocate a special kind of invariant
+  that has two parameters, a mutable "P" part and an immutable "Pc" part. We
+  can think of the invariant as a non-duplicable "box" in which some P is
+  stored, with the property that P always implies Pc.
+
+  When you open the box across an atomic step, you get ▷ P. When closing the
+  box, you do not have to put ▷ P back in. Instead, you may update the mutable
+  parameter to ▷ P' and put ▷ P' in, so long as ▷ P' implies ▷ Pc.
+
+  More precisely, rather than putting in such a P' that implies Pc, we put in
+  P' ∧ Pc.
+
+  If you try to encode this on top of standard Iris invariants using saved
+  predicates, then unfortunately  when opening the box you will only get
+  ▷ ▷ P. This is a problem because even if P is timeless, ▷ P is not.  We can
+  work around this using later credits (see new/proof/own_crash.v for an
+  alternate encoding that uses this route).
+
+  Instead, we can use the mechanism of "invariant schemas" and mutable
+  invariants that Perennial supports to avoid this issue. This file uses the
+  schema that we previously developed in wp_to_wpc.v for showing a generic
+  HOCAP wp can be converted to a HOCAP wpc.
+
+*)
+
+
+
+Section own_crash_inv.
+Context `{!invGS Σ}.
+Context `{!crashGS Σ}.
+Context `{!stagedG Σ}.
+
+Definition own_crash_sch : bi_schema :=
+  (bi_sch_or (bi_sch_and (bi_sch_var_mut 0) (bi_sch_except_0 (bi_sch_fupd ∅ ∅ (bi_sch_var_fixed 0))))
+             (bi_sch_or (bi_sch_sep (bi_sch_var_fixed 2) (bi_sch_var_fixed 3))
+                        (bi_sch_var_fixed 1))).
+
+Lemma own_crash_sch_interp_weak k P Qs_mut γ :
+  bi_schema_interp (S k) (bi_later <$> [P; staged_done γ; C; staged_pending (3/4)%Qp γ])
+                   (bi_later <$> Qs_mut) own_crash_sch ⊣⊢
+                   let Qs := default emp%I (bi_later <$> (Qs_mut !! O)) in
+                      (((Qs ∧ ◇ |k,Some O={∅}=> ▷ P) ∨ ((▷ C ∗ ▷ staged_pending (3/4)%Qp γ)
+                       ∨ (▷ staged_done γ))))%I.
+Proof.
+  remember (S k) as n eqn:Heq.
+  do 2 (rewrite ?bi_schema_interp_unfold /= //=).
+  rewrite Heq.
+  erewrite (bi_sch_fupd_interp); last first.
+  { rewrite ?bi_schema_interp_unfold /= //=. }
+  rewrite list_lookup_fmap. by f_equiv.
+Qed.
+
+Lemma own_crash_sch_interp_strong k P Q γ :
+  bi_schema_interp (S k) (bi_later <$> [P; staged_done γ; C; staged_pending (3/4)%Qp γ])
+                   (bi_later <$> [Q]) own_crash_sch ⊣⊢
+                      (((▷ Q ∧ ◇ |k,Some O={∅}=> ▷ P) ∨ ((▷ C ∗ ▷ staged_pending (3/4)%Qp γ)
+                       ∨ (▷ staged_done γ))))%I.
+Proof.
+  remember (S k) as n eqn:Heq.
+  do 2 (rewrite ?bi_schema_interp_unfold /= //=).
+  rewrite Heq.
+  erewrite (bi_sch_fupd_interp); last first.
+  { rewrite ?bi_schema_interp_unfold /= //=. }
+  do 2 (rewrite ?bi_schema_interp_unfold /= //=).
+Qed.
+
+Definition own_crash_redeem `{!invGS Σ} N Pc : iProp Σ :=
+  ∃ γ, inv_mut 1 N own_crash_sch [Pc; staged_done γ; C; staged_pending (3/4)%Qp γ] ∗
+       staged_pending (3/4)%Qp γ.
+
+Definition own_crash `{!invGS Σ} N P Pc : iProp Σ :=
+  ∃ γ, inv_mut 1 N own_crash_sch [Pc; staged_done γ; C; staged_pending (3/4)%Qp γ] ∗
+       inv_mut_full 1 N own_crash_sch [P] [Pc; staged_done γ; C; staged_pending (3/4)%Qp γ] ∗
+       staged_pending (1/4)%Qp γ.
+
+Lemma own_crash_redeem_cfupd N E Pc :
+  ↑N ⊆ E →
+  own_crash_redeem N Pc -∗ |C={E}=> ▷ Pc.
+Proof.
+  iIntros (?) "H".
+  iDestruct "H" as (γ) "(#Hinv&Hpend34)".
+  iMod (inv_mut_acc with "Hinv") as (Qs) "(H&Hclo)"; first auto.
+  iIntros "HC".
+  rewrite own_crash_sch_interp_weak /=.
+  iDestruct "H" as "[(_&>H)|[Hfalse1|Hfalse2]]".
+  * iApply (fupd_level_fupd _ _ _ (S 0)).
+    iEval (rewrite uPred_fupd_level_eq /uPred_fupd_level_def).
+    iMod (fupd_split_level_intro_mask' _ ∅) as "Hclo''"; first by set_solver+.
+    iMod (fupd_split_level_le with "H") as "H"; first lia.
+    iMod "Hclo''".
+    iSpecialize ("Hclo" with "[HC Hpend34]").
+    { iRight. iLeft. iFrame. }
+    iEval (rewrite uPred_fupd_level_eq /uPred_fupd_level_def) in "Hclo".
+    iMod "Hclo". iFrame "H"; done.
+  * iDestruct "Hfalse1" as "(>_&>?)". iDestruct (pending34_pending34 with "[$] [$]") as "[]".
+  * iDestruct "Hfalse2" as ">?". iDestruct (pending_done with "[$] [$]") as "[]".
+Qed.
+
+Lemma own_crash_alloc_weak N E P Pc :
+  ▷ P ∧ ▷ Pc -∗ |={E}=> own_crash_redeem N Pc ∗ own_crash N P Pc.
+Proof.
+  iIntros "HP".
+  iMod (pending_alloc) as (γpending) "Hpending".
+  iMod (inv_mut_alloc (S 0) N _ own_crash_sch ([ Pc; staged_done γpending; C; staged_pending (3/4)%Qp γpending])%I [P] with "[HP]") as "(#Hinv&Hfull)".
+  {  rewrite own_crash_sch_interp_strong.
+     iLeft. iSplit.
+     - iDestruct "HP" as "($&_)".
+     - iModIntro. iModIntro. iNext. iDestruct "HP" as "(_&$)".
+  }
+  iDestruct (pending_split34 with "Hpending") as "(Hpend34&Hpend14)".
+  iModIntro.
+  iSplitR "Hfull Hpend14".
+  - iExists γpending. rewrite /own_crash_redeem. iFrame. iExact "Hinv".
+  - iExists γpending. rewrite /own_crash. iFrame. iExact "Hinv".
+Qed.
+
+Lemma own_crash_open N E P Pc :
+  ↑N ⊆ E →
+  own_crash N P Pc -∗
+  |NC={E, E ∖ ↑N}=> ▷ P ∗ ∀ P', ▷ P' ∧ ▷ Pc -∗ |={E ∖ ↑N, E}=> own_crash N P' Pc.
+Proof.
+  iIntros (Hin) "H".
+  iDestruct "H" as (γ) "(#Hinv&Hown&Hpend)".
+  rewrite /own_crash.
+  rewrite ncfupd_eq /ncfupd_def. iIntros (q) "HNC".
+  iMod (inv_mut_full_acc with "Hown") as "(H&Hclo)"; first auto.
+  rewrite ?own_crash_sch_interp_strong.
+  iDestruct "H" as "[HQ|Hfalse]"; last first.
+  { iDestruct "Hfalse" as "[(>Hfalse&_)|>Hfalse]".
+    * iDestruct (NC_C with "[$] [$]") as "[]".
+    * iDestruct (pending_done with "[$] [$]") as "[]".
+  }
+  iDestruct "HQ" as "(HQ&_)".
+  iModIntro.
+  iFrame "HQ HNC".
+  iIntros (P') "HP'".
+  iMod ("Hclo" $! [P'] with "[HP']") as "Hfull".
+  { rewrite own_crash_sch_interp_strong. iLeft. iSplit; first auto.
+    - iDestruct "HP'" as "($&_)".
+    - iModIntro. iModIntro. iDestruct "HP'" as "(_&$)".
+  }
+  iModIntro. iFrame. iFrame "Hinv".
+Qed.
+
+End own_crash_inv.
+
+
+Section wpc.
+Context `{!irisGS Λ Σ, !generationGS Λ Σ, stagedG Σ}.
+
+(* Demo of how allocating a own_crash gives you ▷ of the "crash invariant" in the postcondition *)
+
+Lemma wpc_own_crash_alloc_weak s E N (P Pc : iProp Σ) (e : expr Λ) Φ Φc :
+  ↑N ⊆ E →
+  ▷ P ∧ ▷ Pc -∗
+  (own_crash N P Pc -∗ WPC e @ s ; E {{ Φ }} {{ ▷ Pc -∗ Φc }}) -∗
+  WPC e @ s; E {{ Φ }} {{ Φc }}.
+Proof.
+  iIntros (?) "HP Hwp".
+  iMod (own_crash_alloc_weak N E P Pc with "[$]") as "(Hredeem&H)".
+  iSpecialize ("Hwp" with "[$]").
+  iApply (wpc_strong_mono with "Hwp"); auto.
+  iSplit.
+  { eauto. }
+  { iIntros "Hpc".
+    iMod (own_crash_redeem_cfupd with "Hredeem"); first by auto.
+    iModIntro. iApply "Hpc". auto.
+  }
+Qed.
+End wpc.