minerva 0.2.0

Causal ordering for distributed systems
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210

extern crate alloc;

use alloc::collections::BTreeSet;

use super::super::super::super::support::dot;
use crate::metis::{Dot, DotSet, DotStore};

/// A store paired with a context that covers every store dot.
#[derive(Clone, Debug)]
pub(super) struct Covered<S> {
    store: S,
    context: DotSet,
}

impl<S: DotStore> Covered<S> {
    pub(super) fn new(store: S, context: DotSet) -> Self {
        let covered = Self { store, context };
        covered.assert_covered();
        covered
    }

    pub(super) fn store(&self) -> &S {
        &self.store
    }

    pub(super) fn context(&self) -> &DotSet {
        &self.context
    }

    fn assert_covered(&self) {
        for held in self.store.dots() {
            let (station, counter) = (held.station(), held.counter());
            assert!(
                self.context.contains(held),
                "harness input is uncovered at ({station}, {counter})"
            );
        }
    }
}

/// A dot survives causal merge iff both stores hold it, or one holds it and the
/// other side's context has never seen it.
fn check_survivor_law<S: DotStore>(a: &Covered<S>, b: &Covered<S>) {
    let merged = a.store.causal_merge(&a.context, &b.store, &b.context);
    let a_dots: BTreeSet<Dot> = a.store.dots().collect();
    let b_dots: BTreeSet<Dot> = b.store.dots().collect();
    let merged_dots: BTreeSet<Dot> = merged.dots().collect();
    for station in 0u32..4 {
        for counter in 1u64..9 {
            let probe = dot(station, counter);
            let in_a = a_dots.contains(&probe);
            let in_b = b_dots.contains(&probe);
            let survives = (in_a && (in_b || !b.context.contains(probe)))
                || (in_b && (in_a || !a.context.contains(probe)));
            assert_eq!(
                merged_dots.contains(&probe),
                survives,
                "survivor law disagrees at ({station}, {counter})"
            );
        }
    }
}

fn check_commutative<S: DotStore + PartialEq + core::fmt::Debug>(a: &Covered<S>, b: &Covered<S>) {
    let ab = a.store.causal_merge(&a.context, &b.store, &b.context);
    let ba = b.store.causal_merge(&b.context, &a.store, &a.context);
    assert_eq!(ab, ba, "causal_merge is not commutative");
}

fn check_associative<S: DotStore + PartialEq + core::fmt::Debug>(
    a: &Covered<S>,
    b: &Covered<S>,
    c: &Covered<S>,
) {
    let bc = b.store.causal_merge(&b.context, &c.store, &c.context);
    let bc_ctx = b.context.merge(&c.context);
    let left = a.store.causal_merge(&a.context, &bc, &bc_ctx);

    let ab = a.store.causal_merge(&a.context, &b.store, &b.context);
    let ab_ctx = a.context.merge(&b.context);
    let right = ab.causal_merge(&ab_ctx, &c.store, &c.context);

    assert_eq!(left, right, "causal_merge is not associative");
}

fn check_idempotent<S: DotStore + Clone + PartialEq + core::fmt::Debug>(a: &Covered<S>) {
    let merged = a.store.causal_merge(&a.context, &a.store, &a.context);
    assert_eq!(merged, a.store, "causal_merge is not idempotent");
}

/// The in-place fold agrees with the pure merge on every covered input: the
/// one law `causal_merge_from` carries (S182). An override is a cost seam,
/// never a semantic choice, and this check holds the default body and every
/// override to the same output.
fn check_in_place_agrees<S: DotStore + Clone + PartialEq + core::fmt::Debug>(
    a: &Covered<S>,
    b: &Covered<S>,
) {
    let pure = a.store.causal_merge(&a.context, &b.store, &b.context);
    let mut folded = a.store.clone();
    folded.causal_merge_from(&a.context, &b.store, &b.context);
    assert_eq!(
        folded, pure,
        "causal_merge_from disagrees with causal_merge"
    );
}

fn check_bottom_identity<S: DotStore + Clone + PartialEq + core::fmt::Debug>(a: &Covered<S>) {
    let bottom = S::default();
    let empty = DotSet::new();
    let right = a.store.causal_merge(&a.context, &bottom, &empty);
    assert_eq!(right, a.store, "bottom is not a right identity");
    let left = bottom.causal_merge(&empty, &a.store, &a.context);
    assert_eq!(left, a.store, "bottom is not a left identity");
}

/// Bottom carries no state at all, so it carries no support; the converse is
/// instance-specific (a recording plane keeps a store non-bottom with empty
/// support, the revised law 1 wording, S190), so only this direction is a
/// generic law.
fn check_bottom_is_support_empty<S: DotStore>(a: &Covered<S>) {
    assert!(
        S::default().is_bottom(),
        "the default store is not the bottom"
    );
    if a.store.is_bottom() {
        assert!(
            a.store.dots().next().is_none(),
            "a bottom store must carry no support"
        );
    }
}

/// `support()` is the have-set fold of `dots()`: the compact form and the
/// enumeration agree, for the default body and every override alike (the
/// arc-10 survivor half's one law).
fn check_support_is_the_dots_fold<S: DotStore>(a: &Covered<S>) {
    let support = a.store.support();
    let want: BTreeSet<Dot> = a.store.dots().collect();
    let got: BTreeSet<Dot> = support.dots().collect();
    assert_eq!(got, want, "support disagrees with the dots enumeration");
}

/// The witnessed selection's two generic laws: a bottom witness licenses no
/// sub-selection (the read equals the bare `novel_to`), and no witness moves
/// SUPPORT selection (the witness reaches recording planes only).
fn check_witnessed_selection<S: DotStore + Clone + PartialEq + core::fmt::Debug>(
    a: &Covered<S>,
    b: &Covered<S>,
) {
    let bare = a.store.novel_to(b.context());
    let bottomed = a.store.novel_to_witnessed(b.context(), &DotSet::new());
    assert_eq!(bottomed, bare, "a bottom witness must equal the bare read");

    let witnessed = a.store.novel_to_witnessed(b.context(), &a.store.support());
    let want: BTreeSet<Dot> = bare.dots().collect();
    let got: BTreeSet<Dot> = witnessed.dots().collect();
    assert_eq!(got, want, "a witness must not move support selection");
}

/// Restriction is fiber selection and commutes with causal merge under
/// restricted contexts.
fn check_restriction<S: DotStore + Clone + PartialEq + core::fmt::Debug>(
    a: &Covered<S>,
    b: &Covered<S>,
    roster: &[u32],
) {
    let kept = a.store.restrict(roster.iter().copied());
    let roster_set: BTreeSet<u32> = roster.iter().copied().collect();
    let want: BTreeSet<Dot> = a
        .store
        .dots()
        .filter(|held| roster_set.contains(&held.station()))
        .collect();
    let got: BTreeSet<Dot> = kept.dots().collect();
    assert_eq!(got, want, "restrict is not fiber selection");

    let merged = a.store.causal_merge(&a.context, &b.store, &b.context);
    let restrict_then_merge = a.store.restrict(roster.iter().copied()).causal_merge(
        &a.context.restrict(roster.iter().copied()),
        &b.store.restrict(roster.iter().copied()),
        &b.context.restrict(roster.iter().copied()),
    );
    assert_eq!(
        merged.restrict(roster.iter().copied()),
        restrict_then_merge,
        "restrict does not commute with causal_merge"
    );
}

pub(super) fn check_all_laws<S: DotStore + Clone + PartialEq + core::fmt::Debug>(
    a: &Covered<S>,
    b: &Covered<S>,
    c: &Covered<S>,
    roster: &[u32],
) {
    a.assert_covered();
    b.assert_covered();
    c.assert_covered();
    check_survivor_law(a, b);
    check_commutative(a, b);
    check_associative(a, b, c);
    check_idempotent(a);
    check_bottom_identity(a);
    check_bottom_is_support_empty(a);
    check_in_place_agrees(a, b);
    check_support_is_the_dots_fold(a);
    check_witnessed_selection(a, b);
    check_restriction(a, b, roster);
}