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//! release_assert: a guard against `std.debug.assert` used as input validation.
//!
//! `std.debug.assert` is the right tool for an *internal invariant* — a fact the
//! function itself established, held to as a crash-on-violation. It is the wrong
//! tool for a rule about data the function did not produce, and it is wrong in
//! two different ways depending on how the binary was built. Neither of them is
//! "the input gets rejected", which is what the code reads as if it does.
//!
//! Under ReleaseSafe — which build.zig now makes the default for `--release`,
//! pinned by `src/ci_test.zig`'s "the shipped release mode keeps assertions
//! alive" — the assert survives and **panics**. On the write path that is a good
//! trade and deliberately chosen: a crash beats a durable bad frame in an
//! append-only store. On a *peer-reachable* path it is not a trade at all, it is
//! a remote denial of service — anyone who can hand this process a malformed
//! card can halt it. A parser reached from the sync wire must reject its input,
//! not assert about it, however safe the build mode is.
//!
//! Under ReleaseFast or ReleaseSmall the assert compiles to nothing and the
//! malformed input flows straight through into the work the assert was meant to
//! stop. The ReleaseSafe pin makes this the less likely build, not an impossible
//! one: it sets a default, not a ceiling, and `--release=fast` still selects it
//! deliberately. A guard that only holds while nobody passes a flag is not a
//! guard.
//!
//! So the same finding names a DoS in the default build and a missing check in a
//! fast one, and the same one-line fix — `return error.X` — closes both.
//!
//! Three defects of exactly this shape were found and fixed recently:
//!
//!   * `putShun` asserted a bound on caller-supplied bytes and, with the assert
//!     compiled out, wrote a durable frame that could never be opened again.
//!   * `ShunSet.contains` asserted a card's shape and crashed on a peer's
//!     malformed card instead of rejecting it.
//!   * `gatherCandidates` asserted a caller-supplied bound and, without it, left
//!     a scan unbounded.
//!
//! One of each: `ShunSet.contains` is the panic-on-a-peer's-input shape that
//! ReleaseSafe does nothing to help, and the other two are the compiled-out
//! shape. All three were `pub fn`s that already returned an error union — the
//! correct fix in each case was `return error.X` rather than `assert`. This
//! guard exists so the fourth one is caught by `zig build test` instead of by a
//! user or a peer.
//!
//! What it flags — all of these must hold at once:
//!
//!   1. the enclosing function is `pub fn` (it is reachable from outside),
//!   2. its return type is an error union (`!` or a named `…Error…` type), so
//!      rejecting the input costs nothing structurally,
//!   3. the assert expression names one of that function's own parameters
//!      (`self`, `gpa`, `io`, `allocator`, `arena` are conventional plumbing and
//!      do not count),
//!   4. that parameter is *untrusted*: a slice type, or a name from the
//!      `untrusted_names` list below, and
//!   5. the assert is NOT already dominated by a `return` on that parameter.
//!
//! Rules 3 and 5 are what make this signal rather than noise, and `container.zig`
//! is the worked example of each — it holds exactly two asserts and neither is a
//! finding, for two different correct reasons:
//!
//!   * `encode`'s assert names the parameter `bytes`, but the function opens with
//!     `if (bytes.len > …) return error.InputTooLarge`. The rejection already
//!     happened; the assert restates an established fact. Rule 5 clears it.
//!   * `decode`'s assert names only locals (`out`, `total`, `declared`). It is a
//!     postcondition about the function's own output, not a rule about input.
//!     Rule 3 clears it.
//!
//! `delta.applyCopy` is the same shape as the first (and is private besides, so
//! rule 1 excludes it too). Because those zeros are load-bearing — a parser that
//! silently stopped seeing asserts would also produce them — the test
//! "container.zig's two asserts are attributed and correctly cleared" below pins
//! the attribution directly rather than trusting the absence.
//!
//! The baseline is keyed on **(file, function name)** with a COUNT, deliberately
//! not on line numbers: line numbers churn on every edit above them and would
//! turn this into a maintenance tax. The count is what stops a function that
//! already asserts badly — the likeliest place for the next bad assert — from
//! absorbing a new one for free. A count that rises fails; a count that falls
//! also fails, so the list can only shrink.
//!
//! The parse is driven by the Zig tokenizer, so an `assert` written inside a
//! comment or a string literal is not read as a call, and a `{` inside one
//! cannot skew a function's body extent. It is still a heuristic, not semantic
//! analysis: it does not resolve aliases (`const assert = std.debug.assert`) or
//! types. That is a knowing trade — the shape it does catch is the shape that
//! produced all three defects.

const std = @import("std");

/// The repository root, wired in build.zig's `build_unit_test_module`. A test
/// binary's working directory is not the checkout, so the src tree is opened by
/// this absolute path. Same handle src/tidy.zig and src/packaging_test.zig use.
const repo_options = @import("repo_options");

/// Parameter names that are plumbing, not data: an assert on one of these is
/// about the call's own wiring, not about untrusted content.
const plumbing_names = [_][]const u8{ "self", "gpa", "io", "allocator", "arena" };

/// Substrings that mark a parameter as carrying data the function did not
/// produce. A slice-typed parameter is untrusted regardless of its name; these
/// catch the non-slice carriers (a length, an index, a scalar id read off the
/// wire) whose names say where the value came from.
const untrusted_names = [_][]const u8{
    "hash",   "name",   "card",     "payload",  "bytes",
    "buf",    "slice",  "priority", "deltasrc", "token",
    "reason", "digest", "wire",     "input",
};

/// Files not scanned. Test and simulation sources assert about fixtures they
/// themselves built — the parameter is trusted by construction — and the three
/// harness roots (`e2e`, `smoke`, `dst`) are drivers, not library surface.
const excluded_prefixes = [_][]const u8{"sim/"};
const excluded_files = [_][]const u8{ "e2e.zig", "smoke.zig", "dst.zig" };

/// Vacuity floor: the walk must find at least this many scannable `.zig` files
/// or the tree was not actually scanned and every check below would pass by
/// finding nothing. ~45 files survive the exclusions today; 30 is a floor with
/// headroom, not a running count to keep in sync.
const min_expected_files = 30;

/// Read cap per source file. The largest src file is well under 1 MiB; 8 MiB is
/// a tripwire against a runaway read, not an expected size.
const max_source_bytes = 8 << 20;

/// One baselined function and how many undominated asserts on untrusted input it
/// held when the guard landed.
const Baseline = struct { file: []const u8, function: []const u8, count: usize };

/// The pre-existing population, as measured by this guard's own parse. Each
/// entry is a `pub fn` returning an error union that asserts on untrusted input
/// without a dominating `return` — an input path that panics where it should
/// reject (and, in a `--release=fast` build, does not check at all).
///
/// These are recorded rather than fixed in one sweep because each fix is an API
/// decision (which error, and what every caller does with it). The 29 entries in
/// `sync.zig` are the ones to fix first: they are wire-card parsers, so under the
/// ReleaseSafe default they are live remote-DoS surface rather than a latent
/// fast-build hole.
///
/// Counts may only fall. Raising one is not a fix — see issue #19.
const baseline = [_]Baseline{
    .{ .file = "annotate.zig", .function = "attribute", .count = 1 },
    .{ .file = "artifact.zig", .function = "resolvePrefix", .count = 2 },
    .{ .file = "checkout.zig", .function = "read", .count = 1 },
    .{ .file = "cli.zig", .function = "composeAmendTags", .count = 1 },
    .{ .file = "cli.zig", .function = "composeTagAddCards", .count = 1 },
    .{ .file = "delta.zig", .function = "create", .count = 2 },
    .{ .file = "logstore.zig", .function = "intern", .count = 1 },
    .{ .file = "logstore.zig", .function = "pickBase", .count = 2 },
    .{ .file = "logstore.zig", .function = "put", .count = 1 },
    .{ .file = "logstore.zig", .function = "putArtifact", .count = 2 },
    .{ .file = "logstore.zig", .function = "putArtifactPacked", .count = 2 },
    .{ .file = "logstore.zig", .function = "putContainer", .count = 1 },
    .{ .file = "logstore.zig", .function = "putDelta", .count = 1 },
    .{ .file = "logstore.zig", .function = "putDeltaContainer", .count = 1 },
    .{ .file = "logstore.zig", .function = "putZlib", .count = 1 },
    .{ .file = "logstore.zig", .function = "resolveHexPrefix", .count = 1 },
    .{ .file = "main.zig", .function = "mintControlArtifact", .count = 1 },
    .{ .file = "repoimage.zig", .function = "serialize", .count = 1 },
    .{ .file = "repostate.zig", .function = "bisectSpan", .count = 2 },
    .{ .file = "repostate.zig", .function = "currentTrunk", .count = 1 },
    .{ .file = "repostate.zig", .function = "ingestControlTags", .count = 1 },
    .{ .file = "repostate.zig", .function = "openLeafOf", .count = 2 },
    .{ .file = "repostate.zig", .function = "openLeavesOf", .count = 2 },
    .{ .file = "repostate.zig", .function = "putWire", .count = 1 },
    .{ .file = "repostate.zig", .function = "putWireDelta", .count = 2 },
    .{ .file = "repostate.zig", .function = "resolve", .count = 1 },
    .{ .file = "repostate.zig", .function = "resolveName", .count = 1 },
    .{ .file = "repoview.zig", .function = "buildFromImage", .count = 1 },
    .{ .file = "repoview.zig", .function = "buildFromRecordBody", .count = 1 },
    .{ .file = "sync.zig", .function = "apply", .count = 2 },
    .{ .file = "sync.zig", .function = "consumeConfigPayload", .count = 1 },
    .{ .file = "sync.zig", .function = "consumeContentPayload", .count = 1 },
    .{ .file = "sync.zig", .function = "consumeReply", .count = 1 },
    .{ .file = "sync.zig", .function = "consumeUvfilePayload", .count = 1 },
    .{ .file = "sync.zig", .function = "fossilUnquote", .count = 1 },
    .{ .file = "sync.zig", .function = "parseCfileCard", .count = 3 },
    .{ .file = "sync.zig", .function = "parseConfigCard", .count = 2 },
    .{ .file = "sync.zig", .function = "parseFileCard", .count = 3 },
    .{ .file = "sync.zig", .function = "parseGimmeCard", .count = 2 },
    .{ .file = "sync.zig", .function = "parseIgotCard", .count = 2 },
    .{ .file = "sync.zig", .function = "parseUvfileCard", .count = 2 },
    .{ .file = "sync.zig", .function = "parseUvgimmeCard", .count = 2 },
    .{ .file = "sync.zig", .function = "parseUvigotCard", .count = 2 },
    .{ .file = "sync.zig", .function = "readLine", .count = 2 },
    .{ .file = "sync.zig", .function = "recordPeerHas", .count = 1 },
    .{ .file = "sync.zig", .function = "recordPeerLacks", .count = 1 },
    .{ .file = "sync.zig", .function = "writeCiLock", .count = 3 },
    .{ .file = "sync.zig", .function = "writeCiLockFail", .count = 2 },
    .{ .file = "sync.zig", .function = "writeCiUnlock", .count = 2 },
    .{ .file = "sync.zig", .function = "writeClientVersion", .count = 1 },
    .{ .file = "sync.zig", .function = "writeCloneCard", .count = 1 },
    .{ .file = "sync.zig", .function = "writeConfigCard", .count = 2 },
    .{ .file = "sync.zig", .function = "writeFlagPragma", .count = 2 },
    .{ .file = "sync.zig", .function = "writeLogin", .count = 3 },
    .{ .file = "sync.zig", .function = "writePushPullCard", .count = 3 },
    .{ .file = "sync.zig", .function = "writeReqconfig", .count = 3 },
    .{ .file = "sync.zig", .function = "writeScalarConfig", .count = 1 },
    .{ .file = "sync.zig", .function = "writeServerVersion", .count = 1 },
    .{ .file = "undo.zig", .function = "record", .count = 1 },
};

test "no new std.debug.assert guards a release-build rule on untrusted input" {
    const io = std.testing.io;

    var arena_state = std.heap.ArenaAllocator.init(std.testing.allocator);
    defer arena_state.deinit();
    const arena = arena_state.allocator();

    const src_abs = try std.fmt.allocPrint(arena, "{s}/src", .{repo_options.root});

    var src_dir = std.Io.Dir.cwd().openDir(io, src_abs, .{ .iterate = true }) catch |err| switch (err) {
        // The only legitimate skip: the wired root is not a directory here, so
        // the machine cannot answer the question. Anything else is a real fault.
        error.FileNotFound, error.NotDir => return error.SkipZigTest,
        else => {
            std.debug.print("\nrelease_assert: cannot open {s}: {s}\n", .{ src_abs, @errorName(err) });
            return error.ReleaseAssertWalkFailed;
        },
    };
    defer src_dir.close(io);

    var files: std.ArrayList([]const u8) = .empty;
    {
        var walker = try src_dir.walk(arena);
        defer walker.deinit();
        while (try walker.next(io)) |entry| {
            if (entry.kind != .file) continue;
            if (!std.mem.endsWith(u8, entry.path, ".zig")) continue;
            if (isExcluded(entry.path)) continue;
            try files.append(arena, try arena.dupe(u8, entry.path));
        }
    }
    std.mem.sort([]const u8, files.items, {}, lessThanPath);

    if (files.items.len < min_expected_files) {
        std.debug.print("\nrelease_assert: walked {s} and found only {d} scannable .zig files; " ++
            "expected at least {d}. The walk is not seeing the source tree — this guard " ++
            "would pass vacuously.\n", .{ src_abs, files.items.len, min_expected_files });
        return error.ReleaseAssertWalkFailed;
    }

    var scan: Scan = .empty;
    for (files.items) |file| {
        const source = try src_dir.readFileAllocOptions(io, file, arena, .limited(max_source_bytes), .of(u8), 0);
        try scanFile(arena, file, source, &scan);
    }

    var failed = false;

    // Forward direction: more asserts in a function than the baseline records.
    // A brand-new offender in a clean function and a second offender bolted onto
    // an already-listed one are the same failure and read the same way.
    for (scan.entries.values()) |entry| {
        const recorded = baselineCount(entry.file, entry.function);
        if (entry.sites.items.len <= recorded) continue;
        failed = true;
        std.debug.print(
            \\
            \\release_assert: src/{s}: `{s}` has {d} assert(s) on untrusted input; the baseline records {d}.
            \\
        , .{ entry.file, entry.function, entry.sites.items.len, recorded });
        for (entry.sites.items) |site| {
            std.debug.print("      line {d} (`{s}`): {s}\n", .{ site.line, site.param, site.source_line });
        }
        std.debug.print(
            \\      An assert is not a rejection. Under the default ReleaseSafe build these
            \\      panic, so if that data can reach `{s}` from a peer or a repository file,
            \\      that is a remote denial of service. Under an explicitly chosen
            \\      `--release=fast` build they compile to nothing and the data flows through
            \\      unvalidated instead. One fix closes both: `{s}` already returns an error
            \\      union — return an error.
            \\          if (!condition) return error.SomethingSpecific;
            \\      If the fact really is an invariant this function established, establish it
            \\      first (a preceding `return` on that parameter) and the assert then restates
            \\      it, which this guard accepts.
            \\      Every site in the function is listed above, not just the new one — the
            \\      baseline records a count, not positions. See issue #19.
            \\
        , .{ entry.function, entry.function });
    }

    // Reverse direction: fewer than recorded means someone fixed one. Say so as
    // progress, and require the number to come down so the list can only shrink.
    var key_buffer: [key_max]u8 = undefined;
    for (baseline) |entry| {
        const actual = if (scan.entries.get(entryKey(&key_buffer, entry.file, entry.function))) |found|
            found.sites.items.len
        else
            0;
        if (actual >= entry.count) continue;
        failed = true;
        if (actual == 0) {
            std.debug.print(
                \\
                \\release_assert: the baseline lists src/{s} `{s}` with {d} assert(s), but it now
                \\      has none. That is one fewer input path that panics instead of rejecting —
                \\      remove the entry from the baseline in src/release_assert_test.zig so the
                \\      list keeps shrinking.
                \\      (If instead the function was renamed or moved, update the entry to match.)
                \\
            , .{ entry.file, entry.function, entry.count });
        } else {
            std.debug.print(
                \\
                \\release_assert: the baseline lists src/{s} `{s}` with {d} assert(s), but it now
                \\      has {d}. That is progress — lower the count in the baseline in
                \\      src/release_assert_test.zig so it keeps shrinking.
                \\
            , .{ entry.file, entry.function, entry.count, actual });
        }
    }

    if (failed) return error.ReleaseAssertOnUntrustedInput;
}

/// A source fixture with a known answer, so the parser's attribution is pinned by
/// something that cannot drift underneath it. Every shape here previously had to
/// be reasoned about by hand; now it is executable.
const fixture =
    \\const std = @import("std");
    \\
    \\pub fn flagged(card: []const u8) !void {
    \\    std.debug.assert(card.len > 4);
    \\}
    \\
    \\pub fn cleared(card: []const u8) !void {
    \\    if (card.len < 4) return error.Short;
    \\    std.debug.assert(card.len > 4);
    \\}
    \\
    \\pub fn bracesInText(payload: []const u8) !void {
    \\    // an unbalanced brace in a comment: {
    \\    const s = "an unbalanced brace in a string: {";
    \\    _ = s;
    \\    switch (payload.len) {
    \\        0 => {},
    \\        else => {},
    \\    }
    \\    errdefer {
    \\        _ = payload;
    \\    }
    \\    std.debug.assert(payload.len > 1);
    \\}
    \\
    \\pub fn outer(bytes: []const u8) !void {
    \\    const Inner = struct {
    \\        pub fn nested(digest: []const u8) !void {
    \\            std.debug.assert(digest.len == 32);
    \\        }
    \\    };
    \\    try Inner.nested(bytes);
    \\    std.debug.assert(bytes.len != 0);
    \\}
    \\
    \\pub fn notAnErrorUnion(buf: []const u8) void {
    \\    std.debug.assert(buf.len > 0);
    \\}
    \\
    \\fn notPublic(buf: []const u8) !void {
    \\    std.debug.assert(buf.len > 0);
    \\}
    \\
    \\pub fn onlyLocals(wire: []const u8) !void {
    \\    const n = wire.len;
    \\    const doubled = n * 2;
    \\    std.debug.assert(doubled >= 0);
    \\}
    \\
    \\pub fn twoAsserts(hash: []const u8) !void {
    \\    std.debug.assert(hash.len == 40);
    \\    std.debug.assert(hash[0] != 0);
    \\}
;

test "the scanner attributes asserts to the right function" {
    var arena_state = std.heap.ArenaAllocator.init(std.testing.allocator);
    defer arena_state.deinit();
    const arena = arena_state.allocator();

    var scan: Scan = .empty;
    try scanFile(arena, "fixture.zig", fixture, &scan);

    // Vacuity: every `std.debug.assert` in the fixture must have been seen and
    // attributed, or the filters below would be judging an empty set.
    try std.testing.expectEqual(@as(usize, 10), scan.asserts_seen);

    // Attribution, independent of whether the assert is a finding. `nested` is
    // the inner `pub fn` of a struct declared inside `outer`, so it exercises the
    // innermost-span rule; `notPublic` has no enclosing *candidate* and resolves
    // to "".
    const expected_owners = [_][]const u8{
        "flagged",    "cleared",    "bracesInText", "nested",
        "outer",      "",           "",             "onlyLocals",
        "twoAsserts", "twoAsserts",
    };
    try std.testing.expectEqual(expected_owners.len, scan.attributions.items.len);
    for (expected_owners, scan.attributions.items) |want, got| {
        try std.testing.expectEqualStrings(want, got.function);
        // Self-check the line number rather than hand-counting it: the line the
        // scanner reported must itself hold the assert.
        try std.testing.expect(std.mem.indexOf(u8, nthLine(fixture, got.line), "std.debug.assert") != null);
    }

    // Findings, with counts. `cleared` is dominated (rule 5), `notAnErrorUnion`
    // and `notPublic` fail rules 2 and 1, and `onlyLocals` names no parameter
    // (rule 3) even though `wire` is in scope and untrusted.
    const expected = [_]struct { function: []const u8, count: usize }{
        .{ .function = "flagged", .count = 1 },
        .{ .function = "bracesInText", .count = 1 },
        .{ .function = "nested", .count = 1 },
        .{ .function = "outer", .count = 1 },
        .{ .function = "twoAsserts", .count = 2 },
    };
    try std.testing.expectEqual(expected.len, scan.entries.count());
    var key_buffer: [key_max]u8 = undefined;
    for (expected) |want| {
        const got = scan.entries.get(entryKey(&key_buffer, "fixture.zig", want.function)) orelse {
            std.debug.print("\nrelease_assert fixture: expected a finding in `{s}`, got none\n", .{want.function});
            return error.FixtureMismatch;
        };
        try std.testing.expectEqual(want.count, got.sites.items.len);
    }
}

test "container.zig's two asserts are attributed and correctly cleared" {
    const io = std.testing.io;

    var arena_state = std.heap.ArenaAllocator.init(std.testing.allocator);
    defer arena_state.deinit();
    const arena = arena_state.allocator();

    const path = try std.fmt.allocPrint(arena, "{s}/src/container.zig", .{repo_options.root});
    const source = std.Io.Dir.cwd().readFileAllocOptions(io, path, arena, .limited(max_source_bytes), .of(u8), 0) catch |err| switch (err) {
        error.FileNotFound => return error.SkipZigTest,
        else => return err,
    };

    var scan: Scan = .empty;
    try scanFile(arena, "container.zig", source, &scan);

    // The file holds exactly two asserts, one per public function. Pinning the
    // attribution is the point: the zero findings below are only meaningful if
    // the scanner actually resolved both, rather than losing them to a body-extent
    // bug and reporting nothing.
    try std.testing.expectEqual(@as(usize, 2), scan.asserts_seen);
    try std.testing.expectEqual(@as(usize, 2), scan.attributions.items.len);
    try std.testing.expectEqualStrings("encode", scan.attributions.items[0].function);
    try std.testing.expectEqualStrings("decode", scan.attributions.items[1].function);

    // Neither is a finding: `encode`'s is dominated by its opening
    // `return error.InputTooLarge` on `bytes` (rule 5), and `decode`'s names only
    // locals (rule 3). See the module doc.
    try std.testing.expectEqual(@as(usize, 0), scan.entries.count());
}

/// The `line`-th line of `source`, 1-indexed.
fn nthLine(source: []const u8, line: usize) []const u8 {
    var it = std.mem.splitScalar(u8, source, '\n');
    var n: usize = 1;
    while (it.next()) |text| : (n += 1) {
        if (n == line) return text;
    }
    return "";
}

/// One assert that is a finding.
const Site = struct {
    line: usize,
    param: []const u8,
    source_line: []const u8,
};

/// All findings for one (file, function) pair.
const Entry = struct {
    file: []const u8,
    function: []const u8,
    sites: std.ArrayList(Site),
};

/// Every assert the scanner resolved, finding or not. Kept so the fixture tests
/// can pin attribution independently of the filters, and so a parser that stopped
/// seeing asserts cannot masquerade as a clean tree.
const Attribution = struct { line: usize, function: []const u8 };

const Scan = struct {
    entries: std.StringArrayHashMapUnmanaged(Entry),
    attributions: std.ArrayList(Attribution),
    asserts_seen: usize,

    const empty: Scan = .{ .entries = .empty, .attributions = .empty, .asserts_seen = 0 };
};

/// The map key for one (file, function) pair, written into a caller-owned
/// buffer. The map compares keys by bytes, so a lookup key only has to outlive
/// the call; `scanFile` dupes into the arena on insert.
///
/// A path plus an identifier cannot approach 512 bytes in this tree, and the
/// alternative to a fixed buffer — allocating a key for every lookup — buys
/// nothing. Truncation would silently merge two functions' counts, so it fails
/// loudly instead.
const key_max = 512;
fn entryKey(buffer: *[key_max]u8, file: []const u8, function: []const u8) []const u8 {
    return std.fmt.bufPrint(buffer, "{s}\x00{s}", .{ file, function }) catch
        @panic("release_assert: (file, function) key exceeds 512 bytes");
}

fn lessThanPath(_: void, a: []const u8, b: []const u8) bool {
    return std.mem.order(u8, a, b) == .lt;
}

fn isExcluded(path: []const u8) bool {
    if (std.mem.endsWith(u8, path, "_test.zig")) return true;
    for (excluded_prefixes) |prefix| {
        if (std.mem.startsWith(u8, path, prefix)) return true;
    }
    for (excluded_files) |file| {
        if (std.mem.eql(u8, path, file)) return true;
    }
    return false;
}

fn baselineCount(file: []const u8, function: []const u8) usize {
    for (baseline) |entry| {
        if (std.mem.eql(u8, entry.file, file) and std.mem.eql(u8, entry.function, function)) return entry.count;
    }
    return 0;
}

const Param = struct { name: []const u8, untrusted: bool };

/// One `pub fn` with an error-union return type, and the byte span of its body.
const Function = struct {
    name: []const u8,
    params: []const Param,
    body_start: usize,
    body_end: usize,
};

/// Tokenize `source`, collect every candidate `pub fn`, then attribute each
/// `std.debug.assert(...)` call to the innermost one containing it and apply the
/// untrusted-parameter and dominance rules.
fn scanFile(
    arena: std.mem.Allocator,
    file: []const u8,
    source: [:0]const u8,
    scan: *Scan,
) !void {
    var tokens: std.ArrayList(std.zig.Token) = .empty;
    {
        var tokenizer = std.zig.Tokenizer.init(source);
        while (true) {
            const token = tokenizer.next();
            try tokens.append(arena, token);
            if (token.tag == .eof) break;
        }
    }
    const toks = tokens.items;

    var functions: std.ArrayList(Function) = .empty;
    for (toks, 0..) |token, i| {
        if (token.tag != .keyword_pub) continue;
        if (i + 1 >= toks.len or toks[i + 1].tag != .keyword_fn) continue;
        const function = try parseFunction(arena, source, toks, i + 1) orelse continue;
        try functions.append(arena, function);
    }

    var i: usize = 0;
    while (i + 5 < toks.len) : (i += 1) {
        if (!isDottedCall(source, toks, i, "std", "debug", "assert")) continue;
        const arg_end = matchingParen(toks, i + 5) orelse continue;
        const assert_start = toks[i].loc.start;

        scan.asserts_seen += 1;
        const enclosing = innermost(functions.items, assert_start);
        try scan.attributions.append(arena, .{
            .line = lineNumber(source, assert_start),
            .function = if (enclosing) |function| function.name else "",
        });

        const owner = enclosing orelse continue;
        for (owner.params) |param| {
            if (!param.untrusted) continue;
            if (!spanMentionsIdentifier(source, toks, i + 6, arg_end, param.name)) continue;
            if (dominated(source, owner.body_start, assert_start, param.name)) continue;

            const site: Site = .{
                .line = lineNumber(source, assert_start),
                .param = param.name,
                .source_line = std.mem.trim(u8, lineAt(source, assert_start), " \t"),
            };
            var key_buffer: [key_max]u8 = undefined;
            const gop = try scan.entries.getOrPut(arena, entryKey(&key_buffer, file, owner.name));
            if (!gop.found_existing) {
                gop.key_ptr.* = try arena.dupe(u8, gop.key_ptr.*);
                gop.value_ptr.* = .{ .file = file, .function = owner.name, .sites = .empty };
            }
            try gop.value_ptr.sites.append(arena, site);
            // One finding per assert, not per matching parameter: an assert that
            // names two untrusted parameters is still one thing to fix.
            break;
        }
    }
}

/// True when tokens `[i..i+5]` spell `a.b.c` followed by `(`.
fn isDottedCall(
    source: []const u8,
    toks: []const std.zig.Token,
    i: usize,
    a: []const u8,
    b: []const u8,
    c: []const u8,
) bool {
    if (toks[i].tag != .identifier) return false;
    if (!std.mem.eql(u8, tokenText(source, toks[i]), a)) return false;
    if (toks[i + 1].tag != .period) return false;
    if (toks[i + 2].tag != .identifier or !std.mem.eql(u8, tokenText(source, toks[i + 2]), b)) return false;
    if (toks[i + 3].tag != .period) return false;
    if (toks[i + 4].tag != .identifier or !std.mem.eql(u8, tokenText(source, toks[i + 4]), c)) return false;
    return toks[i + 5].tag == .l_paren;
}

fn tokenText(source: []const u8, token: std.zig.Token) []const u8 {
    return source[token.loc.start..token.loc.end];
}

/// Index of the `)` matching the `(` at `open`, or null if unbalanced.
fn matchingParen(toks: []const std.zig.Token, open: usize) ?usize {
    var depth: usize = 0;
    var i = open;
    while (i < toks.len) : (i += 1) {
        switch (toks[i].tag) {
            .l_paren => depth += 1,
            .r_paren => {
                depth -= 1;
                if (depth == 0) return i;
            },
            .eof => return null,
            else => {},
        }
    }
    return null;
}

/// Index of the `}` matching the `{` at `open`, or null if unbalanced.
fn matchingBrace(toks: []const std.zig.Token, open: usize) ?usize {
    var depth: usize = 0;
    var i = open;
    while (i < toks.len) : (i += 1) {
        switch (toks[i].tag) {
            .l_brace => depth += 1,
            .r_brace => {
                depth -= 1;
                if (depth == 0) return i;
            },
            .eof => return null,
            else => {},
        }
    }
    return null;
}

/// Parse the `pub fn` whose `fn` keyword is at `fn_index`. Returns null when it
/// is not a candidate: no body (a prototype), or a return type that is not an
/// error union — a function that cannot report a rejection is out of scope here.
fn parseFunction(
    arena: std.mem.Allocator,
    source: []const u8,
    toks: []const std.zig.Token,
    fn_index: usize,
) !?Function {
    var i = fn_index + 1;
    if (i >= toks.len or toks[i].tag != .identifier) return null;
    const name = tokenText(source, toks[i]);
    i += 1;
    if (i >= toks.len or toks[i].tag != .l_paren) return null;

    const params_open = i;
    const params_close = matchingParen(toks, params_open) orelse return null;

    // The return type runs from just past `)` to the `{` that opens the body.
    // A `struct`/`union`/`enum`/`opaque`/`error` brace group inside the return
    // type is skipped so it is not mistaken for the body.
    var body_open: ?usize = null;
    var j = params_close + 1;
    while (j < toks.len) : (j += 1) {
        switch (toks[j].tag) {
            .semicolon, .eof => return null, // prototype, or a malformed parse
            .l_brace => {
                const introducer = if (j == 0) std.zig.Token.Tag.invalid else toks[j - 1].tag;
                const is_container = switch (introducer) {
                    .keyword_struct, .keyword_union, .keyword_enum, .keyword_opaque, .keyword_error => true,
                    else => false,
                };
                if (is_container) {
                    j = matchingBrace(toks, j) orelse return null;
                    continue;
                }
                body_open = j;
                break;
            },
            else => {},
        }
    }
    const open = body_open orelse return null;
    const close = matchingBrace(toks, open) orelse return null;

    if (!returnsError(source, toks, params_close + 1, open)) return null;

    var params: std.ArrayList(Param) = .empty;
    try parseParams(arena, source, toks, params_open, params_close, &params);
    if (params.items.len == 0) return null;

    return .{
        .name = name,
        .params = params.items,
        .body_start = toks[open].loc.end,
        .body_end = toks[close].loc.start,
    };
}

/// An error union return type: an explicit `!`, or a named type whose spelling
/// carries `Error` (the `Foo.Error` / `SyncError` convention this repo uses).
fn returnsError(source: []const u8, toks: []const std.zig.Token, from: usize, to: usize) bool {
    var i = from;
    while (i < to) : (i += 1) {
        if (toks[i].tag == .bang) return true;
        if (toks[i].tag == .identifier and
            std.mem.indexOf(u8, tokenText(source, toks[i]), "Error") != null) return true;
    }
    return false;
}

/// Split the parameter list at top-level commas and record each parameter's name
/// and whether it is untrusted. Parameters without a `name: type` shape
/// (`anytype` alone, `...`) contribute nothing and are skipped.
fn parseParams(
    arena: std.mem.Allocator,
    source: []const u8,
    toks: []const std.zig.Token,
    open: usize,
    close: usize,
    out: *std.ArrayList(Param),
) !void {
    var start = open + 1;
    var i = start;
    var depth: usize = 0;
    while (i < close) : (i += 1) {
        switch (toks[i].tag) {
            .l_paren, .l_bracket, .l_brace => depth += 1,
            .r_paren, .r_bracket, .r_brace => depth -|= 1,
            .comma => if (depth == 0) {
                try pushParam(arena, source, toks, start, i, out);
                start = i + 1;
            },
            else => {},
        }
    }
    if (start < close) try pushParam(arena, source, toks, start, close, out);
}

fn pushParam(
    arena: std.mem.Allocator,
    source: []const u8,
    toks: []const std.zig.Token,
    from: usize,
    to: usize,
    out: *std.ArrayList(Param),
) !void {
    var i = from;
    while (i < to and (toks[i].tag == .keyword_comptime or toks[i].tag == .keyword_noalias)) i += 1;
    if (i >= to or toks[i].tag != .identifier) return;
    const name = tokenText(source, toks[i]);
    if (i + 1 >= to or toks[i + 1].tag != .colon) return;
    if (i + 2 >= to) return; // `name:` with no type — malformed, nothing to judge

    for (plumbing_names) |plumbing| {
        if (std.mem.eql(u8, name, plumbing)) return;
    }

    const type_text = std.mem.trim(u8, source[toks[i + 2].loc.start..toks[to - 1].loc.end], " \t\n");
    var untrusted = std.mem.startsWith(u8, type_text, "[]");
    if (!untrusted) {
        for (untrusted_names) |needle| {
            if (std.mem.indexOf(u8, name, needle) != null) {
                untrusted = true;
                break;
            }
        }
    }
    try out.append(arena, .{ .name = name, .untrusted = untrusted });
}

/// The function with the smallest body span containing `offset`, so an assert in
/// a nested `pub fn` is attributed to the nested one.
fn innermost(functions: []const Function, offset: usize) ?Function {
    var best: ?Function = null;
    for (functions) |function| {
        if (offset < function.body_start or offset >= function.body_end) continue;
        const current = best orelse {
            best = function;
            continue;
        };
        if (function.body_end - function.body_start < current.body_end - current.body_start) best = function;
    }
    return best;
}

/// True when some identifier token in `[from..to)` is exactly `name`.
fn spanMentionsIdentifier(
    source: []const u8,
    toks: []const std.zig.Token,
    from: usize,
    to: usize,
    name: []const u8,
) bool {
    var i = from;
    while (i < to) : (i += 1) {
        if (toks[i].tag != .identifier) continue;
        if (std.mem.eql(u8, tokenText(source, toks[i]), name)) return true;
    }
    return false;
}

/// Rule 5. True when an earlier line of the same body both mentions `param` and
/// contains `return` — the function has already had its chance to reject the
/// input, so a following assert restates an established fact rather than
/// standing in for validation.
///
/// This is deliberately line-shaped rather than flow-sensitive: it is the
/// pattern a reviewer reads off the page, and over-clearing here costs a missed
/// finding while under-clearing would cost a false alarm on correct code.
fn dominated(source: []const u8, body_start: usize, assert_start: usize, param: []const u8) bool {
    if (assert_start <= body_start) return false;
    const before = source[body_start..assert_start];
    // Drop the partial final line: the assert's own line is not "earlier".
    const cut = std.mem.lastIndexOfScalar(u8, before, '\n') orelse return false;
    var it = std.mem.splitScalar(u8, before[0..cut], '\n');
    while (it.next()) |line| {
        if (std.mem.indexOf(u8, line, "return") == null) continue;
        if (mentionsIdentifier(line, param)) return true;
    }
    return false;
}

/// Substring search for `name` bounded by non-identifier characters, so `hash`
/// does not match inside `hash_index`.
fn mentionsIdentifier(haystack: []const u8, name: []const u8) bool {
    var from: usize = 0;
    while (std.mem.indexOfPos(u8, haystack, from, name)) |at| {
        const before_ok = at == 0 or !isIdentChar(haystack[at - 1]);
        const after = at + name.len;
        const after_ok = after >= haystack.len or !isIdentChar(haystack[after]);
        if (before_ok and after_ok) return true;
        from = at + 1;
    }
    return false;
}

fn isIdentChar(c: u8) bool {
    return std.ascii.isAlphanumeric(c) or c == '_';
}

fn lineNumber(source: []const u8, offset: usize) usize {
    return std.mem.count(u8, source[0..offset], "\n") + 1;
}

fn lineAt(source: []const u8, offset: usize) []const u8 {
    const start = if (std.mem.lastIndexOfScalar(u8, source[0..offset], '\n')) |at| at + 1 else 0;
    const rest = source[start..];
    const end = std.mem.indexOfScalar(u8, rest, '\n') orelse rest.len;
    return rest[0..end];
}