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commands: $project computes, renames and narrows

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The last three cases of the corpus, which is now 70 pass / 0 fail -- every
answer byte-identical to mongod 8.3.7 across the accumulators, the expressions
and the document stages.

The fix turned out to need nothing from `query.project`, which `find` shares
and which I had expected to have to rewrite. A nested spec *is* a dotted path:
`{n: {x: 1}}` and `{"n.x": 1}` are the same projection, and dotted paths are
something the existing projection already narrows correctly. So `$project` is
flattened into inclusion/exclusion flags plus a list of computed fields, and
both halves reuse machinery that was already there -- `query.project` for the
flags, `set_path` from the document stages for the computed fields. A bare path
(`{value: "$a"}`) is a rename, which is a computed field like any other.

Both shapes used to read as *falsy*, which flipped the whole projection into
its exclusion branch and returned the entire document minus the field. That was
recorded during M2 as broken rather than unimplemented; this is the fix it was
waiting for.

One case `query.project` genuinely cannot express, so it is built directly: a
projection that only computes keeps `_id` and nothing else, and with no non-`_id`
flag that function reads the spec as an exclusion and returns everything. It
cost two failures and a `id_only` flag to find, which is what a recorded corpus
is for -- the answer is obvious once seen and not before.

`$project` now goes through the same `Rewrite` path as `$addFields`, `$unset`,
`$replaceRoot` and `$unwind`, so its own branch is gone. Its refusal shrank to
the one shape mongod also refuses, mixing inclusion with exclusion, judged on
the flattened flags so a nested spec is treated like a dotted one.

191/191 unit tests in ReleaseFast and ReleaseSafe, 83/83 fuzz, e2e 49, e2e2
concurrent, e2e3 16, e2e4 17, e2e6 72, e2e7 86, crud corpus unchanged at
201/90/196.
This commit was merged in pull request #3.
This commit is contained in:
A.Shakhmatov
2026-08-09 22:50:31 +03:00
parent 1a5386ff00
commit fc611a2c64
2 changed files with 126 additions and 90 deletions
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207
src/commands.zig
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@@ -2296,34 +2296,6 @@ fn cmd_aggregate(ctx: *Context, msg: *wire.Message, reply: *wire.Reply) !void {
} else if (std.mem.eql(u8, stage_name, "$limit")) {
const n = try stage_count(reply, stage[0].value, "$limit") orelse return;
end = @min(end, start + n);
} else if (std.mem.eql(u8, stage_name, "$project")) {
const pp = doc_arg(stage[0].value) orelse return bad_value(reply, "$project requires a document");
if (try refuse_unprojectable(reply, pp)) return;
// Applied here rather than remembered for the emit. It used to set
// a variable that only the last `$project` in a pipeline could win
// and that no later stage could see -- so a `$match` after a
// `$project` still matched on a field the projection had removed.
const arena = reply.arena_alloc();
var projected: std.ArrayListUnmanaged(*const bson.Document) = .empty;
errdefer projected.deinit(ctx.gpa);
try projected.ensureTotalCapacity(ctx.gpa, end - start);
if (in_trees) {
for (trees.items[start..end]) |d| {
projected.appendAssumeCapacity(try projected_tree(arena, d, pp));
}
} else {
for (offs.items[start..end]) |off| {
projected.appendAssumeCapacity(try projected_tree(arena, try doc_tree(arena, coll, off), pp));
}
}
offs.deinit(ctx.gpa);
offs = .empty;
trees.deinit(ctx.gpa);
trees = projected;
projected = .empty;
in_trees = true;
start = 0;
end = trees.items.len;
} else if (std.mem.eql(u8, stage_name, "$group")) {
const gp = doc_arg(stage[0].value) orelse return bad_value(reply, "$group requires a document");
const src: Stream = if (in_trees)
@@ -2343,7 +2315,7 @@ fn cmd_aggregate(ctx: *Context, msg: *wire.Message, reply: *wire.Reply) !void {
end = trees.items.len;
} else if (std.mem.eql(u8, stage_name, "$addFields") or std.mem.eql(u8, stage_name, "$set") or
std.mem.eql(u8, stage_name, "$unset") or std.mem.eql(u8, stage_name, "$replaceRoot") or
std.mem.eql(u8, stage_name, "$unwind"))
std.mem.eql(u8, stage_name, "$unwind") or std.mem.eql(u8, stage_name, "$project"))
{
// The stages that rewrite a document, all one shape: read the
// window, build a new list, replace the stream. The design review
@@ -2512,55 +2484,18 @@ fn count_only_pipeline(reply: *wire.Reply, stages: []const bson.Value) !?CountSh
return .{ .filter = filter, .id_value = id_value, .accs = accs.items };
}
/// Refuse a `$project` this engine cannot carry out, answering the client.
/// Returns true when it did.
/// Refuse a `$project` that mixes inclusion with exclusion, which mongod
/// refuses too -- so this is parity rather than a limitation of this server.
/// Judged on the *flattened* flags, so a nested spec is treated the same way a
/// dotted one is.
///
/// The stage takes inclusion and exclusion flags and nothing else. A computed
/// field (`{y: {$literal: 5}}`) needs the expression evaluator that does not
/// exist yet, and a nested spec (`{a: {b: 1}}`) needs a narrowing
/// `query.project` does not do. Both used to be read as *falsy*, which put the
/// whole projection into its exclusion branch: `{$project: {y: {$literal: 5}}}`
/// returned every document with `y` removed, where mongod adds a computed `y`.
/// Measured on a live server, not inferred.
///
/// Codes read off mongod 8.3.7. It refuses the empty and the mixed forms too,
/// so those two are parity rather than a limitation of this server.
fn refuse_unprojectable(reply: *wire.Reply, pp: []const bson.Pair) !bool {
if (pp.len == 0) {
try reply.put_error(
@intFromEnum(ErrorCode.location_project_empty),
"Location51272",
"Invalid $project :: caused by :: projection specification must have at least one field",
);
return true;
}
/// The rest of what this used to refuse -- computed fields and nested specs --
/// is implemented now. They were refused because both read as falsy, which put
/// the whole projection into its exclusion branch and returned the entire
/// document minus that field.
fn refuse_mixed_projection(reply: *wire.Reply, flags: []const bson.Pair) !bool {
var include: ?bool = null;
for (pp) |p| {
switch (p.value) {
.bool, .int32, .int64, .double => {},
else => {
// A `$`-led document is an expression mongod evaluates and
// names in its own message; anything else is a nested spec,
// where the honest message is the one this server can stand
// behind.
const detail = if (p.value == .doc and p.value.doc.len > 0 and
p.value.doc[0].key.len > 0 and p.value.doc[0].key[0] == '$')
try std.fmt.allocPrint(
reply.arena_alloc(),
"Invalid $project :: caused by :: Unknown expression {s}",
.{p.value.doc[0].key},
)
else
try std.fmt.allocPrint(
reply.arena_alloc(),
"Invalid $project :: caused by :: field '{s}' must be an inclusion or " ++
"exclusion flag: this server projects no computed or nested fields",
.{p.key},
);
try reply.put_error(@intFromEnum(ErrorCode.location_project_unknown_expression), "Location31325", detail);
return true;
},
}
for (flags) |p| {
// `_id` is the one field that may be excluded from an inclusion
// projection, so it never decides which kind this is.
if (std.mem.eql(u8, p.key, "_id")) continue;
@@ -3002,6 +2937,83 @@ fn compile_switch(reply: *wire.Reply, arena: std.mem.Allocator, spec: bson.Value
const EvalError = error{ DivideByZero, SwitchNoDefault, NonNumericArithmetic, ReplaceRootNotDocument } ||
std.mem.Allocator.Error || error{ EndOfStream, Overflow, InvalidBson };
/// A `$project` taken apart: the inclusion/exclusion flags flattened to dotted
/// paths, and the computed fields as expressions.
///
/// Flattening is what lets a nested spec work without touching
/// `query.project`, which `find` shares: `{n: {x: 1}}` *is* `{"n.x": 1}`, and a
/// dotted path is something the projection already narrows correctly. The
/// nested form used to read as *falsy*, which flipped the whole projection into
/// its exclusion branch and returned every document minus that field.
const ProjectSpec = struct {
flags: []const bson.Pair,
computed: []const Expr.Field,
/// A projection that only computes -- `{$project: {b: <expr>}}` -- keeps
/// `_id` and nothing else. `query.project` cannot say that: with no
/// non-`_id` flag it reads the spec as an *exclusion* and returns the whole
/// document, so this case is built directly instead.
id_only: bool = false,
};
fn compile_project(
reply: *wire.Reply,
arena: std.mem.Allocator,
pp: []const bson.Pair,
) !?ProjectSpec {
var flags: std.ArrayListUnmanaged(bson.Pair) = .empty;
var computed: std.ArrayListUnmanaged(Expr.Field) = .empty;
if (try flatten_project(reply, arena, pp, "", &flags, &computed)) return null;
if (flags.items.len == 0 and computed.items.len == 0) {
try reply.put_error(
@intFromEnum(ErrorCode.location_project_empty),
"Location51272",
"Invalid $project :: caused by :: projection specification must have at least one field",
);
return null;
}
return ProjectSpec{
.flags = flags.items,
.computed = computed.items,
.id_only = flags.items.len == 0,
};
}
/// Walk a projection spec into flags and computed fields. Returns true when it
/// has already answered the client.
fn flatten_project(
reply: *wire.Reply,
arena: std.mem.Allocator,
pp: []const bson.Pair,
prefix: []const u8,
flags: *std.ArrayListUnmanaged(bson.Pair),
computed: *std.ArrayListUnmanaged(Expr.Field),
) CompileError!bool {
for (pp) |p| {
const key = if (prefix.len == 0)
p.key
else
try std.fmt.allocPrint(arena, "{s}.{s}", .{ prefix, p.key });
switch (p.value) {
.bool, .int32, .int64, .double => try flags.append(arena, .{ .key = key, .value = p.value }),
.doc => |d| {
// `{a: {$op: ...}}` computes; `{a: {b: 1}}` narrows.
if (d.len > 0 and d[0].key.len > 0 and d[0].key[0] == '$') {
const e = (try compile_expr(reply, arena, p.value, "a $project value")) orelse return true;
try computed.append(arena, .{ .key = key, .value = e });
} else if (try flatten_project(reply, arena, d, key, flags, computed)) {
return true;
}
},
// A bare path is a rename, which is a computed field like any other.
else => {
const e = (try compile_expr(reply, arena, p.value, "a $project value")) orelse return true;
try computed.append(arena, .{ .key = key, .value = e });
},
}
}
return false;
}
/// A compiled document-rewriting stage.
const Rewrite = union(enum) {
/// `$addFields` and `$set`, which are the same stage under two names.
@@ -3009,6 +3021,7 @@ const Rewrite = union(enum) {
unset: []const []const u8,
replace_root: Expr,
unwind: UnwindSpec,
project: ProjectSpec,
};
fn compile_rewrite(
@@ -3019,6 +3032,17 @@ fn compile_rewrite(
spec: bson.Value,
) !?Rewrite {
_ = ctx;
if (std.mem.eql(u8, name, "$project")) {
const pp = doc_arg(spec) orelse {
try bad_value(reply, "$project requires a document");
return null;
};
const parsed = (try compile_project(reply, arena, pp)) orelse return null;
// Mixed inclusion and exclusion is judged on the *flattened* flags, so
// a nested spec is treated the same way a dotted one is.
if (try refuse_mixed_projection(reply, parsed.flags)) return null;
return Rewrite{ .project = parsed };
}
if (std.mem.eql(u8, name, "$unwind")) {
return Rewrite{ .unwind = (try unwind_spec(reply, spec)) orelse return null };
}
@@ -3081,6 +3105,23 @@ fn apply_rewrite(
};
try out.append(gpa, try tree_of(ec.arena, pairs));
},
.project => |spec| {
var kept: std.ArrayListUnmanaged(bson.Pair) = .empty;
if (spec.id_only) {
if (bson.get_pair(doc.pairs, "_id")) |id| try kept.append(ec.arena, .{ .key = "_id", .value = id });
} else {
query.project(ec.arena, doc, &.{ .arena = undefined, .pairs = spec.flags }, &kept) catch
return error.OutOfMemory;
}
var pairs: []const bson.Pair = kept.items;
for (spec.computed) |f| {
// Same rule as `$addFields`: an expression that resolves to
// nothing leaves the field out rather than setting it null.
const v = (try eval_expr(ec, f.value)) orelse continue;
pairs = try set_path(ec.arena, pairs, f.key, v);
}
try out.append(gpa, try tree_of(ec.arena, pairs));
},
.unwind => |spec| {
const found = try eval_expr(ec, .{ .path = spec.path });
const items: []const bson.Value = switch (found orelse bson.Value.null) {
@@ -5301,15 +5342,13 @@ test "$project is a stage, not a note about how to print the answer" {
},
.code = 31254,
},
// A computed field and a nested spec stood here until the expression
// evaluator and the flattening landed; both work now. What is left is
// the one shape mongod refuses too.
.{
.name = "a computed field",
.spec = &.{.{ .key = "y", .value = .{ .doc = &.{.{ .key = "$literal", .value = .{ .int32 = 5 } }} } }},
.code = 31325,
},
.{
.name = "a nested spec",
.spec = &.{.{ .key = "a", .value = .{ .doc = &.{.{ .key = "b", .value = .{ .int32 = 1 } }} } }},
.code = 31325,
.name = "an unknown expression",
.spec = &.{.{ .key = "y", .value = .{ .doc = &.{.{ .key = "$bogusExpr", .value = .{ .int32 = 5 } }} } }},
.code = 168,
},
};
for (cases) |c| {