feat(conflict): Add Tango request and signature intersection (#884)

# Why?
The Tango-backed conflict analyzer needs three building blocks that
don't depend on how it is wired: the interfaces for the two things a
deployment supplies, the request it sends to Tango, and the check that
decides whether two signatures conflict. This PR adds them separately so
the interface shapes can be reviewed before the analyzer uses them.

## What?
Everything is added under `submitqueue/extension/conflict/tango`:
- Adds `ChangedTargetsRequest`: the remote, the base revision, every
change URI in batch order, and the merge strategy.
- Adds `ChangedTargetsClient`, which returns the affected targets of a
request as label and distance pairs. A deployment supplies an adapter
that owns the Tango client and its stream, collects the response and
resolves each target's label. Any failure to produce a complete answer
is an error, and an empty result is valid.
- Adds `Base` and `BaseResolver`: how the analyzer learns the newest
revision of a queue's target branch. A deployment supplies the
implementation.
- Adds `changedTargetsRequestFor`, which flattens the URIs of a batch's
changes, in order, into a request.
- Adds `sharesTarget`, which reports whether two signatures have a
target ID in common. Both must be sorted by ID, so the check is one pass
over each, and it stops at the first shared ID.
- Adds gomock mocks for the two interfaces, generated in the same
package.
- Nothing calls this code yet. The analyzer that uses it is the next PR
in this stack.
- Tests cover flattening of multiple changes with multiple URIs, an
empty change list, and intersection with empty inputs, shared first,
middle and last IDs, different lengths, IDs above the signed range
(checking they order as unsigned), and distance being ignored.

## Test Plan
`./tool/bazel test //submitqueue/extension/conflict/tango/...`
`go test -race ./submitqueue/extension/conflict/tango/`
`make check-gazelle`
`make check-mocks`
`make lint`
CI

## Issue

Author: manjari25

submitqueue/extension/conflict/tango/BUILD.bazel

@@ -4,25 +4,34 @@ go_library(
     name = "go_default_library",
     srcs = [
         "cache.go",
+        "intersect.go",
+        "mock_request.go",
         "mock_store.go",
+        "request.go",
         "signature.go",
         "store.go",
         "target_label.go",
         "target_signature.go",
     ],
     importpath = "github.com/uber/submitqueue/submitqueue/extension/conflict/tango",
     visibility = ["//visibility:public"],
-    deps = ["@org_uber_go_mock//gomock:go_default_library"],
+    deps = [
+        "//platform/base/change:go_default_library",
+        "@org_uber_go_mock//gomock:go_default_library",
+    ],
 )
 
 go_test(
     name = "go_default_test",
     srcs = [
         "cache_test.go",
+        "intersect_test.go",
+        "request_test.go",
         "signature_test.go",
     ],
     embed = [":go_default_library"],
     deps = [
+        "//platform/base/change:go_default_library",
         "@com_github_stretchr_testify//assert:go_default_library",
         "@com_github_stretchr_testify//require:go_default_library",
     ],

submitqueue/extension/conflict/tango/intersect.go

@@ -0,0 +1,32 @@
+// Copyright (c) 2026 Uber Technologies, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package tango
+
+// sharesTarget reports whether two signatures have a target ID in common. Both
+// must be sorted ascending by ID, so the check is one pass over each.
+func sharesTarget(a, b []SignatureTarget) bool {
+	i, j := 0, 0
+	for i < len(a) && j < len(b) {
+		switch {
+		case a[i].ID < b[j].ID:
+			i++
+		case a[i].ID > b[j].ID:
+			j++
+		default:
+			return true
+		}
+	}
+	return false
+}

submitqueue/extension/conflict/tango/intersect_test.go

@@ -0,0 +1,61 @@
+// Copyright (c) 2026 Uber Technologies, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package tango
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func targetsWithIDs(ids ...uint64) []SignatureTarget {
+	targets := make([]SignatureTarget, len(ids))
+	for i, id := range ids {
+		targets[i] = SignatureTarget{ID: id}
+	}
+	return targets
+}
+
+func TestSharesTarget(t *testing.T) {
+	const highBit = uint64(1) << 63
+
+	tests := []struct {
+		name string
+		a, b []SignatureTarget
+		want bool
+	}{
+		{name: "both empty"},
+		{name: "one empty", a: targetsWithIDs(1, 2)},
+		{name: "disjoint", a: targetsWithIDs(1, 3, 5), b: targetsWithIDs(2, 4, 6)},
+		{name: "same first ID", a: targetsWithIDs(1, 3), b: targetsWithIDs(1, 9), want: true},
+		{name: "same last ID", a: targetsWithIDs(1, 7), b: targetsWithIDs(2, 7), want: true},
+		{name: "same middle ID", a: targetsWithIDs(1, 4, 9), b: targetsWithIDs(2, 4, 8), want: true},
+		{name: "different lengths", a: targetsWithIDs(5), b: targetsWithIDs(1, 2, 3, 4, 5, 6), want: true},
+		{name: "IDs above the signed range order as unsigned", a: targetsWithIDs(1, highBit), b: targetsWithIDs(2, highBit+1)},
+		{name: "same ID above the signed range", a: targetsWithIDs(1, highBit), b: targetsWithIDs(highBit), want: true},
+		{
+			name: "distance does not affect the result",
+			a:    []SignatureTarget{{ID: 4, Distance: 0}},
+			b:    []SignatureTarget{{ID: 4, Distance: 9}},
+			want: true,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			assert.Equal(t, tt.want, sharesTarget(tt.a, tt.b))
+			assert.Equal(t, tt.want, sharesTarget(tt.b, tt.a))
+		})
+	}
+}

submitqueue/extension/conflict/tango/mock_request.go

@@ -0,0 +1,53 @@
+// Copyright (c) 2026 Uber Technologies, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package tango
+
+//go:generate mockgen -source=request.go -destination=mock_request.go -package=tango
+
+import (
+	"context"
+
+	"github.com/uber/submitqueue/platform/base/change"
+)
+
+// ChangedTargetsRequest asks which targets change when ChangeURIs are applied
+// on top of a base revision.
+type ChangedTargetsRequest struct {
+	// BaseRevision is the full commit SHA the changes are applied on top of.
+	BaseRevision string
+
+	// ChangeURIs lists every change to apply, in batch order.
+	ChangeURIs []string
+}
+
+// ChangedTargetsClient returns the targets a request affects. An adapter owns
+// the Tango client and its stream, collects the response, and resolves each
+// target's label; any failure to produce a complete answer is an error. It also
+// owns the Tango repository remote and computation strategy, which the request
+// does not carry. ChangeURIs are passed through as given, so a URI scheme Tango
+// does not support is an error.
+type ChangedTargetsClient interface {
+	// ChangedTargets returns the affected targets in no particular order, and a
+	// label may repeat. An empty result is valid and means nothing is affected.
+	ChangedTargets(ctx context.Context, request ChangedTargetsRequest) ([]LabelDistance, error)
+}
+
+func changedTargetsRequestFor(baseRevision string, changes []change.Change) ChangedTargetsRequest {
+	var uris []string
+	for _, c := range changes {
+		uris = append(uris, c.URIs...)
+	}
+	return ChangedTargetsRequest{BaseRevision: baseRevision, ChangeURIs: uris}
+}

submitqueue/extension/conflict/tango/request.go

@@ -0,0 +1,51 @@
+// Copyright (c) 2026 Uber Technologies, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package tango
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+
+	"github.com/uber/submitqueue/platform/base/change"
+)
+
+func TestChangedTargetsRequestFor(t *testing.T) {
+	baseRevision := "9f2c1d0e3a4b5c6d7e8f9a0b1c2d3e4f5a6b7c8d"
+
+	tests := []struct {
+		name    string
+		changes []change.Change
+		want    []string
+	}{
+		{name: "no changes", want: nil},
+		{name: "one change", changes: []change.Change{{URIs: []string{"a"}}}, want: []string{"a"}},
+		{
+			name:    "URIs are flattened in batch order",
+			changes: []change.Change{{URIs: []string{"a", "b"}}, {URIs: []string{"c"}}, {URIs: []string{"d", "e"}}},
+			want:    []string{"a", "b", "c", "d", "e"},
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := changedTargetsRequestFor(baseRevision, tt.changes)
+
+			assert.Equal(t, ChangedTargetsRequest{
+				BaseRevision: baseRevision,
+				ChangeURIs:   tt.want,
+			}, got)
+		})
+	}
+}