Fix buf dep graph --format json silently dropping dependencies

cmd/buf/internal/command/dep/depgraph/depgraph.go

@@ -253,9 +253,9 @@ func (e *externalModule) addDeps(
 		depExternalModule, ok := moduleFullNameOrOpaqueIDToExternalModule[depFullNameOrOpaqueID]
 		if ok {
 			// If this dependency has already been seen, we can simply update our current module
-			// and return early.
+			// and continue to the next dependency.
 			e.Deps = append(e.Deps, depExternalModule)
-			return nil
+			continue
 		}
 		// Otherwise, we create a new external module for our direct dependency. However, we do
 		// not add it to our map yet, we only add it once all transitive dependencies have been

cmd/buf/internal/command/dep/depgraph/depgraph_test.go

@@ -0,0 +1,89 @@
+// Copyright 2020-2026 Buf 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 depgraph
+
+import (
+	"bytes"
+	"encoding/json"
+	"path/filepath"
+	"testing"
+
+	"buf.build/go/app/appcmd"
+	"buf.build/go/app/appcmd/appcmdtesting"
+	"buf.build/go/app/appext"
+	"github.com/bufbuild/buf/cmd/buf/internal/internaltesting"
+	"github.com/stretchr/testify/require"
+)
+
+// TestJSONFormatSharedDep is a regression test for a bug where addDeps had
+// a "return nil" instead of "continue" when a dependency was already in the
+// seen map, causing all subsequent dependencies to be silently dropped.
+//
+// The workspace has three local modules: alpha -> common, zeta -> {alpha, common}.
+// Because modules are walked alphabetically, both of zeta's deps are already
+// in the seen map when zeta is processed, which triggered the early return.
+func TestJSONFormatSharedDep(t *testing.T) {
+	t.Parallel()
+	var stdout bytes.Buffer
+	appcmdtesting.Run(
+		t,
+		func(name string) *appcmd.Command {
+			return NewCommand(name, appext.NewBuilder(name))
+		},
+		appcmdtesting.WithExpectedExitCode(0),
+		appcmdtesting.WithEnv(internaltesting.NewEnvFunc(t)),
+		appcmdtesting.WithStdout(&stdout),
+		appcmdtesting.WithArgs(
+			"--format", "json",
+			filepath.Join("testdata", "shared_dep_workspace"),
+		),
+	)
+	var got []externalModule
+	require.NoError(t, json.Unmarshal(stdout.Bytes(), &got))
+	clearDigests(got)
+	expected := []externalModule{
+		{
+			Name:  "alpha",
+			Local: true,
+			Deps: []externalModule{
+				{Name: "common", Local: true},
+			},
+		},
+		{
+			Name:  "common",
+			Local: true,
+		},
+		{
+			Name:  "zeta",
+			Local: true,
+			Deps: []externalModule{
+				{Name: "alpha", Local: true, Deps: []externalModule{
+					{Name: "common", Local: true},
+				}},
+				{Name: "common", Local: true},
+			},
+		},
+	}
+	require.Equal(t, expected, got)
+}
+
+// clearDigests zeros out Digest fields so we can compare structures without
+// depending on content hashes.
+func clearDigests(modules []externalModule) {
+	for i := range modules {
+		modules[i].Digest = ""
+		clearDigests(modules[i].Deps)
+	}
+}

cmd/buf/internal/command/dep/depgraph/testdata/shared_dep_workspace/alpha/alpha.proto

@@ -0,0 +1,9 @@
+syntax = "proto3";
+
+package alpha.v1;
+
+import "common.proto";
+
+message Foo {
+  common.v1.Shared shared = 1;
+}

cmd/buf/internal/command/dep/depgraph/testdata/shared_dep_workspace/buf.yaml

@@ -0,0 +1,5 @@
+version: v2
+modules:
+  - path: alpha
+  - path: common
+  - path: zeta

cmd/buf/internal/command/dep/depgraph/testdata/shared_dep_workspace/common/common.proto

@@ -0,0 +1,7 @@
+syntax = "proto3";
+
+package common.v1;
+
+message Shared {
+  string value = 1;
+}

cmd/buf/internal/command/dep/depgraph/testdata/shared_dep_workspace/zeta/zeta.proto

@@ -0,0 +1,11 @@
+syntax = "proto3";
+
+package zeta.v1;
+
+import "common.proto";
+import "alpha.proto";
+
+message Bar {
+  common.v1.Shared shared = 1;
+  alpha.v1.Foo foo = 2;
+}