fix: avoid re-using AuthInstanceID for sub agents (#22196)

Parent agents were re-using AuthInstanceID when spawning child agents.
This caused GetWorkspaceAgentByInstanceID to return the most recently
created sub agent instead of the parent when the parent tried to refetch
its own manifest.

Fix by not reusing AuthInstanceID for sub agents, and updating
GetWorkspaceAgentByInstanceID to filter them out entirely.

coderd/agentapi/subagent.go

@@ -128,7 +128,7 @@ func (a *SubAgentAPI) CreateSubAgent(ctx context.Context, req *agentproto.Create
 		Name:                     agentName,
 		ResourceID:               parentAgent.ResourceID,
 		AuthToken:                uuid.New(),
-		AuthInstanceID:           parentAgent.AuthInstanceID,
+		AuthInstanceID:           sql.NullString{},
 		Architecture:             req.Architecture,
 		EnvironmentVariables:     pqtype.NullRawMessage{},
 		OperatingSystem:          req.OperatingSystem,

coderd/agentapi/subagent_test.go

@@ -175,6 +175,52 @@ func TestSubAgentAPI(t *testing.T) {
 		}
 	})
 
+	// Context: https://github.com/coder/coder/pull/22196
+	t.Run("CreateSubAgentDoesNotInheritAuthInstanceID", func(t *testing.T) {
+		t.Parallel()
+
+		var (
+			log   = testutil.Logger(t)
+			clock = quartz.NewMock(t)
+
+			db, org     = newDatabaseWithOrg(t)
+			user, agent = newUserWithWorkspaceAgent(t, db, org)
+		)
+
+		// Given: The parent agent has an AuthInstanceID set
+		ctx := testutil.Context(t, testutil.WaitShort)
+		parentAgent, err := db.GetWorkspaceAgentByID(dbauthz.AsSystemRestricted(ctx), agent.ID)
+		require.NoError(t, err)
+		require.True(t, parentAgent.AuthInstanceID.Valid, "parent agent should have an AuthInstanceID")
+		require.NotEmpty(t, parentAgent.AuthInstanceID.String)
+
+		api := newAgentAPI(t, log, db, clock, user, org, agent)
+
+		// When: We create a sub agent
+		createResp, err := api.CreateSubAgent(ctx, &proto.CreateSubAgentRequest{
+			Name:            "sub-agent",
+			Directory:       "/workspaces/test",
+			Architecture:    "amd64",
+			OperatingSystem: "linux",
+		})
+		require.NoError(t, err)
+
+		subAgentID, err := uuid.FromBytes(createResp.Agent.Id)
+		require.NoError(t, err)
+
+		// Then: The sub-agent must NOT re-use the parent's AuthInstanceID.
+		subAgent, err := db.GetWorkspaceAgentByID(dbauthz.AsSystemRestricted(ctx), subAgentID)
+		require.NoError(t, err)
+		assert.False(t, subAgent.AuthInstanceID.Valid, "sub-agent should not have an AuthInstanceID")
+		assert.Empty(t, subAgent.AuthInstanceID.String, "sub-agent AuthInstanceID string should be empty")
+
+		// Double-check: looking up by the parent's instance ID must
+		// still return the parent, not the sub-agent.
+		lookedUp, err := db.GetWorkspaceAgentByInstanceID(dbauthz.AsSystemRestricted(ctx), parentAgent.AuthInstanceID.String)
+		require.NoError(t, err)
+		assert.Equal(t, parentAgent.ID, lookedUp.ID, "instance ID lookup should still return the parent agent")
+	})
+
 	type expectedAppError struct {
 		index int32
 		field string