cmd/relui: publish task start message to pubsub

This change adds a new handler for starting a task by sending a message
to pubsub. A test pubsub server is used in unit testing. The exact
message used to start a task, as well as updating a task to mark it as
started, is still under design and likely to change dramatically. This
change is intended to unblock worker development.

For golang/go#40279

Co-authored-by: Carlos Amedee <carlos@golang.org>
Change-Id: I59bb5c5261a9a5d17e52597c1835a2a980cf91f8
Reviewed-on: https://go-review.googlesource.com/c/build/+/257239
Trust: Alexander Rakoczy <alex@golang.org>
Run-TryBot: Alexander Rakoczy <alex@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
diff --git a/cmd/relui/main.go b/cmd/relui/main.go
index 420b9f3..4e6e03c 100644
--- a/cmd/relui/main.go
+++ b/cmd/relui/main.go
@@ -41,6 +41,7 @@
 	}
 	http.Handle("/workflows/create", http.HandlerFunc(s.createWorkflowHandler))
 	http.Handle("/workflows/new", http.HandlerFunc(s.newWorkflowHandler))
+	http.Handle("/tasks/start", http.HandlerFunc(s.startTaskHandler))
 	http.Handle("/", fileServerHandler(relativeFile("./static"), http.HandlerFunc(s.homeHandler)))
 	port := os.Getenv("PORT")
 	if port == "" {
diff --git a/cmd/relui/protos/relui.pb.go b/cmd/relui/protos/relui.pb.go
index 27d41e5..e85ff43 100644
--- a/cmd/relui/protos/relui.pb.go
+++ b/cmd/relui/protos/relui.pb.go
@@ -56,7 +56,7 @@
 	BuildableTasks []*BuildableTask `protobuf:"bytes,2,rep,name=buildable_tasks,json=buildableTasks,proto3" json:"buildable_tasks,omitempty"`
 	// params are parameters provided when creating a workflow.
 	Params map[string]string `protobuf:"bytes,3,rep,name=params,proto3" json:"params,omitempty" protobuf_key:"bytes,1,opt,name=key,proto3" protobuf_val:"bytes,2,opt,name=value,proto3"`
-	// id is a unique identifier generated by relui when a workflow is created. Do not set.
+	// id is a unique identifier generated by relui when a workflow is created.
 	Id                   string   `protobuf:"bytes,4,opt,name=id,proto3" json:"id,omitempty"`
 	XXX_NoUnkeyedLiteral struct{} `json:"-"`
 	XXX_unrecognized     []byte   `json:"-"`
@@ -132,7 +132,7 @@
 	// task_type is a unique type for a task, such as FetchGerritSource. Types are used by task runners to identify
 	// how to execute a task.
 	TaskType string `protobuf:"bytes,6,opt,name=task_type,json=taskType,proto3" json:"task_type,omitempty"`
-	// id is a unique identifier generated by relui when a buildable task is created. Do not set.
+	// id is a unique identifier generated by relui when a buildable task is created.
 	Id                   string   `protobuf:"bytes,7,opt,name=id,proto3" json:"id,omitempty"`
 	XXX_NoUnkeyedLiteral struct{} `json:"-"`
 	XXX_unrecognized     []byte   `json:"-"`
@@ -213,6 +213,65 @@
 	return ""
 }
 
+// StartBuildableTaskRequest is a message sent to workers to start working on a BuildableTask for a Workflow.
+type StartBuildableTaskRequest struct {
+	// workflow_id is the workflow to which the BuildableTask belongs.
+	WorkflowId string `protobuf:"bytes,1,opt,name=workflow_id,json=workflowId,proto3" json:"workflow_id,omitempty"`
+	// buildable_task_id is the id of the BuildableTask to be started.
+	BuildableTaskId string `protobuf:"bytes,2,opt,name=buildable_task_id,json=buildableTaskId,proto3" json:"buildable_task_id,omitempty"`
+	// buildable_task_type is the type of the BuildableTask to be started.
+	BuildableTaskType    string   `protobuf:"bytes,3,opt,name=buildable_task_type,json=buildableTaskType,proto3" json:"buildable_task_type,omitempty"`
+	XXX_NoUnkeyedLiteral struct{} `json:"-"`
+	XXX_unrecognized     []byte   `json:"-"`
+	XXX_sizecache        int32    `json:"-"`
+}
+
+func (m *StartBuildableTaskRequest) Reset()         { *m = StartBuildableTaskRequest{} }
+func (m *StartBuildableTaskRequest) String() string { return proto.CompactTextString(m) }
+func (*StartBuildableTaskRequest) ProtoMessage()    {}
+func (*StartBuildableTaskRequest) Descriptor() ([]byte, []int) {
+	return fileDescriptor_6de8859f82adce0a, []int{2}
+}
+
+func (m *StartBuildableTaskRequest) XXX_Unmarshal(b []byte) error {
+	return xxx_messageInfo_StartBuildableTaskRequest.Unmarshal(m, b)
+}
+func (m *StartBuildableTaskRequest) XXX_Marshal(b []byte, deterministic bool) ([]byte, error) {
+	return xxx_messageInfo_StartBuildableTaskRequest.Marshal(b, m, deterministic)
+}
+func (m *StartBuildableTaskRequest) XXX_Merge(src proto.Message) {
+	xxx_messageInfo_StartBuildableTaskRequest.Merge(m, src)
+}
+func (m *StartBuildableTaskRequest) XXX_Size() int {
+	return xxx_messageInfo_StartBuildableTaskRequest.Size(m)
+}
+func (m *StartBuildableTaskRequest) XXX_DiscardUnknown() {
+	xxx_messageInfo_StartBuildableTaskRequest.DiscardUnknown(m)
+}
+
+var xxx_messageInfo_StartBuildableTaskRequest proto.InternalMessageInfo
+
+func (m *StartBuildableTaskRequest) GetWorkflowId() string {
+	if m != nil {
+		return m.WorkflowId
+	}
+	return ""
+}
+
+func (m *StartBuildableTaskRequest) GetBuildableTaskId() string {
+	if m != nil {
+		return m.BuildableTaskId
+	}
+	return ""
+}
+
+func (m *StartBuildableTaskRequest) GetBuildableTaskType() string {
+	if m != nil {
+		return m.BuildableTaskType
+	}
+	return ""
+}
+
 // LocalStorage is the persisted data of relui. It is used in development mode for saving application state.
 type LocalStorage struct {
 	// workflows are a list of user-created workflows.
@@ -226,7 +285,7 @@
 func (m *LocalStorage) String() string { return proto.CompactTextString(m) }
 func (*LocalStorage) ProtoMessage()    {}
 func (*LocalStorage) Descriptor() ([]byte, []int) {
-	return fileDescriptor_6de8859f82adce0a, []int{2}
+	return fileDescriptor_6de8859f82adce0a, []int{3}
 }
 
 func (m *LocalStorage) XXX_Unmarshal(b []byte) error {
@@ -266,7 +325,7 @@
 func (m *GitSource) String() string { return proto.CompactTextString(m) }
 func (*GitSource) ProtoMessage()    {}
 func (*GitSource) Descriptor() ([]byte, []int) {
-	return fileDescriptor_6de8859f82adce0a, []int{3}
+	return fileDescriptor_6de8859f82adce0a, []int{4}
 }
 
 func (m *GitSource) XXX_Unmarshal(b []byte) error {
@@ -306,6 +365,7 @@
 	proto.RegisterType((*Workflow)(nil), "protos.Workflow")
 	proto.RegisterMapType((map[string]string)(nil), "protos.Workflow.ParamsEntry")
 	proto.RegisterType((*BuildableTask)(nil), "protos.BuildableTask")
+	proto.RegisterType((*StartBuildableTaskRequest)(nil), "protos.StartBuildableTaskRequest")
 	proto.RegisterType((*LocalStorage)(nil), "protos.LocalStorage")
 	proto.RegisterType((*GitSource)(nil), "protos.GitSource")
 }
@@ -313,32 +373,36 @@
 func init() { proto.RegisterFile("relui.proto", fileDescriptor_6de8859f82adce0a) }
 
 var fileDescriptor_6de8859f82adce0a = []byte{
-	// 426 bytes of a gzipped FileDescriptorProto
-	0x1f, 0x8b, 0x08, 0x00, 0x00, 0x00, 0x00, 0x00, 0x02, 0xff, 0x6c, 0x52, 0x4d, 0x8f, 0xd3, 0x30,
-	0x10, 0xc5, 0xe9, 0x6e, 0xd9, 0x4c, 0x96, 0x52, 0x0c, 0x08, 0x8b, 0x0f, 0xa9, 0xf4, 0x54, 0xed,
-	0xa1, 0xa0, 0xc2, 0x01, 0x38, 0xac, 0x54, 0xa0, 0xe2, 0xb0, 0xa8, 0x8b, 0x9c, 0x54, 0x3d, 0x46,
-	0x6e, 0xe3, 0x56, 0x51, 0xbd, 0x71, 0x64, 0x3b, 0xac, 0xf2, 0x6b, 0xf9, 0x0b, 0xfc, 0x04, 0x64,
-	0xd7, 0x66, 0x4b, 0xc5, 0x29, 0xe3, 0xf7, 0xc6, 0x2f, 0xf3, 0xde, 0x18, 0x12, 0xc5, 0x45, 0x53,
-	0x8e, 0x6b, 0x25, 0x8d, 0xc4, 0x5d, 0xf7, 0xd1, 0xc3, 0x5f, 0x08, 0xce, 0x96, 0x52, 0xed, 0x36,
-	0x42, 0xde, 0x62, 0x0c, 0x27, 0x15, 0xbb, 0xe1, 0x04, 0x0d, 0xd0, 0x28, 0xa6, 0xae, 0xc6, 0x97,
-	0xf0, 0x70, 0xd5, 0x94, 0xa2, 0x60, 0x2b, 0xc1, 0x73, 0xc3, 0xf4, 0x4e, 0x93, 0x68, 0xd0, 0x19,
-	0x25, 0x93, 0xa7, 0x7b, 0x25, 0x3d, 0xfe, 0x1c, 0xe8, 0x8c, 0xe9, 0x1d, 0xed, 0xad, 0x0e, 0x8f,
-	0x1a, 0xbf, 0x87, 0x6e, 0xcd, 0x14, 0xbb, 0xd1, 0xa4, 0xe3, 0xae, 0xbd, 0x0c, 0xd7, 0xc2, 0x5f,
-	0xc7, 0x3f, 0x1c, 0x3d, 0xab, 0x8c, 0x6a, 0xa9, 0xef, 0xc5, 0x3d, 0x88, 0xca, 0x82, 0x9c, 0xb8,
-	0x39, 0xa2, 0xb2, 0x78, 0xfe, 0x11, 0x92, 0x83, 0x36, 0xdc, 0x87, 0xce, 0x8e, 0xb7, 0x7e, 0x4e,
-	0x5b, 0xe2, 0x27, 0x70, 0xfa, 0x93, 0x89, 0x86, 0x93, 0xc8, 0x61, 0xfb, 0xc3, 0xa7, 0xe8, 0x03,
-	0x1a, 0xfe, 0x46, 0xf0, 0xe0, 0x9f, 0x11, 0xff, 0x6b, 0xf3, 0x15, 0x40, 0xc1, 0x6b, 0x5e, 0x15,
-	0x3a, 0x97, 0x95, 0x17, 0x89, 0x3d, 0x72, 0x5d, 0xe1, 0x0b, 0xe8, 0x6a, 0xc3, 0x4c, 0x63, 0x5d,
-	0xa0, 0x51, 0x6f, 0x82, 0x83, 0x0b, 0x2b, 0x98, 0x3a, 0x86, 0xfa, 0x0e, 0xfc, 0x1a, 0xce, 0x99,
-	0x32, 0xe5, 0x86, 0xad, 0x4d, 0xde, 0x28, 0xe1, 0x5d, 0x24, 0x01, 0x5b, 0x28, 0x81, 0xdf, 0x02,
-	0x6c, 0x4b, 0x93, 0x6b, 0xd9, 0xa8, 0x35, 0x27, 0xa7, 0x03, 0x34, 0x4a, 0x26, 0x8f, 0x82, 0xe4,
-	0xb7, 0xd2, 0xa4, 0x8e, 0xa0, 0xf1, 0x36, 0x94, 0xf8, 0x05, 0xc4, 0x36, 0xfc, 0xdc, 0xb4, 0x35,
-	0x27, 0x5d, 0xa7, 0x78, 0x66, 0x81, 0xac, 0xad, 0xb9, 0x4f, 0xeb, 0x7e, 0x48, 0x6b, 0x78, 0x09,
-	0xe7, 0xdf, 0xe5, 0x9a, 0x89, 0xd4, 0x48, 0xc5, 0xb6, 0x1c, 0x8f, 0x21, 0xbe, 0xf5, 0x69, 0x6b,
-	0x82, 0xdc, 0x1a, 0xfa, 0xc7, 0x6b, 0xa0, 0x77, 0x2d, 0xc3, 0x37, 0x10, 0xff, 0x1d, 0xc2, 0x66,
-	0x6d, 0x5d, 0xf8, 0xac, 0x1b, 0x25, 0x2c, 0xa2, 0xf8, 0xc6, 0x87, 0x64, 0xcb, 0x8b, 0x25, 0xc0,
-	0x5d, 0x10, 0xf8, 0x19, 0x3c, 0xce, 0xa6, 0xe9, 0x55, 0x9e, 0x66, 0xd3, 0x6c, 0x91, 0xe6, 0x8b,
-	0xf9, 0xd5, 0xfc, 0x7a, 0x39, 0xef, 0xdf, 0x3b, 0x26, 0xbe, 0xd0, 0xd9, 0x34, 0x9b, 0x7d, 0xed,
-	0xa3, 0x63, 0x22, 0xcd, 0xa6, 0xd4, 0x12, 0xd1, 0x6a, 0xff, 0x4c, 0xdf, 0xfd, 0x09, 0x00, 0x00,
-	0xff, 0xff, 0xa2, 0xed, 0x2b, 0x74, 0xbc, 0x02, 0x00, 0x00,
+	// 486 bytes of a gzipped FileDescriptorProto
+	0x1f, 0x8b, 0x08, 0x00, 0x00, 0x00, 0x00, 0x00, 0x02, 0xff, 0x6c, 0x53, 0xdd, 0x6e, 0xd3, 0x30,
+	0x14, 0xc6, 0xe9, 0x56, 0x96, 0x93, 0xd1, 0xb5, 0x1e, 0x88, 0xf0, 0x27, 0x4a, 0xae, 0xaa, 0x5e,
+	0x04, 0x54, 0xb8, 0x00, 0x2e, 0x26, 0x15, 0xa8, 0xd0, 0x34, 0xd4, 0xa1, 0x24, 0x55, 0x2f, 0x23,
+	0xb7, 0x71, 0xab, 0xa8, 0x5e, 0x12, 0x6c, 0x87, 0xa9, 0x6f, 0xc2, 0xdb, 0xf1, 0x0a, 0x3c, 0x02,
+	0xb2, 0x6b, 0xb3, 0xa6, 0xe2, 0x2a, 0xc7, 0xe7, 0xfb, 0x7c, 0xfc, 0x9d, 0xef, 0x9c, 0x80, 0xc7,
+	0x29, 0xab, 0xf3, 0xb0, 0xe2, 0xa5, 0x2c, 0x71, 0x5b, 0x7f, 0x44, 0xf0, 0x1b, 0xc1, 0xc9, 0xbc,
+	0xe4, 0x9b, 0x15, 0x2b, 0x6f, 0x31, 0x86, 0xa3, 0x82, 0xdc, 0x50, 0x1f, 0xf5, 0xd1, 0xc0, 0x8d,
+	0x74, 0x8c, 0x2f, 0xe0, 0x6c, 0x51, 0xe7, 0x2c, 0x23, 0x0b, 0x46, 0x53, 0x49, 0xc4, 0x46, 0xf8,
+	0x4e, 0xbf, 0x35, 0xf0, 0x46, 0x8f, 0x76, 0x95, 0x44, 0xf8, 0xc9, 0xc2, 0x09, 0x11, 0x9b, 0xa8,
+	0xb3, 0xd8, 0x3f, 0x0a, 0xfc, 0x0e, 0xda, 0x15, 0xe1, 0xe4, 0x46, 0xf8, 0x2d, 0x7d, 0xed, 0xb9,
+	0xbd, 0x66, 0x5f, 0x0d, 0xbf, 0x6b, 0x78, 0x52, 0x48, 0xbe, 0x8d, 0x0c, 0x17, 0x77, 0xc0, 0xc9,
+	0x33, 0xff, 0x48, 0xeb, 0x70, 0xf2, 0xec, 0xe9, 0x07, 0xf0, 0xf6, 0x68, 0xb8, 0x0b, 0xad, 0x0d,
+	0xdd, 0x1a, 0x9d, 0x2a, 0xc4, 0x0f, 0xe1, 0xf8, 0x27, 0x61, 0x35, 0xf5, 0x1d, 0x9d, 0xdb, 0x1d,
+	0x3e, 0x3a, 0xef, 0x51, 0xf0, 0x07, 0xc1, 0x83, 0x86, 0xc4, 0xff, 0xb6, 0xf9, 0x02, 0x20, 0xa3,
+	0x15, 0x2d, 0x32, 0x91, 0x96, 0x85, 0x29, 0xe2, 0x9a, 0xcc, 0x75, 0x81, 0x87, 0xd0, 0x16, 0x92,
+	0xc8, 0x5a, 0x75, 0x81, 0x06, 0x9d, 0x11, 0xb6, 0x5d, 0xa8, 0x82, 0xb1, 0x46, 0x22, 0xc3, 0xc0,
+	0xaf, 0xe0, 0x94, 0x70, 0x99, 0xaf, 0xc8, 0x52, 0xa6, 0x35, 0x67, 0xa6, 0x0b, 0xcf, 0xe6, 0x66,
+	0x9c, 0xe1, 0x37, 0x00, 0xeb, 0x5c, 0xa6, 0xa2, 0xac, 0xf9, 0x92, 0xfa, 0xc7, 0x7d, 0x34, 0xf0,
+	0x46, 0x3d, 0x5b, 0xf2, 0x6b, 0x2e, 0x63, 0x0d, 0x44, 0xee, 0xda, 0x86, 0xf8, 0x19, 0xb8, 0xca,
+	0xfc, 0x54, 0x6e, 0x2b, 0xea, 0xb7, 0x75, 0xc5, 0x13, 0x95, 0x48, 0xb6, 0x15, 0x35, 0x6e, 0xdd,
+	0xb7, 0x6e, 0x05, 0xbf, 0x10, 0x3c, 0x89, 0x25, 0xe1, 0xb2, 0x39, 0x1a, 0xfa, 0xa3, 0xa6, 0x42,
+	0xe2, 0x97, 0xe0, 0xdd, 0x1a, 0xef, 0xd3, 0x3c, 0x33, 0x2e, 0x80, 0x4d, 0x5d, 0x66, 0x78, 0x08,
+	0xbd, 0xe6, 0xc8, 0x15, 0x6d, 0x67, 0xc9, 0x59, 0x63, 0xba, 0x97, 0x19, 0x0e, 0xe1, 0xfc, 0x80,
+	0xab, 0x15, 0xb6, 0x34, 0xbb, 0xd7, 0x60, 0x2b, 0xa9, 0xc1, 0x05, 0x9c, 0x7e, 0x2b, 0x97, 0x84,
+	0xc5, 0xb2, 0xe4, 0x64, 0x4d, 0x71, 0x08, 0xae, 0x7d, 0x59, 0xf8, 0x48, 0x6f, 0x48, 0xf7, 0x70,
+	0x43, 0xa2, 0x3b, 0x4a, 0xf0, 0x1a, 0xdc, 0x7f, 0xfe, 0xa8, 0x35, 0x50, 0x06, 0x9b, 0x35, 0xa8,
+	0x39, 0x53, 0x19, 0x4e, 0x57, 0x46, 0xac, 0x0a, 0x87, 0x73, 0x80, 0xbb, 0x19, 0xe1, 0xc7, 0x70,
+	0x9e, 0x8c, 0xe3, 0xab, 0x34, 0x4e, 0xc6, 0xc9, 0x2c, 0x4e, 0x67, 0xd3, 0xab, 0xe9, 0xf5, 0x7c,
+	0xda, 0xbd, 0x77, 0x08, 0x7c, 0x8e, 0x26, 0xe3, 0x64, 0xf2, 0xa5, 0x8b, 0x0e, 0x81, 0x38, 0x19,
+	0x47, 0x0a, 0x70, 0x16, 0xbb, 0x3f, 0xe8, 0xed, 0xdf, 0x00, 0x00, 0x00, 0xff, 0xff, 0xd3, 0xc2,
+	0xf2, 0x91, 0x57, 0x03, 0x00, 0x00,
 }
diff --git a/cmd/relui/protos/relui.proto b/cmd/relui/protos/relui.proto
index 7bad5db..49f3df6 100644
--- a/cmd/relui/protos/relui.proto
+++ b/cmd/relui/protos/relui.proto
@@ -47,6 +47,18 @@
   string id = 7;
 }
 
+// StartBuildableTaskRequest is a message sent to workers to start working on a BuildableTask for a Workflow.
+message StartBuildableTaskRequest {
+  // workflow_id is the workflow to which the BuildableTask belongs.
+  string workflow_id = 1;
+
+  // buildable_task_id is the id of the BuildableTask to be started.
+  string buildable_task_id = 2;
+
+  // buildable_task_type is the type of the BuildableTask to be started.
+  string buildable_task_type = 3;
+}
+
 // LocalStorage is the persisted data of relui. It is used in development mode for saving application state.
 message LocalStorage {
   // workflows are a list of user-created workflows.
diff --git a/cmd/relui/store.go b/cmd/relui/store.go
index acb2a4c..8dc838c 100644
--- a/cmd/relui/store.go
+++ b/cmd/relui/store.go
@@ -17,8 +17,10 @@
 
 // store is a persistence adapter for saving data.
 type store interface {
-	Workflows() []*reluipb.Workflow
 	AddWorkflow(workflow *reluipb.Workflow) error
+	BuildableTask(workflowId, id string) *reluipb.BuildableTask
+	Workflow(id string) *reluipb.Workflow
+	Workflows() []*reluipb.Workflow
 }
 
 var _ store = (*fileStore)(nil)
@@ -57,11 +59,33 @@
 	return nil
 }
 
-// Workflows returns all workflows stored.
+// Workflows returns all reluipb.Workflows stored.
 func (f *fileStore) Workflows() []*reluipb.Workflow {
 	return f.localStorage().GetWorkflows()
 }
 
+// Workflow returns a single reluipb.Workflow found by its id. If it is not found, it returns nil.
+func (f *fileStore) Workflow(id string) *reluipb.Workflow {
+	for _, w := range f.Workflows() {
+		if w.GetId() == id {
+			return w
+		}
+	}
+	return nil
+}
+
+// BuildableTask returns a single reluipb.BuildableTask found by the reluipb.Workflow id and its id.
+// If it is not found, it returns nil.
+func (f *fileStore) BuildableTask(workflowId, id string) *reluipb.BuildableTask {
+	wf := f.Workflow(workflowId)
+	for _, t := range wf.GetBuildableTasks() {
+		if t.GetId() == id {
+			return t
+		}
+	}
+	return nil
+}
+
 // localStorage returns a deep copy of data stored in fileStore.
 func (f *fileStore) localStorage() *reluipb.LocalStorage {
 	f.mu.Lock()
diff --git a/cmd/relui/web.go b/cmd/relui/web.go
index 4915253..3a9b7fd 100644
--- a/cmd/relui/web.go
+++ b/cmd/relui/web.go
@@ -122,6 +122,28 @@
 	http.Redirect(w, r, "/", http.StatusSeeOther)
 }
 
+func (s *server) startTaskHandler(w http.ResponseWriter, r *http.Request) {
+	wf := s.store.Workflow(r.PostFormValue("workflow.id"))
+	bt := s.store.BuildableTask(r.PostFormValue("workflow.id"), r.PostFormValue("task.id"))
+	if bt == nil {
+		http.Error(w, http.StatusText(http.StatusNotFound), http.StatusNotFound)
+		return
+	}
+	res := s.topic.Publish(r.Context(), &pubsub.Message{
+		Data: []byte((&reluipb.StartBuildableTaskRequest{
+			WorkflowId:        wf.GetId(),
+			BuildableTaskId:   bt.GetId(),
+			BuildableTaskType: bt.GetTaskType(),
+		}).String()),
+	})
+	if _, err := res.Get(r.Context()); err != nil {
+		log.Printf("Error publishing task start: %v", err)
+		http.Error(w, http.StatusText(http.StatusInternalServerError), http.StatusInternalServerError)
+		return
+	}
+	http.Redirect(w, r, "/", http.StatusSeeOther)
+}
+
 // relativeFile returns the path to the provided file or directory,
 // conditionally prepending a relative path depending on the environment.
 //
diff --git a/cmd/relui/web_test.go b/cmd/relui/web_test.go
index 7cab6e9..54991d5 100644
--- a/cmd/relui/web_test.go
+++ b/cmd/relui/web_test.go
@@ -5,6 +5,7 @@
 package main
 
 import (
+	"context"
 	"io/ioutil"
 	"net/http"
 	"net/http/httptest"
@@ -12,8 +13,12 @@
 	"strings"
 	"testing"
 
+	"cloud.google.com/go/pubsub"
+	"cloud.google.com/go/pubsub/pstest"
 	"github.com/google/go-cmp/cmp"
 	reluipb "golang.org/x/build/cmd/relui/protos"
+	"google.golang.org/api/option"
+	"google.golang.org/grpc"
 )
 
 func TestFileServerHandler(t *testing.T) {
@@ -171,3 +176,141 @@
 		})
 	}
 }
+
+// newPSTest creates a new pstest.Server and returns a pubsub.Client connected to it and the server.
+//
+// cleanup will close the client, connection, and test server.
+func newPSTest(ctx context.Context, t *testing.T) (c *pubsub.Client, s *pstest.Server, cleanup func()) {
+	t.Helper()
+	s = pstest.NewServer()
+	conn, err := grpc.DialContext(ctx, s.Addr, grpc.WithInsecure())
+	if err != nil {
+		s.Close()
+		t.Fatalf("grpc.DialContext(_, %q, %v) = _, %v, wanted no error", s.Addr, grpc.WithInsecure(), err)
+	}
+	c, err = pubsub.NewClient(ctx, "relui-test", option.WithGRPCConn(conn))
+	if err != nil {
+		s.Close()
+		conn.Close()
+		t.Fatalf("pubsub.NewClient(_, %q, %v) = _, %v, wanted no error", "relui-test", option.WithGRPCConn(conn), err)
+	}
+	return c, s, func() {
+		c.Close()
+		conn.Close()
+		s.Close()
+	}
+}
+
+func TestServerStartTaskHandler(t *testing.T) {
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+	client, pssrv, cleanup := newPSTest(ctx, t)
+	defer cleanup()
+	topic, err := client.CreateTopic(ctx, "relui-test-topic")
+	if err != nil {
+		t.Fatalf("client.CreateTopic(_, %q) = _, %v", "relui-test-topic", err)
+	}
+
+	s := server{store: newFileStore(""), topic: topic}
+	wf := &reluipb.Workflow{
+		Id:   "someworkflow",
+		Name: "test_workflow",
+		BuildableTasks: []*reluipb.BuildableTask{{
+			Name:     "test_task",
+			TaskType: "TestTask",
+			Id:       "sometask",
+		}},
+		Params: map[string]string{"GitObject": "master"},
+	}
+	if s.store.AddWorkflow(wf) != nil {
+		t.Fatalf("store.AddWorkflow(%v) = %v, wanted no error", wf, err)
+	}
+	want := &reluipb.StartBuildableTaskRequest{
+		WorkflowId:        "someworkflow",
+		BuildableTaskId:   "sometask",
+		BuildableTaskType: "TestTask",
+	}
+	params := url.Values{"workflow.id": []string{"someworkflow"}, "task.id": []string{"sometask"}}
+	req := httptest.NewRequest(http.MethodPost, "/tasks/start", strings.NewReader(params.Encode()))
+	req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
+
+	w := httptest.NewRecorder()
+	s.startTaskHandler(w, req)
+	resp := w.Result()
+
+	if resp.StatusCode != http.StatusSeeOther {
+		t.Errorf("resp.StatusCode = %d, wanted %d", resp.StatusCode, http.StatusSeeOther)
+	}
+	if resp.Header.Get("Location") != "/" {
+		t.Errorf("resp.Header.Get(%q) = %q, wanted %q", "Location", resp.Header.Get("Location"), "/")
+	}
+	if len(pssrv.Messages()) != 1 {
+		t.Fatalf("len(pssrv.Messages()) = %d, wanted %d", len(pssrv.Messages()), 1)
+	}
+	msg := pssrv.Messages()[0]
+	if string(msg.Data) != want.String() {
+		t.Errorf("msg.Data = %q, wanted %q", string(msg.Data), "hello world")
+	}
+}
+
+func TestStartTaskHandlerErrors(t *testing.T) {
+	wf := &reluipb.Workflow{
+		Id:   "someworkflow",
+		Name: "test_workflow",
+		BuildableTasks: []*reluipb.BuildableTask{{
+			Name:     "test_task",
+			TaskType: "TestTask",
+			Id:       "sometask",
+		}},
+		Params: map[string]string{"GitObject": "master"},
+	}
+
+	cases := []struct {
+		desc     string
+		params   url.Values
+		wantCode int
+	}{
+		{
+			desc:     "task not found",
+			params:   url.Values{"workflow.id": []string{"someworkflow"}, "task.id": []string{"notexist"}},
+			wantCode: http.StatusNotFound,
+		},
+		{
+			desc:     "pubsub publish failure",
+			params:   url.Values{"workflow.id": []string{"someworkflow"}, "task.id": []string{"sometask"}},
+			wantCode: http.StatusInternalServerError,
+		},
+	}
+	for _, c := range cases {
+		t.Run(c.desc, func(t *testing.T) {
+			ctx, cancel := context.WithCancel(context.Background())
+			defer cancel()
+			client, pssrv, cleanup := newPSTest(ctx, t)
+			defer cleanup()
+			topic, err := client.CreateTopic(ctx, "relui-test-topic")
+			if err != nil {
+				t.Fatalf("client.CreateTopic(_, %q) = _, %v", "relui-test-topic", err)
+			}
+			// Simulate pubsub failure by stopping publishing.
+			topic.Stop()
+
+			s := server{store: newFileStore(""), topic: topic}
+			if s.store.AddWorkflow(wf) != nil {
+				t.Fatalf("store.AddWorkflow(%v) = %v, wanted no error", wf, err)
+			}
+			req := httptest.NewRequest(http.MethodPost, "/tasks/start", strings.NewReader(c.params.Encode()))
+			req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
+
+			w := httptest.NewRecorder()
+			s.startTaskHandler(w, req)
+			resp := w.Result()
+
+			if resp.StatusCode != c.wantCode {
+				t.Errorf("resp.StatusCode = %d, wanted %d", resp.StatusCode, c.wantCode)
+			}
+			if len(pssrv.Messages()) != 0 {
+				t.Fatalf("len(pssrv.Messages()) = %d, wanted %d", len(pssrv.Messages()), 0)
+			}
+		})
+	}
+}