Manager: add endpoint to fetch task log tail

It returns 2048 bytes at most. It'll likely be less than that, as it will
ignore the first bytes until the very first newline (to avoid returning
cut-off lines). If the log file itself is 2048 bytes or smaller, return the
entire file.

internal/manager/api_impl/api_impl.go

@@ -103,6 +103,7 @@ type JobCompiler interface {
 type LogStorage interface {
 	Write(logger zerolog.Logger, jobID, taskID string, logText string) error
 	RotateFile(logger zerolog.Logger, jobID, taskID string)
+	Tail(jobID, taskID string) (string, error)
 }
 
 type ConfigService interface {

internal/manager/api_impl/jobs.go

@@ -12,6 +12,7 @@ import (
 	"git.blender.org/flamenco/internal/manager/job_compilers"
 	"git.blender.org/flamenco/internal/manager/persistence"
 	"git.blender.org/flamenco/internal/manager/webupdates"
+	"git.blender.org/flamenco/internal/uuid"
 	"git.blender.org/flamenco/pkg/api"
 )
 
@@ -178,6 +179,35 @@ func (f *Flamenco) SetTaskStatus(e echo.Context, taskID string) error {
 	return e.NoContent(http.StatusNoContent)
 }
 
+func (f *Flamenco) FetchTaskLogTail(e echo.Context, taskID string) error {
+	logger := requestLogger(e)
+	ctx := e.Request().Context()
+
+	logger = logger.With().Str("task", taskID).Logger()
+	if !uuid.IsValid(taskID) {
+		logger.Warn().Msg("fetchTaskLogTail: bad task ID ")
+		return sendAPIError(e, http.StatusBadRequest, "bad task ID")
+	}
+
+	dbTask, err := f.persist.FetchTask(ctx, taskID)
+	if err != nil {
+		if errors.Is(err, persistence.ErrTaskNotFound) {
+			return sendAPIError(e, http.StatusNotFound, "no such task")
+		}
+		logger.Error().Err(err).Msg("error fetching task")
+		return sendAPIError(e, http.StatusInternalServerError, "error fetching task: %v", err)
+	}
+
+	tail, err := f.logStorage.Tail(dbTask.Job.UUID, taskID)
+	if err != nil {
+		logger.Error().Err(err).Msg("unable to fetch task log tail")
+		return sendAPIError(e, http.StatusInternalServerError, "error fetching task log tail: %v", err)
+	}
+
+	logger.Debug().Msg("fetched task tail")
+	return e.String(http.StatusOK, tail)
+}
+
 func jobDBtoAPI(dbJob *persistence.Job) api.Job {
 	apiJob := api.Job{
 		SubmittedJob: api.SubmittedJob{

internal/manager/api_impl/mocks/api_impl_mock.gen.go

@@ -378,6 +378,21 @@ func (mr *MockLogStorageMockRecorder) RotateFile(arg0, arg1, arg2 interface{}) *
 	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "RotateFile", reflect.TypeOf((*MockLogStorage)(nil).RotateFile), arg0, arg1, arg2)
 }
 
+// Tail mocks base method.
+func (m *MockLogStorage) Tail(arg0, arg1 string) (string, error) {
+	m.ctrl.T.Helper()
+	ret := m.ctrl.Call(m, "Tail", arg0, arg1)
+	ret0, _ := ret[0].(string)
+	ret1, _ := ret[1].(error)
+	return ret0, ret1
+}
+
+// Tail indicates an expected call of Tail.
+func (mr *MockLogStorageMockRecorder) Tail(arg0, arg1 interface{}) *gomock.Call {
+	mr.mock.ctrl.T.Helper()
+	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "Tail", reflect.TypeOf((*MockLogStorage)(nil).Tail), arg0, arg1)
+}
+
 // Write mocks base method.
 func (m *MockLogStorage) Write(arg0 zerolog.Logger, arg1, arg2, arg3 string) error {
 	m.ctrl.T.Helper()

internal/manager/task_logs/task_logs.go

@@ -3,7 +3,9 @@ package task_logs
 // SPDX-License-Identifier: GPL-3.0-or-later
 
 import (
+	"bytes"
 	"fmt"
+	"io"
 	"os"
 	"path"
 	"path/filepath"
@@ -12,6 +14,11 @@ import (
 	"github.com/rs/zerolog/log"
 )
 
+const (
+	// tailSize is the maximum number of bytes read by the Tail() function.
+	tailSize int64 = 2048
+)
+
 // Storage can write data to task logs, rotate logs, etc.
 type Storage struct {
 	BasePath string // Directory where task logs are stored.
@@ -105,3 +112,54 @@ func (s *Storage) filepath(jobID, taskID string) string {
 	filename := fmt.Sprintf("task-%v.txt", taskID)
 	return path.Join(dirpath, filename)
 }
+
+// Tail reads the final few lines of a task log.
+func (s *Storage) Tail(jobID, taskID string) (string, error) {
+	filepath := s.filepath(jobID, taskID)
+
+	file, err := os.Open(filepath)
+	if err != nil {
+		return "", fmt.Errorf("unable to open log file for reading: %w", err)
+	}
+
+	fileSize, err := file.Seek(0, os.SEEK_END)
+	if err != nil {
+		return "", fmt.Errorf("unable to seek to end of log file: %w", err)
+	}
+
+	// Number of bytes to read.
+	var (
+		buffer   []byte
+		numBytes int
+	)
+	if fileSize <= tailSize {
+		// The file is small, just read all of it.
+		_, err = file.Seek(0, os.SEEK_SET)
+		if err != nil {
+			return "", fmt.Errorf("unable to seek to start of log file: %w", err)
+		}
+		buffer, err = io.ReadAll(file)
+	} else {
+		// Read the last 'tailSize' number of bytes.
+		_, err = file.Seek(-tailSize, os.SEEK_END)
+		if err != nil {
+			return "", fmt.Errorf("unable to seek in log file: %w", err)
+		}
+		buffer = make([]byte, tailSize)
+		numBytes, err = file.Read(buffer)
+
+		// Try to remove contents up to the first newline, as it's very likely we just
+		// seeked into the middle of a line.
+		firstNewline := bytes.IndexByte(buffer, byte('\n'))
+		if 0 <= firstNewline && firstNewline < numBytes-1 {
+			buffer = buffer[firstNewline+1:]
+		} else {
+			// The file consists of a single line of text; don't strip the first line.
+		}
+	}
+	if err != nil {
+		return "", fmt.Errorf("error reading log file: %w", err)
+	}
+
+	return string(buffer), nil
+}

internal/manager/task_logs/task_logs_test.go

@@ -3,6 +3,7 @@ package task_logs
 // SPDX-License-Identifier: GPL-3.0-or-later
 
 import (
+	"fmt"
 	"io/ioutil"
 	"os"
 	"path/filepath"
@@ -75,3 +76,60 @@ func TestLogRotation(t *testing.T) {
 	_, err = os.Stat(filename)
 	assert.True(t, os.IsNotExist(err))
 }
+
+func TestLogTail(t *testing.T) {
+	s := tempStorage()
+	defer os.RemoveAll(s.BasePath)
+
+	jobID := "25c5a51c-e0dd-44f7-9f87-74f3d1fbbd8c"
+	taskID := "20ff9d06-53ec-4019-9e2e-1774f05f170a"
+
+	err := s.Write(zerolog.Nop(), jobID, taskID, "Just a single line")
+	assert.NoError(t, err)
+	contents, err := s.Tail(jobID, taskID)
+	assert.NoError(t, err)
+	assert.Equal(t, "Just a single line\n", string(contents))
+
+	// A short file shouldn't do any line stripping.
+	err = s.Write(zerolog.Nop(), jobID, taskID, "And another line!")
+	assert.NoError(t, err)
+	contents, err = s.Tail(jobID, taskID)
+	assert.NoError(t, err)
+	assert.Equal(t, "Just a single line\nAnd another line!\n", string(contents))
+
+	bigString := ""
+	for lineNum := 1; lineNum < 1000; lineNum++ {
+		bigString += fmt.Sprintf("This is line #%d\n", lineNum)
+	}
+	err = s.Write(zerolog.Nop(), jobID, taskID, bigString)
+	assert.NoError(t, err)
+
+	contents, err = s.Tail(jobID, taskID)
+	assert.NoError(t, err)
+	assert.Equal(t,
+		"This is line #887\nThis is line #888\nThis is line #889\nThis is line #890\nThis is line #891\n"+
+			"This is line #892\nThis is line #893\nThis is line #894\nThis is line #895\nThis is line #896\n"+
+			"This is line #897\nThis is line #898\nThis is line #899\nThis is line #900\nThis is line #901\n"+
+			"This is line #902\nThis is line #903\nThis is line #904\nThis is line #905\nThis is line #906\n"+
+			"This is line #907\nThis is line #908\nThis is line #909\nThis is line #910\nThis is line #911\n"+
+			"This is line #912\nThis is line #913\nThis is line #914\nThis is line #915\nThis is line #916\n"+
+			"This is line #917\nThis is line #918\nThis is line #919\nThis is line #920\nThis is line #921\n"+
+			"This is line #922\nThis is line #923\nThis is line #924\nThis is line #925\nThis is line #926\n"+
+			"This is line #927\nThis is line #928\nThis is line #929\nThis is line #930\nThis is line #931\n"+
+			"This is line #932\nThis is line #933\nThis is line #934\nThis is line #935\nThis is line #936\n"+
+			"This is line #937\nThis is line #938\nThis is line #939\nThis is line #940\nThis is line #941\n"+
+			"This is line #942\nThis is line #943\nThis is line #944\nThis is line #945\nThis is line #946\n"+
+			"This is line #947\nThis is line #948\nThis is line #949\nThis is line #950\nThis is line #951\n"+
+			"This is line #952\nThis is line #953\nThis is line #954\nThis is line #955\nThis is line #956\n"+
+			"This is line #957\nThis is line #958\nThis is line #959\nThis is line #960\nThis is line #961\n"+
+			"This is line #962\nThis is line #963\nThis is line #964\nThis is line #965\nThis is line #966\n"+
+			"This is line #967\nThis is line #968\nThis is line #969\nThis is line #970\nThis is line #971\n"+
+			"This is line #972\nThis is line #973\nThis is line #974\nThis is line #975\nThis is line #976\n"+
+			"This is line #977\nThis is line #978\nThis is line #979\nThis is line #980\nThis is line #981\n"+
+			"This is line #982\nThis is line #983\nThis is line #984\nThis is line #985\nThis is line #986\n"+
+			"This is line #987\nThis is line #988\nThis is line #989\nThis is line #990\nThis is line #991\n"+
+			"This is line #992\nThis is line #993\nThis is line #994\nThis is line #995\nThis is line #996\n"+
+			"This is line #997\nThis is line #998\nThis is line #999\n",
+		string(contents),
+	)
+}