Worker: make sure long lines are broken on character boundaries

When a command (like `blender` or `ffmpeg`) outputs lines that are longer
than our buffer, they are broken into buffer-sized chunks. Extra code has
been added to ensure those chunks consist of valid UTF-8 characters.

internal/worker/cli_runner/cli_runner.go

@@ -12,8 +12,9 @@ import (
 	"github.com/rs/zerolog"
 )
 
-// The buffer size used to read stdout/stderr output from subprocesses.
-// Effectively this determines the maximum line length that can be handled.
+// The buffer size used to read stdout/stderr output from subprocesses, in
+// bytes. Effectively this determines the maximum line length that can be
+// handled in one go. Lines that are longer will be broken up.
 const StdoutBufferSize = 40 * 1024
 
 // CLIRunner is a wrapper around exec.CommandContext() to allow mocking.
@@ -60,6 +61,12 @@ func (cli *CLIRunner) RunWithTextOutput(
 	// of simply returning, because the function must be run to completion in
 	// order to wait for processes (and not create defunct ones).
 	var returnErr error = nil
+
+	// If a line longer than our buffer is received, it will be trimmed to the
+	// bufffer length. This means that it may not end on a valid character
+	// boundary. Any leftover bytes are collected here, and prepended to the next
+	// line.
+	leftovers := []byte{}
 readloop:
 	for {
 		lineBytes, isPrefix, readErr := reader.ReadLine()
@@ -73,12 +80,24 @@ readloop:
 			break readloop
 		}
 
+		// Prepend any leftovers from the previous line to the received bytes.
+		if len(leftovers) > 0 {
+			lineBytes = append(leftovers, lineBytes...)
+			leftovers = []byte{}
+		}
+		// Make sure long lines are broken on character boundaries.
+		lineBytes, leftovers = splitOnCharacterBoundary(lineBytes)
+
 		line := string(lineBytes)
 		if isPrefix {
+			prefix := []rune(line)
+			if len(prefix) > 256 {
+				prefix = prefix[:256]
+			}
 			logger.Warn().
-				Str("line", fmt.Sprintf("%s...", line[:256])).
-				Int("lineLength", len(line)).
-				Msg("unexpectedly long line read, truncating")
+				Str("line", fmt.Sprintf("%s...", string(prefix))).
+				Int("bytesRead", len(lineBytes)).
+				Msg("unexpectedly long line read, will be split up")
 		}
 
 		logger.Debug().Msg(line)

internal/worker/cli_runner/strings.go

@@ -0,0 +1,31 @@
+package cli_runner
+
+import (
+	"unicode/utf8"
+)
+
+// splitOnCharacterBoundary splits `b` such that `valid` + `tail` = `b` and
+// `valid` is valid UTF-8.
+func splitOnCharacterBoundary(b []byte) (valid []byte, tail []byte) {
+	totalLength := len(b)
+	tailBytes := 0
+	for {
+		valid = b[:totalLength-tailBytes]
+		r, size := utf8.DecodeLastRune(valid)
+		switch {
+		case r == utf8.RuneError && size == 0:
+			// valid is empty, which means 'b' consists of only non-UTF8 bytes.
+			return valid, b
+		case r == utf8.RuneError && size == 1:
+			// The last bytes do not form a valid rune. See what happens if we move
+			// one byte from `valid` to `tail`.
+			tailBytes++
+			continue
+		case r == utf8.RuneError:
+			// This shouldn't happen, RuneError should only be returned with size 0 or 1.
+			panic(size)
+		default:
+			return valid, b[totalLength-tailBytes:]
+		}
+	}
+}

internal/worker/cli_runner/strings_test.go

@@ -0,0 +1,33 @@
+package cli_runner
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestSplitOnCharacterBoundary(t *testing.T) {
+	// Test with strings, as those are easier to type.
+	tests := []struct {
+		name      string
+		input     string
+		wantValid string
+		wantTail  string
+	}{
+		{"empty", "", "", ""},
+		{"trivial", "abc", "abc", ""},
+		{"valid", "Stüvel", "Stüvel", ""},
+		{"cats", "🐈🐈\xf0\x9f\x90\x88", "🐈🐈🐈", ""},
+		{"truncated-cats-1", "🐈🐈\xf0\x9f\x90", "🐈🐈", "\xf0\x9f\x90"},
+		{"truncated-cats-2", "🐈🐈\xf0\x9f", "🐈🐈", "\xf0\x9f"},
+		{"truncated-cats-3", "🐈🐈\xf0", "🐈🐈", "\xf0"},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			gotValid, gotTail := splitOnCharacterBoundary([]byte(tt.input))
+			assert.Equal(t, tt.input, string(gotValid)+string(gotTail))
+			assert.Equal(t, tt.wantValid, string(gotValid))
+			assert.Equal(t, tt.wantTail, string(gotTail))
+		})
+	}
+}