feat: agent reads bounded workload logs

agent/internal/workloads/logs.go

@@ -0,0 +1,83 @@
+package workloads
+
+import (
+	"context"
+	"fmt"
+	"os/exec"
+	"strconv"
+	"strings"
+	"time"
+)
+
+const (
+	// MaxLogLines and MaxLogBytes are BOTH enforced, whichever binds first.
+	//
+	// A line count alone does not bound size: 500 lines of a container printing
+	// 4KB JSON blobs is 2MB travelling over the bus. This is the same reasoning
+	// that gave workflow logs a per-line cap as well as a per-run one.
+	MaxLogLines = 500
+	MaxLogBytes = 256 * 1024
+
+	logTimeout = 60 * time.Second
+)
+
+// Logs returns a bounded snapshot of a workload's recent output.
+//
+// There is no follow mode. The browser console already offers a real terminal
+// on the same server where `docker logs -f` works properly, with its own
+// scrollback and cancellation. A snapshot answers "why did this restart",
+// which is the question that sends people to the console in the first place.
+func Logs(ctx context.Context, kind, id string, tail int) (string, bool, error) {
+	if tail <= 0 || tail > MaxLogLines {
+		tail = MaxLogLines
+	}
+
+	ctx, cancel := context.WithTimeout(ctx, logTimeout)
+	defer cancel()
+
+	var cmd *exec.Cmd
+	switch kind {
+	case "container":
+		cmd = exec.CommandContext(ctx, "docker", "logs",
+			"--tail", strconv.Itoa(tail), "--timestamps", id)
+	case "unit":
+		cmd = exec.CommandContext(ctx, "journalctl", "-u", id,
+			"-n", strconv.Itoa(tail), "--no-pager", "--output=short-iso")
+	default:
+		return "", false, fmt.Errorf("unknown workload kind %q", kind)
+	}
+
+	// docker logs writes container stderr to our stderr, so both streams must
+	// be captured or half the output silently disappears.
+	out, err := cmd.CombinedOutput()
+	if err != nil && len(out) == 0 {
+		return "", false, fmt.Errorf("read logs for %s: %s", id, errText(err))
+	}
+
+	text, truncated := capLog(string(out))
+	return text, truncated, nil
+}
+
+// capLog enforces both limits, trimming from the FRONT: the most recent lines
+// are the ones worth keeping.
+func capLog(s string) (string, bool) {
+	truncated := false
+
+	lines := strings.Split(s, "\n")
+	if len(lines) > MaxLogLines {
+		lines = lines[len(lines)-MaxLogLines:]
+		truncated = true
+	}
+	s = strings.Join(lines, "\n")
+
+	if len(s) > MaxLogBytes {
+		s = s[len(s)-MaxLogBytes:]
+		// Drop the leading partial line left by a byte-wise cut.
+		if i := strings.IndexByte(s, '\n'); i >= 0 {
+			s = s[i+1:]
+		}
+		truncated = true
+	}
+
+	return s, truncated
+}