fix: render on a live context so the encoders are not killed at Start

evaluator.go

@@ -113,8 +113,14 @@ func Evaluate(ctx context.Context, tape string, out io.Writer, opts ...Evaluator
 	}
 
 	// Begin recording frames as we are now in a recording state.
-	ctx, cancel := context.WithCancel(ctx)
-	ch := v.Record(ctx)
+	//
+	// This context belongs to the recorder alone. teardown cancels it to stop
+	// the frame capture, and encoding runs after teardown, so the encoders must
+	// not inherit it: makeMedia builds its ffmpeg commands with
+	// exec.CommandContext, and a command started on a cancelled context is
+	// killed at Start before it writes anything.
+	recordCtx, cancel := context.WithCancel(ctx)
+	ch := v.Record(recordCtx)
 
 	// Clean up temporary files at the end.
 	defer func() {
@@ -141,9 +147,9 @@ func Evaluate(ctx context.Context, tape string, out io.Writer, opts ...Evaluator
 	}()
 
 	for _, cmd := range cmds[offset:] {
-		if ctx.Err() != nil {
+		if recordCtx.Err() != nil {
 			teardown()
-			return []error{ctx.Err()}
+			return []error{recordCtx.Err()}
 		}
 
 		// When changing the FontFamily, FontSize, LineHeight, Padding

render_test.go

@@ -0,0 +1,31 @@
+package main
+
+import (
+	"context"
+	"path/filepath"
+	"testing"
+)
+
+// TestRenderReportsEncodeFailure locks in two halves of the same regression.
+//
+// Render builds its encoders with exec.CommandContext, so a command started on
+// a context that is already done is killed at Start and writes nothing. Before
+// this was fixed, Evaluate handed Render the recording context, which teardown
+// had just cancelled, and Render logged the empty output and returned nil. VHS
+// printed "Creating out.gif...", exited 0, and produced no file
+// (https://github.com/charmbracelet/vhs/issues/787).
+//
+// A cancelled context stands in here for any failing encode: whatever the
+// cause, Render must report it rather than return nil.
+func TestRenderReportsEncodeFailure(t *testing.T) {
+	ctx, cancel := context.WithCancel(context.Background())
+	cancel()
+
+	v := New()
+	v.Options.Video.Output.GIF = filepath.Join(t.TempDir(), "out.gif")
+	v.totalFrames = 1
+
+	if err := v.Render(ctx); err == nil {
+		t.Fatal("Render returned nil for an encode that never ran; encode failures must reach the caller")
+	}
+}

vhs.go

@@ -568,6 +568,7 @@ func (vhs *VHS) Render(ctx context.Context) error {
 		out, err := cmd.CombinedOutput()
 		if err != nil {
 			log.Println(string(out))
+			return fmt.Errorf("%s: %w", filepath.Base(cmd.Path), err)
 		}
 	}