fix(render): bind identity into the epoch, titles into reserved strips, one layout per frame

Four review findings on the Task 7 renderer:

- InstanceStore::epoch() is derived from gl::Buffer::generation() -- advanced by
  every allocation, deletion and move -- plus the slot the instances were written
  to, so the orphan fallback of grow_persistent() can no longer leave the vertex
  array's attribute pointers bound to a deleted buffer: the epoch reports the new
  identity and upload() re-points them before the draw.
- PlotLayout reserves y_title_area/x_title_area outside axes (sized from the text
  metrics) and draw_titles() draws in them, so a title can no longer land on the
  data, whatever the envelope does.
- draw() computes plot.layout() once and hands it to the helpers, instead of six
  times per frame, each of which rebuilt the ticks and the grid.
- A refused fence is no longer dropped: mark_drawn() falls back to the orphan path
  and reports the failure rather than rotating through an unsynchronised slot.

New smoke cases force both refusals through the GL entry points (glMapBufferRange,
glFenceSync) and assert the pixels plus the live transfer mode; the title case
asserts the title colour is absent from the bands inside axes and present in both
strips. All three fail against the pre-fix sources.

include/plot/render/buffer.hpp

@@ -71,9 +71,13 @@ class InstanceStore {
 
   /// Records that the draw the caller just issued read the current slot, so the
   /// fence guarding that slot's reuse ends up in the command stream behind the
-  /// draw. A no-op in `Orphan` mode, where nothing reads the store after the
-  /// orphan.
-  void mark_drawn() noexcept;
+  /// draw. True when the slot is guarded, and always in `Orphan` mode, where
+  /// nothing reads the store after the orphan.
+  ///
+  /// False when the driver refused the fence: the slot would stay unsynchronised,
+  /// so the store drops the mapped ring for the orphan path instead of rotating
+  /// through it (the caller reports the failure -- see `Renderer`).
+  bool mark_drawn() noexcept;
 
   /// Byte offset the current slot's instances start at: 0 in `Orphan` mode,
   /// `slot * slot_bytes` in `PersistentMapped` mode. The vertex array re-points
@@ -84,7 +88,14 @@ class InstanceStore {
 
   /// Changes whenever the staged instances moved (a new slot) or the GL buffer
   /// was recreated, i.e. whenever attribute pointers into this store went stale.
-  u32 epoch() const noexcept { return epoch_; }
+  ///
+  /// Derived from the two things that can make them stale rather than bumped by
+  /// hand at every site that replaces the buffer object: `gl::Buffer::generation()`
+  /// advances on every allocation and deletion (so a fallback that re-creates the
+  /// buffer cannot leave the vertex array bound to a deleted one), and `slot_epoch_`
+  /// advances on every upload that lands in a new slot. Both counters only ever
+  /// increase, so their sum changes whenever either does, and never repeats.
+  u32 epoch() const noexcept { return buffer_.generation() + slot_epoch_; }
 
   void bind() const noexcept { buffer_.bind(); }
   bool persistent() const noexcept { return mode_ == UploadMode::PersistentMapped; }
@@ -103,7 +114,9 @@ class InstanceStore {
   u32 capacity_ = 0;        ///< instances the orphan store can hold
   usize slot_bytes_ = 0;    ///< bytes per slot of the persistent ring
   u32 slot_ = kSlots - 1u;  ///< last slot written; an upload advances it first
-  u32 epoch_ = 0;
+  /// Times an upload landed in a new slot. Kept apart from the buffer's own
+  /// identity (see `epoch()`) so neither has to know about the other.
+  u32 slot_epoch_ = 0;
   u8* mapped_ = nullptr;    ///< base of the mapped ring, null in `Orphan` mode
   gl::Fence fences_[kSlots];
 };
@@ -119,7 +132,7 @@ class LineBuffer {
   void upload(const f32* interleaved, u32 count) noexcept {
     store_.upload(interleaved, count, sizeof(LineInstance));
   }
-  void mark_drawn() noexcept { store_.mark_drawn(); }
+  bool mark_drawn() noexcept { return store_.mark_drawn(); }
   usize slot_offset() const noexcept { return store_.slot_offset(); }
   u32 epoch() const noexcept { return store_.epoch(); }
   bool persistent() const noexcept { return store_.persistent(); }
@@ -138,7 +151,7 @@ class TextBuffer {
   void upload(const f32* interleaved, u32 count) noexcept {
     store_.upload(interleaved, count, sizeof(TextInstance));
   }
-  void mark_drawn() noexcept { store_.mark_drawn(); }
+  bool mark_drawn() noexcept { return store_.mark_drawn(); }
   usize slot_offset() const noexcept { return store_.slot_offset(); }
   u32 epoch() const noexcept { return store_.epoch(); }
   bool persistent() const noexcept { return store_.persistent(); }

include/plot/render/renderer.hpp

@@ -58,7 +58,9 @@ struct FrameStats {
 /// persistent-mapped, fence-guarded ring on 4.3+/ARB_buffer_storage, and the
 /// orphan-and-refill baseline on 3.3. Setting `PLOT_FORCE_GL33_UPLOAD=1` in the
 /// environment forces the baseline path, which is how both are exercised on one
-/// machine.
+/// machine. A driver that refuses the storage, a map or a fence drops the store
+/// that hit it to the baseline path -- the ring is never rotated through without
+/// a guard -- and the frame reports it (see `last_frame_had_gl_error()`).
 ///
 /// Not thread-safe, and confined to the thread that owns the current context.
 class Renderer {
@@ -87,6 +89,10 @@ class Renderer {
   /// the renderer draws with -- set colours and metrics here, not on the plot.
   /// The plot also borrows this renderer's font (when it has no backend of its
   /// own), so the renderer must outlive the plot's use of it.
+  ///
+  /// `Plot::layout()` is computed exactly once, here, and handed to every helper:
+  /// it runs the auto scale, the strip fixed-point loop and the grid build, so
+  /// re-deriving it per helper would rebuild the ticks and the grid per helper.
   void draw(Plot& plot);
 
   /// Ends the frame, draining `glGetError()` in a loop so a stale error cannot be
@@ -107,7 +113,10 @@ class Renderer {
   render::Style& style() noexcept { return style_; }
   const render::Style& style() const noexcept { return style_; }
 
-  /// True when the last `end_frame()` found a GL error. Errors are drained in a
+  /// True when the last frame reported a GL failure: an error drained from
+  /// `glGetError()` in `end_frame()`, or a fence the driver refused, which the
+  /// store reports when the draw that should have been guarded is issued (such a
+  /// frame has already fallen back to the baseline path). Errors are drained in a
   /// loop, so a stale one cannot be attributed to a later frame.
   bool last_frame_had_gl_error() const noexcept { return gl_error_; }
 
@@ -115,9 +124,14 @@ class Renderer {
   const FrameStats& stats() const noexcept { return stats_; }
 
   bool ready() const noexcept { return ready_; }
-  /// True when `init()` selected the persistent-mapped transfer (4.3+ with
-  /// ARB_buffer_storage, and `PLOT_FORCE_GL33_UPLOAD` unset).
-  bool using_persistent_upload() const noexcept { return persistent_; }
+  /// True while both pipelines are uploading through the persistent-mapped ring:
+  /// `init()` selects it from the caps and the entry points (4.3+ with
+  /// ARB_buffer_storage, and `PLOT_FORCE_GL33_UPLOAD` unset), and a driver failure
+  /// later -- a refused map or fence -- leaves the store that hit it on the
+  /// baseline path, which this then says.
+  bool using_persistent_upload() const noexcept {
+    return line_vao_.persistent() && text_vao_.persistent();
+  }
   /// The axis text size the renderer actually draws with: `style().axis_font_px`
   /// rounded to the nearest whole multiple of `render::BitmapFont::kGlyphPx`,
   /// because a 1-bit glyph only scales cleanly by whole factors.
@@ -191,11 +205,15 @@ class Renderer {
 
   void draw_grid(const Plot& plot);
   void draw_axis_lines(const Plot& plot);
-  void draw_tick_labels(const Plot& plot);
-  void draw_titles(const Plot& plot);
-  void draw_traces(const Plot& plot);
-  void draw_cursors(const Plot& plot);
-  void draw_measures(const Plot& plot);
+  /// The chrome helpers take the frame's layout: `plot.layout()` recomputes the
+  /// ticks, the strips and the grid, so it belongs to `draw()` alone.
+  void draw_tick_labels(const Plot& plot, const PlotLayout& layout);
+  void draw_titles(const Plot& plot, const PlotLayout& layout);
+  /// The data helpers take it too; they read the axes rect and the transforms it
+  /// pins.
+  void draw_traces(const Plot& plot, const PlotLayout& layout);
+  void draw_cursors(const Plot& plot, const PlotLayout& layout);
+  void draw_measures(const Plot& plot, const PlotLayout& layout);
   void draw_annotations(const Plot& plot);
 
   render::BitmapFont font_;
@@ -218,7 +236,6 @@ class Renderer {
   u32 fb_w_ = 0;
   u32 fb_h_ = 0;
   bool ready_ = false;
-  bool persistent_ = false;
   bool gl_error_ = false;
   FrameStats stats_{};
 };

include/plot/render/vertex_array.hpp

@@ -31,9 +31,14 @@ class LineVertexArray {
 
   void bind() const noexcept { vao_.bind(); }
   /// Records that the draw just issued read the instance buffer's current slot,
-  /// which is what lets the mapped path reuse it safely. A no-op on the baseline
-  /// path, so a caller always calls it the same way.
-  void mark_drawn() noexcept { buffer_.mark_drawn(); }
+  /// which is what lets the mapped path reuse it safely. True in the baseline
+  /// mode, so a caller always calls it the same way. False only when the driver
+  /// refused the fence: the buffer has then fallen back to the baseline path, and
+  /// `persistent()` says so from the next call on.
+  bool mark_drawn() noexcept { return buffer_.mark_drawn(); }
+  /// True while this array's instance buffer is the mapped ring (see
+  /// `UploadMode`); a failure that falls back clears it.
+  bool persistent() const noexcept { return buffer_.persistent(); }
 
   bool valid() const noexcept { return vao_.id() != 0; }
   u32 instance_count() const noexcept { return instances_; }
@@ -57,7 +62,8 @@ class TextVertexArray {
   void upload(const f32* interleaved, u32 glyphs) noexcept;
 
   void bind() const noexcept { vao_.bind(); }
-  void mark_drawn() noexcept { buffer_.mark_drawn(); }
+  bool mark_drawn() noexcept { return buffer_.mark_drawn(); }
+  bool persistent() const noexcept { return buffer_.persistent(); }
 
   bool valid() const noexcept { return vao_.id() != 0; }
   u32 instance_count() const noexcept { return instances_; }