| 1 | package sheets |
| 2 | |
| 3 | import ( |
| 4 | "html/template" |
| 5 | "sync" |
| 6 | ) |
| 7 | |
| 8 | // previewCache holds rendered thumbnails in memory. |
| 9 | // |
| 10 | // **In memory only, never in a column.** The preview contains no cell |
| 11 | // text — buildPreview cannot read one — so it is not plaintext even |
| 12 | // before you seal it, and a column would be safe. The reason not to |
| 13 | // have one is different and better: a preview column becomes a second |
| 14 | // thing to remember to seal later, and the plaintext-grep gate that |
| 15 | // guards the one rule structurally cannot catch it. A picture of a grid |
| 16 | // contains no fixture words, so that test would pass while the |
| 17 | // obligation quietly went unmet. An untestable obligation is the |
| 18 | // argument, not disclosure. (Docs reached this first and it is theirs.) |
| 19 | // |
| 20 | // **Invalidation is by construction, not by a call.** An entry is a hit |
| 21 | // only when the stored nanosecond timestamp equals the one on the row |
| 22 | // the list just read, so any write that bumps UpdatedAt invalidates it |
| 23 | // and there is no Forget() for a future write path to forget. Every |
| 24 | // write path in this app already bumps it: the cell POST, the format |
| 25 | // POST, import, and the two scriptless paths through publishReport. |
| 26 | // |
| 27 | // Two traps, both learned from Docs and both real: |
| 28 | // |
| 29 | // - Compare UnixNano int64s, NEVER time.Time with ==. A time parsed |
| 30 | // out of SQLite and a time.Now().UTC() in memory carry different |
| 31 | // monotonic and location fields, so == is never true and the cache |
| 32 | // silently never hits — it goes on rendering every row on every |
| 33 | // load and looks exactly like a cache that works. |
| 34 | // - Take the timestamp from the LIST QUERY's row, not from a struct |
| 35 | // that has been through a write path, which may not have written |
| 36 | // the field back. |
| 37 | type previewCache struct { |
| 38 | mu sync.RWMutex |
| 39 | at map[uint64]previewEntry |
| 40 | } |
| 41 | |
| 42 | type previewEntry struct { |
| 43 | nanos int64 |
| 44 | svg template.HTML |
| 45 | } |
| 46 | |
| 47 | // maxPreviewEntries bounds the map. |
| 48 | // |
| 49 | // A cache with no bound is a slow leak keyed on however many sheets an |
| 50 | // instance accumulates. The eviction is deliberately crude — past the |
| 51 | // bound the map is dropped whole — because the alternative is an LRU |
| 52 | // with a linked list, and the thing being protected is a few kilobytes |
| 53 | // of markup that costs two indexed queries to rebuild. |
| 54 | const maxPreviewEntries = 512 |
| 55 | |
| 56 | func newPreviewCache() *previewCache { |
| 57 | return &previewCache{at: map[uint64]previewEntry{}} |
| 58 | } |
| 59 | |
| 60 | // get returns the cached thumbnail for a sheet if it was rendered from |
| 61 | // exactly this version of it. |
| 62 | func (c *previewCache) get(sheetID uint64, nanos int64) (template.HTML, bool) { |
| 63 | if c == nil { |
| 64 | return "", false |
| 65 | } |
| 66 | c.mu.RLock() |
| 67 | e, ok := c.at[sheetID] |
| 68 | c.mu.RUnlock() |
| 69 | if !ok || e.nanos != nanos { |
| 70 | return "", false |
| 71 | } |
| 72 | return e.svg, true |
| 73 | } |
| 74 | |
| 75 | func (c *previewCache) put(sheetID uint64, nanos int64, svg template.HTML) { |
| 76 | if c == nil { |
| 77 | return |
| 78 | } |
| 79 | c.mu.Lock() |
| 80 | defer c.mu.Unlock() |
| 81 | if len(c.at) >= maxPreviewEntries { |
| 82 | c.at = map[uint64]previewEntry{} |
| 83 | } |
| 84 | c.at[sheetID] = previewEntry{nanos: nanos, svg: svg} |
| 85 | } |
| 86 | |
| 87 | // previewFor is what the list calls: cached if it can be, rendered if |
| 88 | // it cannot. |
| 89 | func (a *App) previewFor(s SheetSummary) template.HTML { |
| 90 | nanos := s.UpdatedAt.UnixNano() |
| 91 | if svg, ok := a.previews.get(s.ID, nanos); ok { |
| 92 | return svg |
| 93 | } |
| 94 | svg := previewSVG(a.buildPreview(s.TabID)) |
| 95 | a.previews.put(s.ID, nanos, svg) |
| 96 | return svg |
| 97 | } |
| 98 | |
amadan