Font symbol maps makes startup slower

[adithyagenie]

Issue

With a growing symbol map similar to this size:

symbol-map = [
  # Japanese kana
  { start = "3040", end = "309F", font-family = "Source Han Sans" },
  { start = "30A0", end = "30FF", font-family = "Source Han Sans" },
  { start = "31F0", end = "31FF", font-family = "Source Han Sans" },

  # Chinese phonetic scripts
  { start = "3100", end = "312F", font-family = "Source Han Sans TC" },
  { start = "31A0", end = "31BF", font-family = "Source Han Sans TC" },

  # Korean hangul
  { start = "1100", end = "11FF", font-family = "Source Han Sans K" },
  { start = "3130", end = "318F", font-family = "Source Han Sans K" },
  { start = "A960", end = "A97F", font-family = "Source Han Sans K" },
  { start = "AC00", end = "D7AF", font-family = "Source Han Sans K" },
  { start = "D7B0", end = "D7FF", font-family = "Source Han Sans K" },

  # Shared CJK symbols, punctuation, ideographs, radicals, and fullwidth forms
  { start = "2E80", end = "2EFF", font-family = "Source Han Sans" },
  { start = "2F00", end = "2FDF", font-family = "Source Han Sans" },
  { start = "3000", end = "303F", font-family = "Source Han Sans" },
  { start = "3190", end = "319F", font-family = "Source Han Sans" },
  { start = "31C0", end = "31EF", font-family = "Source Han Sans" },
  { start = "3200", end = "32FF", font-family = "Source Han Sans" },
  { start = "3300", end = "33FF", font-family = "Source Han Sans" },
  { start = "3400", end = "4DBF", font-family = "Source Han Sans" },
  { start = "4E00", end = "9FFF", font-family = "Source Han Sans" },
  { start = "F900", end = "FAFF", font-family = "Source Han Sans" },
  { start = "FE30", end = "FE4F", font-family = "Source Han Sans" },
  { start = "FF00", end = "FFEF", font-family = "Source Han Sans" },
  { start = "20000", end = "2A6DF", font-family = "Source Han Sans" },
  { start = "2A700", end = "2B73F", font-family = "Source Han Sans" },
  { start = "2B740", end = "2B81F", font-family = "Source Han Sans" },
  { start = "2B820", end = "2CEAF", font-family = "Source Han Sans" },
  { start = "2CEB0", end = "2EBEF", font-family = "Source Han Sans" },
  { start = "30000", end = "3134F", font-family = "Source Han Sans" },
  { start = "31350", end = "323AF", font-family = "Source Han Sans" },

  # Emoji and emoji presentation helpers
  { start = "20E3", end = "20E3", font-family = "Noto Color Emoji" },
  { start = "FE00", end = "FE0F", font-family = "Noto Color Emoji" },
  { start = "1F100", end = "1F1FF", font-family = "Noto Color Emoji" },
  { start = "1F300", end = "1F5FF", font-family = "Noto Color Emoji" },
  { start = "1F600", end = "1F64F", font-family = "Noto Color Emoji" },
  { start = "1F680", end = "1F6FF", font-family = "Noto Color Emoji" },
  { start = "1F700", end = "1F77F", font-family = "Noto Color Emoji" },
  { start = "1F780", end = "1F7FF", font-family = "Noto Color Emoji" },
  { start = "1F900", end = "1F9FF", font-family = "Noto Color Emoji" },
]

startup seems to be ~4 seconds slower.

The INFO logs during the wait time seems to be a sequential font search:

2026-07-25T06:06:41.279727Z  INFO sugarloaf::font: Font search: 'Query { families: [Name("Source Han Sans")], weight: Weight(400), stretch: Normal, style: Normal }' style_override=None
2026-07-25T06:06:41.418732Z  INFO sugarloaf::font: Font 'Source Han Sans' found in /nix/store/jh33d5z7i1chzc0wahvmnhq58agy2xpv-source-han-sans-2.005/share/fonts/opentype/source-han-sans/SourceHanSans.ttc

Specs

• Rioterm - v0.4.7 (v0.4.12 wasn't there on Nix yet)
• NixOS 26.05 - Linux 6.18.38
• Vulkan renderer

[renderer]
backend = "Vulkan"
disable-occluded-render = false
disable-unfocused-render = false

Please let me know if any more logs are needed.

[adithyagenie]

A quick diagnostic that GLM gave me, feel free to ignore if it's wrong:

Where

sugarloaf/src/font/mod.rs, FontLibraryData::load:

• :980-1017 — the symbol-map loop calls resolve() once per range, with no deduplication by font-family. The TODO at :965 says it outright: "it will naively just extend fonts from symbol_map without even look if the font has been loaded before."
• resolve() → find_font() (:1712, non-macOS) → db.query(&query) (:1742).
• Database::query is in sugarloaf/src/font/loader.rs:367. For every query it calls font-kit's SystemSource::all_families() (:450), select_family_by_name, and — the expensive part — handle.load() (:425 and :463), which re-parses the font file to read its properties on every call.
• The "Font search:" log is at mod.rs:1737; "Font … found in" at :1758.

Why it's slow

Each Source Han Sans range re-parses SourceHanSans.ttc (large multi-sub-face CJK font) inside handle.load(). Your log shows it exactly — every "Font search:" → "found in" pair is ~138 ms, repeated identically 22 times for the same family:

06:27:46.273  Font search: 'Query { … Source Han Sans … }'
06:27:46.412  Font 'Source Han Sans' found in …/SourceHanSans.ttc   ← +139 ms
06:27:46.412  Font search: 'Query { … Source Han Sans … }'
06:27:46.550  Font 'Source Han Sans' found in …/SourceHanSans.ttc   ← +138 ms
… (20 more, all ~138 ms) …

Noto Color Emoji queries are ~2–4 ms each (small file) — confirming the cost scales with font-file parse time × number of ranges mapping to it, not with fontconfig enumeration overhead (which would be roughly constant per query).

FONT_DATA_CACHE (mod.rs:71, populated by load_from_font_source at :1954) does not help: it only caches sugarloaf's own mmap'd bytes after db.query() has already returned. The 138 ms lives in font-kit's handle.load(), a separate code path with no cache, so the 2nd…22nd Source Han Sans query costs the same as the 1st.

Approximate cost from the log: 22 × 138 ms (SHS) + 6 × 138 ms (SHS K) + 2 × 138 ms (TC) + 10 × ~3 ms (emoji) ≈ 3.8 s, matching the 46.273 → 50.308 stall window. All synchronous on the init thread → single core maxed.