feat(share-card): 字体自动发现 + 启动探测，缺字体不再等到第一次请求才 500
- 系统候选补上 glyf 轮廓的 CJK 字体（文泉驿正黑/微米黑、DroidSansFallback、 AR PL UMing），并把 Noto Sans CJK 压到最后：它是 CFF/OTTO，freetype 必定解析失败， 排前面只会白白多读十几 MB - static/fonts 同时按当前工作目录与可执行文件目录查找，systemd 没配 WorkingDirectory 时也能找到（原先只按 CWD，文件在却报「没有可用字体」） - 启动时探一次字体：命中打 Info 带路径，缺失打 WARN 并给出补救命令 （apt install fonts-wqy-zenhei / 放 static/fonts/ / 配 font_path） - .ttc 先提取首个字面再用（仅对 glyf 轮廓的集合有效，如文泉驿）； 失败结果缓存 1 分钟，避免公开端点被「每请求重新解析十几 MB」放大 实测：本地实例 apt install fonts-wqy-zenhei 后无需任何配置，启动即打出 font=/usr/share/fonts/truetype/wqy/wqy-zenhei.ttc，/share-card 与单题分享图 从 500 变为 200 image/jpeg，中文与二维码渲染正常。
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tamakyi
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package question
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import (
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"encoding/binary"
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"testing"
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"github.com/stretchr/testify/require"
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)
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// buildTTC 拼一个最小的 TrueType Collection：两个字面共用一套表，
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// 表数据刻意按「与目录相反」的顺序摆放，用来证明提取逻辑是按目录里的 offset
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// 搬数据，而不是简单地顺序切片。
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func buildTTC(t *testing.T) []byte {
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t.Helper()
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// 表内容
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headData := []byte{0xDE, 0xAD, 0xBE, 0xEF}
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cmapData := []byte{0x01, 0x02, 0x03, 0x04, 0x05, 0x06}
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const ttcHeaderLen = 16
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const sfntHeaderLen = 12
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const recLen = 16
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const numTables = 2
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sfntStart := ttcHeaderLen
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dirStart := sfntStart + sfntHeaderLen
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dataStart := dirStart + recLen*numTables
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// 先放 cmap 再放 head（与目录顺序相反）
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cmapOff := dataStart
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headOff := cmapOff + len(cmapData)
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buf := make([]byte, headOff+len(headData))
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copy(buf[0:4], "ttcf")
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binary.BigEndian.PutUint32(buf[4:8], 0x00010000) // version
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binary.BigEndian.PutUint32(buf[8:12], 1) // numFonts
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binary.BigEndian.PutUint32(buf[12:16], uint32(sfntStart))
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binary.BigEndian.PutUint32(buf[sfntStart:sfntStart+4], 0x00010000) // sfntVersion
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binary.BigEndian.PutUint16(buf[sfntStart+4:sfntStart+6], numTables)
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binary.BigEndian.PutUint16(buf[sfntStart+6:sfntStart+8], 16) // searchRange（随便填，提取时应重算）
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binary.BigEndian.PutUint16(buf[sfntStart+8:sfntStart+10], 0)
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binary.BigEndian.PutUint16(buf[sfntStart+10:sfntStart+12], 0)
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// 第 1 条记录：head → 数据在末尾
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p := dirStart
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copy(buf[p:p+4], "head")
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binary.BigEndian.PutUint32(buf[p+4:p+8], 0x11111111) // checksum
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binary.BigEndian.PutUint32(buf[p+8:p+12], uint32(headOff))
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binary.BigEndian.PutUint32(buf[p+12:p+16], uint32(len(headData)))
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// 第 2 条记录：cmap → 数据在前
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p = dirStart + recLen
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copy(buf[p:p+4], "cmap")
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binary.BigEndian.PutUint32(buf[p+4:p+8], 0x22222222)
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binary.BigEndian.PutUint32(buf[p+8:p+12], uint32(cmapOff))
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binary.BigEndian.PutUint32(buf[p+12:p+16], uint32(len(cmapData)))
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copy(buf[cmapOff:], cmapData)
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copy(buf[headOff:], headData)
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return buf
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}
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func TestTTCFirstFont(t *testing.T) {
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ttc := buildTTC(t)
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out, err := ttcFirstFont(ttc)
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require.NoError(t, err)
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// sfnt header
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require.Equal(t, uint32(0x00010000), binary.BigEndian.Uint32(out[0:4]))
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require.Equal(t, uint16(2), binary.BigEndian.Uint16(out[4:6]))
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// searchRange = 最大 2 的幂 × 16 = 2 × 16 = 32；entrySelector = 1；rangeShift = 32-32 = 0
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require.Equal(t, uint16(32), binary.BigEndian.Uint16(out[6:8]))
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require.Equal(t, uint16(1), binary.BigEndian.Uint16(out[8:10]))
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require.Equal(t, uint16(0), binary.BigEndian.Uint16(out[10:12]))
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// 目录：tag / checksum / length 原样保留，offset 指向新位置
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type rec struct {
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tag string
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checksum uint32
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off, ln uint32
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}
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var recs []rec
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for i := 0; i < 2; i++ {
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p := 12 + 16*i
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recs = append(recs, rec{
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tag: string(out[p : p+4]),
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checksum: binary.BigEndian.Uint32(out[p+4 : p+8]),
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off: binary.BigEndian.Uint32(out[p+8 : p+12]),
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ln: binary.BigEndian.Uint32(out[p+12 : p+16]),
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})
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}
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require.Equal(t, "head", recs[0].tag)
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require.Equal(t, "cmap", recs[1].tag)
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require.Equal(t, uint32(0x11111111), recs[0].checksum)
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require.Equal(t, uint32(0x22222222), recs[1].checksum)
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// 表数据按新 offset 读出来必须与原始一致
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require.Equal(t, []byte{0xDE, 0xAD, 0xBE, 0xEF},
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out[recs[0].off:recs[0].off+recs[0].ln])
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require.Equal(t, []byte{0x01, 0x02, 0x03, 0x04, 0x05, 0x06},
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out[recs[1].off:recs[1].off+recs[1].ln])
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// 每张表在新文件里都要 4 字节对齐（sfnt 规范要求）
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require.Zero(t, recs[0].off%4)
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require.Zero(t, recs[1].off%4)
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}
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func TestTTCFirstFontRejects(t *testing.T) {
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cases := map[string][]byte{
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"空数据": nil,
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"不是 TTC": []byte("not a font file at all"),
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"头部被截断": []byte("ttcf\x00\x01\x00\x00"),
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"numFonts 为 0": func() []byte {
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b := make([]byte, 16)
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copy(b, "ttcf")
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binary.BigEndian.PutUint32(b[4:8], 0x00010000)
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return b
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}(),
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"子字体偏移越界": func() []byte {
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b := make([]byte, 16)
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copy(b, "ttcf")
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binary.BigEndian.PutUint32(b[4:8], 0x00010000)
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binary.BigEndian.PutUint32(b[8:12], 1)
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binary.BigEndian.PutUint32(b[12:16], 4096)
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return b
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}(),
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}
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for name, data := range cases {
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t.Run(name, func(t *testing.T) {
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_, err := ttcFirstFont(data)
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require.Error(t, err)
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})
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}
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}
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// TestTTCFirstFontRejectsBadTableOffset 表目录里的 offset 指向文件外时必须报错，
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// 而不是把越界切片交给上层（否则就是一次 panic 而不是 500）。
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func TestTTCFirstFontRejectsBadTableOffset(t *testing.T) {
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ttc := buildTTC(t)
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// 第 1 条记录位于 ttcHeader(16) + sfntHeader(12)，其 length 字段在记录内第 12 字节
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p := 16 + 12 + 12
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binary.BigEndian.PutUint32(ttc[p:p+4], 1<<20)
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_, err := ttcFirstFont(ttc)
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require.Error(t, err)
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}
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