Guest fonts: give the runtime Android's fonts for Chinese, Japanese and Korean (#118, #206, #249)

The runtime ships only DejaVu and Adwaita, so Steam and the desktop drew
Chinese, Japanese and Korean (and Thai) as boxes. Android already has Noto
for all of them in /system/fonts.

- /system/fonts is bound read-only at /usr/share/fonts/android, where
  fontconfig finds it with the rest of /usr/share/fonts. Nothing is
  downloaded and the runtime image is not changed.
- At every session start, conf.d/65-droiddeck-language.conf keeps DejaVu
  first for Latin text and puts the Noto CJK face for the device's
  language next (JP, KR, SC, TC or HK), so shared characters get that
  region's shapes.

app/src/main/java/com/droiddeck/launcher/runtime/GuestFonts.kt

@@ -0,0 +1,73 @@
+package com.droiddeck.launcher.runtime
+
+import android.content.Context
+import java.io.File
+import java.util.Locale
+
+/**
+ * Fonts for every script Steam and the desktop may be shown in.
+ *
+ * The runtime ships DejaVu and Adwaita only, which have no Chinese, Japanese, Korean or Thai, so
+ * Steam drew those as boxes (#118, #206, #249). Android already carries Noto for all of them in
+ * /system/fonts; the guest gets that folder read-only at [GUEST_DIR], where fontconfig finds it
+ * with the rest of /usr/share/fonts. Nothing is downloaded and the runtime is not changed.
+ *
+ * One CJK font holds Chinese, Japanese and Korean under one set of characters, and a character
+ * they share is drawn the way the chosen region writes it. [preferenceConf] puts that region's
+ * variant first, after DejaVu, which stays the font for Latin text.
+ */
+object GuestFonts {
+    const val HOST_DIR = "/system/fonts"
+    const val GUEST_DIR = "/usr/share/fonts/android"
+    /** Rewritten at every session start, so it follows the device's language. */
+    const val CONF = "etc/fonts/conf.d/65-droiddeck-language.conf"
+
+    /** The bind spec for Android's fonts, or null on a device without the folder. */
+    @JvmStatic
+    fun bindSpec(): String? = File(HOST_DIR).takeIf { it.isDirectory }?.let { "${it.path}:$GUEST_DIR" }
+
+    /** Writes the guest's font preference for the device's current language; best effort. */
+    @JvmStatic
+    fun prepare(context: Context, root: File) {
+        val conf = File(root, CONF)
+        val text = preferenceConf(context.resources.configuration.locales[0])
+        try {
+            if (conf.isFile && conf.readText() == text) return
+            conf.parentFile?.mkdirs()
+            conf.writeText(text)
+        } catch (e: Exception) {
+            // fontconfig still finds every font; shared characters just take its own default shape.
+        }
+    }
+
+    /** The CJK variant whose shapes match how [locale]'s readers write shared characters. */
+    fun cjkRegion(locale: Locale): String {
+        val script = locale.script
+        return when (locale.language) {
+            "ja" -> "JP"
+            "ko" -> "KR"
+            "zh" -> when {
+                script == "Hans" -> "SC"
+                locale.country == "HK" || locale.country == "MO" -> "HK"
+                script == "Hant" || locale.country == "TW" -> "TC"
+                else -> "SC"
+            }
+            else -> "SC"
+        }
+    }
+
+    fun preferenceConf(locale: Locale): String {
+        val region = cjkRegion(locale)
+        fun alias(generic: String, latin: String, cjk: String) =
+            "  <alias binding=\"same\">\n    <family>$generic</family>\n    <prefer>\n" +
+                "      <family>$latin</family>\n      <family>$cjk $region</family>\n" +
+                "    </prefer>\n  </alias>\n"
+        return "<?xml version=\"1.0\"?>\n<!DOCTYPE fontconfig SYSTEM \"urn:fontconfig:fonts.dtd\">\n" +
+            "<!-- Written by DroidDeck at session start for the device's language (${locale.toLanguageTag()}). -->\n" +
+            "<fontconfig>\n" +
+            alias("sans-serif", "DejaVu Sans", "Noto Sans CJK") +
+            alias("serif", "DejaVu Serif", "Noto Serif CJK") +
+            alias("monospace", "DejaVu Sans Mono", "Noto Sans Mono CJK") +
+            "</fontconfig>\n"
+    }
+}

app/src/main/java/com/droiddeck/launcher/runtime/LinuxRuntime.java

@@ -270,6 +270,12 @@ public final class LinuxRuntime {
         if (extraBinds != null) {
             for (String spec : extraBinds) bind(cmd, spec);
         }
+        // Android's fonts, for the scripts the runtime's own fonts lack (GuestFonts).
+        String fonts = GuestFonts.bindSpec();
+        if (fonts != null) {
+            bind(cmd, fonts);
+            GuestFonts.prepare(context, root);
+        }
         List<String> specs = new ArrayList<>();
         for (int i = 0; i + 1 < cmd.size(); i += 2) specs.add(cmd.get(i + 1));
         return specs;

app/src/test/java/com/droiddeck/launcher/runtime/GuestFontsTest.kt

@@ -0,0 +1,31 @@
+package com.droiddeck.launcher.runtime
+
+import java.util.Locale
+import org.junit.Assert.assertEquals
+import org.junit.Assert.assertTrue
+import org.junit.Test
+
+class GuestFontsTest {
+    private fun region(tag: String) = GuestFonts.cjkRegion(Locale.forLanguageTag(tag))
+
+    @Test fun sharedCharactersTakeTheReadersRegionalShapes() {
+        assertEquals("JP", region("ja"))
+        assertEquals("KR", region("ko"))
+        assertEquals("SC", region("zh-CN"))
+        assertEquals("TC", region("zh-TW"))
+        assertEquals("HK", region("zh-HK"))
+        assertEquals("HK", region("zh-MO"))
+        assertEquals("SC", region("zh-Hans-HK"))
+        assertEquals("TC", region("zh-Hant"))
+        assertEquals("SC", region("en"))
+    }
+
+    @Test fun latinStaysOnDejaVuWithTheRegionsCjkNext() {
+        val conf = GuestFonts.preferenceConf(Locale.JAPANESE)
+        val sans = conf.substringAfter("<family>sans-serif</family>").substringBefore("</alias>")
+        assertTrue(sans, sans.indexOf("DejaVu Sans") in 0 until sans.indexOf("Noto Sans CJK JP"))
+        assertTrue(conf.contains("<family>Noto Serif CJK JP</family>"))
+        assertTrue(conf.contains("<family>Noto Sans Mono CJK JP</family>"))
+        assertTrue(conf.startsWith("<?xml"))
+    }
+}