Read /etc/locale.conf when configuring the distro locale

src/linux/init/config.cpp

@@ -53,6 +53,7 @@ Module Name:
 #define HOSTS_FILE_PATH ETC_FOLDER "hosts"
 #define LANG_ENV "LANG"
 #define LOCALE_FILE_PATH ETC_DEFAULT_FOLDER "locale"
+#define LOCALE_CONF_FILE_PATH ETC_FOLDER "locale.conf"
 #define PATH_ENV "PATH"
 #define RESOLV_CONF_DIRECTORY_MODE 0755
 #define RESOLV_CONF_FILE_MODE 0644
@@ -2514,9 +2515,14 @@ void ConfigUpdateLanguage(EnvironmentBlock& Environment)
 
 Routine Description:
 
-    This routine queries the contents of the /etc/default/locale text file and
+    This routine queries the contents of the locale configuration file and
     if present updates the $LANG environment variable in the environment block.
 
+    Different distributions store this file in different locations:
+        - /etc/default/locale
+        - /etc/locale.conf
+    Both share the same "LANG=" line format, so the first file that exists is used.
+
 Arguments:
 
     Environment - Supplies the environment block pointer to update.
@@ -2530,22 +2536,33 @@ Return Value:
 try
 {
     //
-    // Attempt to open the /etc/default/locale file. If the file does not exist
-    // then the $LANG environment variable will not be updated.
+    // Attempt to open the locale configuration file, trying each known path in turn.
+    // If none of the files exist then the $LANG environment variable will not be updated.
     //
-    // N.B. This file is being opened by root. The only user-visible content
+    // N.B. These files are being opened by root. The only user-visible content
     //      will be the contents of the last line of the file that contains
     //      "LANG=".
     //
 
-    wil::unique_file LocaleFile{fopen(LOCALE_FILE_PATH, "r")};
-    if (!LocaleFile)
+    constexpr const char* LocaleFilePaths[] = {LOCALE_FILE_PATH, LOCALE_CONF_FILE_PATH};
+
+    wil::unique_file LocaleFile;
+    for (const auto* Path : LocaleFilePaths)
     {
+        LocaleFile.reset(fopen(Path, "r"));
+        if (LocaleFile)
+        {
+            break;
+        }
+
         if (errno != ENOENT)
         {
-            LOG_ERROR("fopen({}) failed {}", LOCALE_FILE_PATH, errno);
+            LOG_ERROR("fopen({}) failed {}", Path, errno);
         }
+    }
 
+    if (!LocaleFile)
+    {
         return;
     }
 

test/windows/UnitTests.cpp

@@ -516,6 +516,46 @@ class UnitTests
         }
     }
 
+    WSL2_TEST_METHOD(ConfigUpdateLanguage)
+    {
+        // Validates that init populates $LANG from the distro locale configuration file.
+        // ConfigUpdateLanguage reads /etc/default/locale first, then /etc/locale.conf, and uses
+        // the first file that exists. See ConfigUpdateLanguage in src/linux/init/config.cpp.
+
+        DistroFileChange defaultLocale(L"/etc/default/locale", LxsstuLaunchWsl(L"test -f /etc/default/locale") == 0);
+        DistroFileChange localeConf(L"/etc/locale.conf", LxsstuLaunchWsl(L"test -f /etc/locale.conf") == 0);
+
+        const auto readLang = []() { return LxsstuLaunchWslAndCaptureOutput(L"echo $LANG").first; };
+
+        // Only /etc/default/locale is present (Debian/Ubuntu).
+        {
+            defaultLocale.Delete();
+            localeConf.Delete();
+            defaultLocale.SetContent(L"LANG=de_DE.UTF-8\n");
+            TerminateDistribution();
+            VERIFY_ARE_EQUAL(readLang(), L"de_DE.UTF-8\n");
+        }
+
+        // Only /etc/locale.conf is present (Fedora, Arch, openSUSE, ...).
+        {
+            defaultLocale.Delete();
+            localeConf.Delete();
+            localeConf.SetContent(L"LANG=fr_FR.UTF-8\n");
+            TerminateDistribution();
+            VERIFY_ARE_EQUAL(readLang(), L"fr_FR.UTF-8\n");
+        }
+
+        // Both files are present: /etc/default/locale takes precedence because it is read first.
+        {
+            defaultLocale.Delete();
+            localeConf.Delete();
+            defaultLocale.SetContent(L"LANG=ja_JP.UTF-8\n");
+            localeConf.SetContent(L"LANG=en_US.UTF-8\n");
+            TerminateDistribution();
+            VERIFY_ARE_EQUAL(readLang(), L"ja_JP.UTF-8\n");
+        }
+    }
+
     TEST_METHOD(Dup)
     {
         VERIFY_NO_THROW(LxsstuRunTest(L"/data/test/wsl_unit_tests dup", L"Dup"));