preload/host: Allow disabling spawning the latest version on re-exec

This is now controlled by ZYPAK_SPAWN_LATEST_ON_REEXEC, which will be
set to 0 by default at some point in the future.

Closes #19

README.md

@@ -15,12 +15,21 @@ This requires your Flatpak to be using:
 Now, instead of running your Electron binary directly, call it via
 `zypak-wrapper PATH/TO/MY/ELECTRON/BINARY`.
 
-## Usage with a wrapper script
+## Re-exec behavior
 
-If this is wrapping an application that requries some sort of wrapper script,
-make sure you set `CHROME_WRAPPER=` to the path of said script. Otherwise, if the
-application attempts to re-exec itself (i.e. `chrome://restart`), it won't be using
-the wrapper on re-exec, leading to potentially unexpected behavior.
+By default, Zypak will detect when the app is re-exec'ing itself and attempt to
+spawn the latest version, in order for `chrome://restart` to function. This can
+be disabled by setting `ZYPAK_SPAWN_LATEST_ON_REEXEC=0`. **This will be the
+default in the future once existing browsers are migrated,** because it matches
+more closely with what Electron would expect.
+
+### Usage with a wrapper script
+
+If `ZYPAK_SPAWN_LATEST_ON_REEXEC=1` (the current default) is active, and Zypak
+was invoked by some sort of wrapper script, make sure you set `CHROME_WRAPPER=`
+to the path of said script. Otherwise, if the application attempts to re-exec
+itself (i.e. `chrome://restart`), it won't be using the wrapper on re-exec,
+leading to potentially unexpected behavior.
 
 ## Widevine support
 

src/base/env.cc

@@ -39,11 +39,11 @@ void Env::Set(cstring_view name, cstring_view value, bool overwrite /*= true*/)
 void Env::Clear(cstring_view name) { ZYPAK_ASSERT(unsetenv(name.c_str()) == 0); }
 
 // static
-bool Env::Test(cstring_view name) {
+bool Env::Test(cstring_view name, bool default_value /*= false*/) {
   if (auto env = Get(name)) {
     return !env->empty() && *env != "0" && *env != "false";
   } else {
-    return false;
+    return default_value;
   }
 }
 

src/base/env.h

@@ -23,7 +23,7 @@ class Env {
   // Clears the given environment variable.
   static void Clear(cstring_view name);
   // Tests if the variable is set to a truthy value (i.e. not empty, 0, or false).
-  static bool Test(cstring_view name);
+  static bool Test(cstring_view name, bool default_value = false);
 
   static constexpr cstring_view kZypakBin = "ZYPAK_BIN";
   static constexpr cstring_view kZypakLib = "ZYPAK_LIB";
@@ -38,6 +38,7 @@ class Env {
   static constexpr cstring_view kZypakSettingSandboxFilename = "ZYPAK_SANDBOX_FILENAME";
   static constexpr cstring_view kZypakSettingExposeWidevinePath = "ZYPAK_EXPOSE_WIDEVINE_PATH";
   static constexpr cstring_view kZypakSettingLdPreload = "ZYPAK_LD_PRELOAD";
+  static constexpr cstring_view kZypakSettingSpawnLatestOnReexec = "ZYPAK_SPAWN_LATEST_ON_REEXEC";
 };
 
 }  // namespace zypak

src/preload/host/exec_zypak_sandbox.cc

@@ -49,36 +49,37 @@ DECLARE_OVERRIDE(int, execvp, const char* file, char* const* argv) {
     return -1;
   }
 
+  Env::Clear("LD_PRELOAD");
+
   if (file == SandboxPath::instance()->sandbox_path()) {
-    Env::Clear("LD_PRELOAD");
     file = "zypak-sandbox";
-  } else if (!HasTypeArg(argv)) {
-    Env::Clear("LD_PRELOAD");
-
-    if (IsCurrentExe(file)) {
-      // exec on the host exe, so pass it through the sandbox.
-      // "Leaking" calls to 'new' doesn't matter here since we're about to exec anyway.
-      std::vector<const char*> c_argv;
-      c_argv.push_back("zypak-helper");
+  } else if (!HasTypeArg(argv) && IsCurrentExe(file)) {
+    // exec on the host exe, so pass it through the sandbox.
+    // "Leaking" calls to 'new' doesn't matter here since we're about to exec anyway.
+    std::vector<const char*> c_argv;
+    c_argv.push_back("zypak-helper");
 
-      // Swap out the main binary to the wrapper if one was used, and assume the wrapper will use
-      // zypak-wrapper.sh itself (i.e. we don't need to handle it here).
+    if (Env::Test(Env::kZypakSettingSpawnLatestOnReexec, /*default_value=*/true)) {
       if (auto wrapper = Env::Get("CHROME_WRAPPER")) {
+        // Swap out the main binary to the wrapper if one was used, and assume the wrapper
+        // will use zypak-wrapper.sh itself (i.e. we don't need to handle it here).
         c_argv.push_back("exec-latest");
         c_argv.push_back(wrapper->data());
         argv++;
       } else {
         c_argv.push_back("host-latest");
       }
+    } else {
+      c_argv.push_back("host");
+    }
 
-      for (; *argv != nullptr; argv++) {
-        c_argv.push_back(*argv);
-      }
+    for (; *argv != nullptr; argv++) {
+      c_argv.push_back(*argv);
+    }
 
-      c_argv.push_back(nullptr);
+    c_argv.push_back(nullptr);
 
-      return original("zypak-helper", const_cast<char* const*>(c_argv.data()));
-    }
+    return original("zypak-helper", const_cast<char* const*>(c_argv.data()));
   }
 
   return original(file, argv);

src/preload/host/spawn_strategy/initialize.cc

@@ -23,10 +23,6 @@ int MAIN_OVERRIDE(int argc, char** argv, char** envp) {
   DebugContext::instance()->LoadFromEnvironment();
   DebugContext::instance()->set_name("preload-host-spawn-strategy");
 
-  // Unset LD_PRELOAD so exec'd processes don't call back into here again.
-  // This would particularly break anything that assumes only one thread is running.
-  unsetenv("LD_PRELOAD");
-
   dbus::Bus* bus = dbus::Bus::Acquire();
   ZYPAK_ASSERT(bus);