[Fix] (Client): step aside for processes that ask for software rendering

MobileGL/Init.cpp

@@ -238,6 +238,18 @@ namespace MobileGL {
 #endif
             return true;
         }
+#if defined(__linux__) && !defined(__ANDROID__)
+        // LIBGL_ALWAYS_SOFTWARE is how a process (or whoever launched it) asks the system's GL for a
+        // software renderer. MobileGL is a GPU forwarder, so it steps aside and lets the system's
+        // other implementation give the process what it asked for.
+        {
+            String software;
+            if (MG_ConfigLoader::LookupSetting("LIBGL_ALWAYS_SOFTWARE", software) && !software.empty() &&
+                software != "0" && software != "false") {
+                return false;
+            }
+        }
+#endif
 #if MOBILEGL_BUILD_DISAGGREGATED
         // Not yet: the bring-up would dial the configured server and wait for it. Ask first.
         return MG_Remote::Client::ConfiguredServerAvailable();

MobileGL/Init.h

@@ -32,8 +32,9 @@ namespace MobileGL {
         Bool LookupSetting(const char* key, String& outValue);
     } // namespace MG_ConfigLoader
 
-    // Whether this process can be served right now. False only when MobileGL is configured to
-    // reach a server that is not there (or a split session already failed to come up), so a
+    // Whether this process can be served right now. False when MobileGL is configured to reach a
+    // server that is not there (or a split session already failed to come up), or when the process
+    // asked for software rendering (LIBGL_ALWAYS_SOFTWARE, desktop Linux), so a
     // caller that can choose another implementation - the system's EGL/GLX/GBM loaders - should
     // decline rather than hand out something that cannot draw. Cheap: once true it stays true
     // until the library is torn down, and a probe is one non-blocking connect with a short bound.

MobileGL/MG_Test/Wire/ServerProbeTest.cpp

@@ -194,7 +194,7 @@ namespace {
         Int64 elapsedMs = -1;      // all of the above, end to end
     };
 
-    EntryReport RunEntryPoints(const std::string& control, const std::string& logBase) {
+    EntryReport RunEntryPoints(const std::string& control, const std::string& logBase, bool software = false) {
         EntryReport report{};
         int fds[2] = {-1, -1};
         if (::pipe(fds) != 0) return report;
@@ -206,6 +206,8 @@ namespace {
             ::setenv("MOBILEGL_TRANSPORT", "spawn", 1);
             ::setenv("MOBILEGL_IPC_CONTROL", control.c_str(), 1);
             ::setenv("MOBILEGL_BACKEND_TYPE", "DirectGLES", 1);
+            if (software) ::setenv("LIBGL_ALWAYS_SOFTWARE", "1", 1);
+            else ::unsetenv("LIBGL_ALWAYS_SOFTWARE");
             EntryReport r{};
             const auto start = std::chrono::steady_clock::now();
 
@@ -311,6 +313,13 @@ namespace {
         EXPECT_EQ(r.getDisplay, 1);
         EXPECT_EQ(r.vendorDevices, 1);
         EXPECT_EQ(r.glxLoaded, 1);
+
+        // A process that asked for software rendering is left to the system's other GL, server or not.
+        const EntryReport software = RunEntryPoints(server.endpoint, logBase, true);
+        EXPECT_EQ(software.vendorDisplay, 0);
+        EXPECT_EQ(software.getDisplay, 0);
+        EXPECT_EQ(software.vendorDevices, 0);
+        EXPECT_EQ(software.glxLoaded, 0);
         std::error_code ec;
         std::filesystem::remove(Debug::RoleLogPath(logBase.c_str(), Debug::LogRole::Client), ec);
     }