[Fix] (Pipe): the device-loss debug knob re-arms when set to a new value.

MobileGL/MG_Pipe/PipeApply.cpp

@@ -186,9 +186,22 @@ namespace MobileGL::MG_Pipe {
             }
         }
 #endif
-        if (at <= 0) return false;
+        static std::atomic<long> armedAt{0};
         static std::atomic<long> seen{0};
         static std::atomic<bool> fired{false};
+        if (at <= 0) {
+            // Unset (or 0) disarms: the same value set again counts afresh.
+            if (armedAt.load(std::memory_order_relaxed) != 0) armedAt.store(0, std::memory_order_relaxed);
+            return false;
+        }
+        // A NEW VALUE RE-ARMS. One server process outlives many sessions, so a second loss is
+        // injected by setting the knob again (another value, or 0 and then a value): the count
+        // restarts from the checks made after it was seen.
+        if (long previous = armedAt.load(std::memory_order_acquire);
+            previous != at && armedAt.compare_exchange_strong(previous, at, std::memory_order_acq_rel)) {
+            seen.store(0, std::memory_order_release);
+            fired.store(false, std::memory_order_release);
+        }
         if (fired.load(std::memory_order_acquire)) return false;
         if (seen.fetch_add(1, std::memory_order_acq_rel) + 1 < at) return false;
         if (fired.exchange(true, std::memory_order_acq_rel)) return false;

MobileGL/MG_Pipe/PipeSessionFail.h

@@ -119,8 +119,10 @@ namespace MobileGL::MG_Pipe {
     // A DEBUG KNOB THAT MAKES ONE DEVICE CHECK REPORT A LOSS, so the containment above can be
     // driven without a GPU that faults on demand. MOBILEGL_DEBUG_INJECT_DEVICE_LOST_AT=N (or, on
     // Android, the property debug.mobilegl.inject_device_lost_at) makes the N-th device check the
-    // backends make after the knob was first seen nonzero answer "lost", once per process. The
-    // backends ask at their frame boundaries and readbacks; unset (the default) it is one getenv.
+    // backends make after the knob was first seen nonzero answer "lost", once per value: setting
+    // another value (or 0 and then a value again) re-arms it, so one server process can lose
+    // several sessions in turn. The backends ask at their frame boundaries and readbacks; unset
+    // (the default) it is one getenv.
     bool MGPipeDebugDeviceLossDue();
 #endif
 

MobileGL/MG_Test/Wire/FatalFamilyTest.cpp

@@ -250,3 +250,33 @@ TEST(FatalFunnelSeam, ADeviceLossWithNoArmedSessionDiesThroughSessionFail) {
         << "an unarmed device loss must keep the inproc shape's death";
 #endif
 }
+
+#if !defined(_WIN32)
+// THE DEBUG KNOB RE-ARMS ON A NEW VALUE. One server process serves many sessions, so a device test
+// that loses a second session in the same process sets the knob again. Each value fires once, at its
+// N-th check; the same value set again after 0 counts afresh. In a death-test child, so the knob's
+// process-wide counters start at zero. Red with the old once-per-process latch: the second value
+// never fires (exit code 2).
+TEST(FatalFunnelSeam, TheDeviceLossKnobFiresOncePerValueAndReArmsOnANewOne) {
+    EXPECT_EXIT(
+        {
+            const auto checks = [](int n) {
+                int fired = 0;
+                for (int i = 0; i < n; ++i) fired += MobileGL::MG_Pipe::MGPipeDebugDeviceLossDue() ? 1 << i : 0;
+                return fired;
+            };
+            ::unsetenv("MOBILEGL_DEBUG_INJECT_DEVICE_LOST_AT");
+            if (checks(3) != 0) std::_Exit(1);
+            ::setenv("MOBILEGL_DEBUG_INJECT_DEVICE_LOST_AT", "2", 1);
+            if (checks(4) != 0b0010) std::_Exit(1); // the 2nd check only, then never again
+            ::setenv("MOBILEGL_DEBUG_INJECT_DEVICE_LOST_AT", "1", 1);
+            if (checks(3) != 0b001) std::_Exit(2); // a new value re-arms
+            ::setenv("MOBILEGL_DEBUG_INJECT_DEVICE_LOST_AT", "0", 1);
+            if (checks(2) != 0) std::_Exit(3);
+            ::setenv("MOBILEGL_DEBUG_INJECT_DEVICE_LOST_AT", "1", 1);
+            if (checks(2) != 0b01) std::_Exit(4); // 0 then the same value again re-arms too
+            std::_Exit(0);
+        },
+        ::testing::ExitedWithCode(0), "");
+}
+#endif