From f6fb445b73f0c84f92f74c799effac08e18c4c1c Mon Sep 17 00:00:00 2001
From: Chavi Weingarten <chaviw@google.com>
Date: Tue, 23 Jan 2024 21:10:30 +0000
Subject: Add captureLayersSync function

This captureLayers function in SurfaceFlinger will wait on the
requested binder thread for the screenshot composition to complete
before returning the buffer to the client. This is different than the
existing captureLayers method since the other request is oneway and
invokes the screen capture callback on a different binder thread.

This is needed because there are places in system server that request
screenshots while holding a lock and then wait synchronously on the
results. While waiting on the buffer and holding the lock, additional
two way binder calls can be made into system server that are waiting to
acquire the same lock. If there are enough requests, we may run out of
binder threads and then the screen capture result can't be posted back
to system server because it needs a free binder thread. This will result
in a deadlock because the lock that the screenshot request is holding
can never be unlocked without a free binder thread. Binder threads will
never be freed up because they are waiting to acquire the lock.

The async screencapture code is still useful for cases where there's no
global lock being held while waiting on results or the results is posted
to another thread.

Test: Screenshots
Bug: 321263247
Change-Id: I259173a59f488995e13af8f7dd2ca98c3bbf8639
---
 services/surfaceflinger/SurfaceFlinger.cpp | 13 +++++++++++++
 1 file changed, 13 insertions(+)

(limited to 'services/surfaceflinger/SurfaceFlinger.cpp')

diff --git a/services/surfaceflinger/SurfaceFlinger.cpp b/services/surfaceflinger/SurfaceFlinger.cpp
index fe5b159051..178828a5f6 100644
--- a/services/surfaceflinger/SurfaceFlinger.cpp
+++ b/services/surfaceflinger/SurfaceFlinger.cpp
@@ -116,6 +116,7 @@
 #include <common/FlagManager.h>
 #include <gui/LayerStatePermissions.h>
 #include <gui/SchedulingPolicy.h>
+#include <gui/SyncScreenCaptureListener.h>
 #include <ui/DisplayIdentification.h>
 #include "BackgroundExecutor.h"
 #include "Client.h"
@@ -7842,6 +7843,12 @@ void SurfaceFlinger::captureDisplay(DisplayId displayId, const CaptureArgs& args
                         kAllowProtected, kGrayscale, captureListener);
 }
 
+ScreenCaptureResults SurfaceFlinger::captureLayersSync(const LayerCaptureArgs& args) {
+    sp<SyncScreenCaptureListener> captureListener = sp<SyncScreenCaptureListener>::make();
+    captureLayers(args, captureListener);
+    return captureListener->waitForResults();
+}
+
 void SurfaceFlinger::captureLayers(const LayerCaptureArgs& args,
                                    const sp<IScreenCaptureListener>& captureListener) {
     ATRACE_CALL();
@@ -9686,6 +9693,12 @@ binder::Status SurfaceComposerAIDL::captureDisplayById(
     return binderStatusFromStatusT(NO_ERROR);
 }
 
+binder::Status SurfaceComposerAIDL::captureLayersSync(const LayerCaptureArgs& args,
+                                                      ScreenCaptureResults* outResults) {
+    *outResults = mFlinger->captureLayersSync(args);
+    return binderStatusFromStatusT(NO_ERROR);
+}
+
 binder::Status SurfaceComposerAIDL::captureLayers(
         const LayerCaptureArgs& args, const sp<IScreenCaptureListener>& captureListener) {
     mFlinger->captureLayers(args, captureListener);
-- 
cgit v1.2.3-59-g8ed1b