[FFmpeg-devel] [PATCH] avutils/hwcontext: When deriving a hwdevice, search for existing device in both directions

[Soft Works]

The test /libavutil/tests/hwdevice checks that when deriving a device
from a source device and then deriving back to the type of the source
device, the result is matching the original source device, i.e. the
derivation mechanism doesn't create a new device in this case.

Previously, this test was usually passed, but only due to two different
kind of flaws:

1. The test covers only a single level of derivation (and back)

It derives device Y from device X and then Y back to the type of X and
checks whether the result matches X.

What it doesn't check for, are longer chains of derivation like:

CUDA1 > OpenCL2 > CUDA3 and then back to OpenCL4

In that case, the second derivation returns the first device (CUDA3 ==
CUDA1), but when deriving OpenCL4, hwcontext.c was creating a new
OpenCL4 context instead of returning OpenCL2, because there was no link
from CUDA1 to OpenCL2 (only backwards from OpenCL2 to CUDA1)

If the test would check for two levels of derivation, it would have
failed.

This patch fixes those (yet untested) cases by introducing forward
references (derived_device) in addition to the existing back references
(source_device).

2. hwcontext_qsv didn't properly set the source_device

In case of QSV, hwcontext_qsv creates a source context internally
(vaapi, dxva2 or d3d11va) without calling av_hwdevice_ctx_create_derived
and without setting source_device.

This way, the hwcontext test ran successful, but what practically
happened, was that - for example - deriving vaapi from qsv didn't return
the original underlying vaapi device and a new one was created instead:
Exactly what the test is intended to detect and prevent. It just
couldn't do so, because the original device was hidden (= not set as the
source_device of the QSV device).

This patch properly makes these setting and fixes all derivation
scenarios.

(at a later stage, /libavutil/tests/hwdevice should be extended to check
longer derivation chains as well)

Signed-off-by: softworkz <softworkz at hotmail.com>
---
 libavutil/hwcontext.c          | 16 ++++++++++++++++
 libavutil/hwcontext_internal.h |  6 ++++++
 libavutil/hwcontext_qsv.c      |  7 ++++++-
 3 files changed, 28 insertions(+), 1 deletion(-)

diff --git a/libavutil/hwcontext.c b/libavutil/hwcontext.c
index d13d0f7c9b..3714ce7553 100644
--- a/libavutil/hwcontext.c
+++ b/libavutil/hwcontext.c
@@ -132,6 +132,7 @@ static void hwdevice_ctx_free(void *opaque, uint8_t *data)
         ctx->free(ctx);
 
     av_buffer_unref(&ctx->internal->source_device);
+    av_buffer_unref(&ctx->internal->derived_device);
 
     av_freep(&ctx->hwctx);
     av_freep(&ctx->internal->priv);
@@ -666,6 +667,20 @@ int av_hwdevice_ctx_create_derived_opts(AVBufferRef **dst_ref_ptr,
         tmp_ref = tmp_ctx->internal->source_device;
     }
 
+    tmp_ref = src_ref;
+    while (tmp_ref) {
+        tmp_ctx = (AVHWDeviceContext*)tmp_ref->data;
+        if (tmp_ctx->type == type) {
+            dst_ref = av_buffer_ref(tmp_ref);
+            if (!dst_ref) {
+                ret = AVERROR(ENOMEM);
+                goto fail;
+            }
+            goto done;
+        }
+        tmp_ref = tmp_ctx->internal->derived_device;
+    }
+
     dst_ref = av_hwdevice_ctx_alloc(type);
     if (!dst_ref) {
         ret = AVERROR(ENOMEM);
@@ -683,6 +698,7 @@ int av_hwdevice_ctx_create_derived_opts(AVBufferRef **dst_ref_ptr,
                                                             flags);
             if (ret == 0) {
                 dst_ctx->internal->source_device = av_buffer_ref(src_ref);
+                tmp_ctx->internal->derived_device = av_buffer_ref(dst_ref);
                 if (!dst_ctx->internal->source_device) {
                     ret = AVERROR(ENOMEM);
                     goto fail;
diff --git a/libavutil/hwcontext_internal.h b/libavutil/hwcontext_internal.h
index e6266494ac..cfe525d20c 100644
--- a/libavutil/hwcontext_internal.h
+++ b/libavutil/hwcontext_internal.h
@@ -109,6 +109,12 @@ struct AVHWDeviceInternal {
      * context it was derived from.
      */
     AVBufferRef *source_device;
+
+    /**
+     * A reference to a device context which
+     * was derived from this device.
+     */
+    AVBufferRef *derived_device;
 };
 
 struct AVHWFramesInternal {
diff --git a/libavutil/hwcontext_qsv.c b/libavutil/hwcontext_qsv.c
index 08a6e0ee1c..27d96d116f 100644
--- a/libavutil/hwcontext_qsv.c
+++ b/libavutil/hwcontext_qsv.c
@@ -1268,8 +1268,13 @@ static int qsv_device_create(AVHWDeviceContext *ctx, const char *device,
     child_device = (AVHWDeviceContext*)priv->child_device_ctx->data;
 
     impl = choose_implementation(device);
+    ret = qsv_device_derive_from_child(ctx, impl, child_device, 0);
+    if (ret >= 0) {
+        ctx->internal->source_device = av_buffer_ref(priv->child_device_ctx);
+        child_device->internal->derived_device = av_buffer_create((uint8_t*)ctx, sizeof(*ctx), 0, ctx, 0);
+    }
 
-    return qsv_device_derive_from_child(ctx, impl, child_device, 0);
+    return ret;
 }
 
 const HWContextType ff_hwcontext_type_qsv = {
-- 
2.28.0.windows.1