[PATCH i-g-t] tests/intel/gem_exec_capture: Fix many-* subtests

[Peter Senna Tschudin]

Currently trying to run `gem_exec_capture --run-subtest
many-4K-incremental` or `gem_exec_capture --run-subtest many-4K-zero`
will fail with:

 (gem_exec_capture:81999) i915/gem_engine_topology-CRITICAL: Test
 assertion failure function gem_engine_properties_configure, file
 ../lib/i915/gem_engine_topology.c:577:
 (gem_exec_capture:81999) i915/gem_engine_topology-CRITICAL: Failed assertion: ret == 1
 (gem_exec_capture:81999) i915/gem_engine_topology-CRITICAL: Last errno: 9, Bad file descriptor
 (gem_exec_capture:81999) i915/gem_engine_topology-CRITICAL: error: -1 != 1

This problem happens inside the macro find_first_available_engine()
when:
 1. for_each_ctx_engine() allocates an struct intel_engine_data 'ed'
    inside a for loop. The core of the issue is that ed only exists
    inside the for loop. As soon as the for loop ends, ed is out of scope
    and after it's lifetime.
 2. intel_get_current_engine() sets '*e' to an address of ed. This is ok
    while inside the for loop, and is undefined behavior after the for
    loop ends.
 3. configure_hangs() uses '*e' after the lifetime of 'ed' has ended
    leading to undefined behavior
 4. After the call to find_first_available_engine() __captureN() will
    fail as it expects '*e' to be valid. This is also undefined
    behavior.

Using 'e' outside for_each_ctx_engine() is incorrect and this patch
fixes it. Additionally, this patch moves the 'igt_assert(tmpe);' to
between the assignment and first use of 'tmpe' to prevent code
from trying to use a NULL 'tmpe'.

Reviewed-by: Kamil Konieczny <kamil.konieczny at linux.intel.com>
Signed-off-by: Peter Senna Tschudin <me at petersenna.com>
---

Sending as a new patch instead of v8 of the following series:
 https://patchwork.freedesktop.org/series/131602/

 tests/intel/gem_exec_capture.c | 15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/tests/intel/gem_exec_capture.c b/tests/intel/gem_exec_capture.c
index 57b178f3e..8800fa586 100644
--- a/tests/intel/gem_exec_capture.c
+++ b/tests/intel/gem_exec_capture.c
@@ -662,13 +662,20 @@ static bool needs_recoverable_ctx(int fd)
 
 #define find_first_available_engine(fd, ctx, e, saved) \
 	do { \
+		struct intel_execution_engine2 *tmpe = NULL; \
+		e = NULL; \
+		\
 		ctx = intel_ctx_create_all_physical(fd); \
 		igt_assert(ctx); \
-		for_each_ctx_engine(fd, ctx, e) \
-			for_each_if(gem_class_can_store_dword(fd, e->class)) \
+		for_each_ctx_engine(fd, ctx, tmpe) { \
+			igt_assert(tmpe); \
+			if(gem_class_can_store_dword(fd, tmpe->class)) { \
+				saved = configure_hangs(fd, tmpe, ctx->id); \
+				e = &saved.engine; \
 				break; \
-		igt_assert(e); \
-		saved = configure_hangs(fd, e, ctx->id); \
+			} \
+		} \
+		igt_skip_on_f(e == NULL, "no capable engine found\n"); \
 	} while(0)
 
 static void many(int fd, int dir, uint64_t size, unsigned int flags)
-- 
2.44.0