[PATCH v4] perf fdarray: Fix destructor invocation and event counting in fdarray__filter

[Ian Rogers <irogers@google.com>]

When processing POLLHUP or POLLERR for an event in fdarray__filter, the
function invokes its destructor callback. However, the exact behavior
around unhandled POLLHUP events on control pipe descriptors caused
premature termination due to thread and evlist pollfd index mismatches.
This occurred because they were skipped by the early nonfilterable continue.

Address this by refining the early continue filter to only skip system-wide
perf events (which are nonfilterable but not non_perf_event). Control
descriptors (non_perf_event) now fall through to appropriately have their
fd value unset to -1 on POLLHUP while avoiding their destructors. Finally,
maintain the invariant that the active event counter (nr) increments strictly
and only for completely filterable events.

Fixes: fb4751e79c45 ("perf record: Fix teardown hang on system-wide multi-threaded sessions")
Assisted-by: Gemini:gemini-3.1-pro
Signed-off-by: Ian Rogers <irogers@google.com>
---
 tools/lib/api/fd/array.c | 16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)

diff --git a/tools/lib/api/fd/array.c b/tools/lib/api/fd/array.c
index 67b73481df27..3681ad7c6527 100644
--- a/tools/lib/api/fd/array.c
+++ b/tools/lib/api/fd/array.c
@@ -116,14 +116,23 @@ int fdarray__filter(struct fdarray *fda, short revents,
 		return 0;
 
 	for (fd = 0; fd < fda->nr; ++fd) {
-		if (fda->priv[fd].flags & fdarray_flag__nonfilterable)
+		/*
+		 * System-wide perf events are nonfilterable but not non_perf_event.
+		 * We want to skip them entirely and never process revents on them.
+		 */
+		if ((fda->priv[fd].flags & fdarray_flag__nonfilterable) &&
+		    !(fda->priv[fd].flags & fdarray_flag__non_perf_event))
 			continue;
 
 		if (!fda->entries[fd].events)
 			continue;
 
 		if (fda->entries[fd].revents & revents) {
-			if (entry_destructor)
+			/*
+			 * Control descriptors are non_perf_event and don't need
+			 * their perf-specific destructors triggered.
+			 */
+			if (entry_destructor && !(fda->priv[fd].flags & fdarray_flag__non_perf_event))
 				entry_destructor(fda, fd, arg);
 
 			/*
@@ -136,7 +145,8 @@ int fdarray__filter(struct fdarray *fda, short revents,
 			continue;
 		}
 
-		++nr;
+		if (!(fda->priv[fd].flags & fdarray_flag__nonfilterable))
+			++nr;
 	}
 
 	return nr;
-- 
2.55.0.699.gb54405d56f-goog