autoprewarm worker failing to load

Hi,

089480c077056 seems to have broken pg_prewarm. When pg_prewarm
is added to shared_preload_libraries, each new connection results in
thousands of errors such as this:

2022-07-27 04:25:14.325 UTC [2903955] LOG: background worker
"autoprewarm leader" (PID 2904146) exited with exit code 1
2022-07-27 04:25:14.325 UTC [2904148] ERROR: could not find function
"autoprewarm_main" in file
"/home/ubuntu/proj/tempdel/lib/postgresql/pg_prewarm.so"

Checking pg_prewarm.so the function 'autoprewarm_main' visibility
switched from GLOBAL to LOCAL. Per [1], using PGDLLEXPORT
makes it GLOBAL again, which appears to fix the issue:

Before commit (089480c077056) -
ubuntu:~/proj/tempdel$ readelf -sW lib/postgresql/pg_prewarm.so | grep main
103: 0000000000003d79 609 FUNC GLOBAL DEFAULT 14 autoprewarm_main
109: 00000000000045ad 873 FUNC GLOBAL DEFAULT 14 autoprewarm_database_main
128: 0000000000003d79 609 FUNC GLOBAL DEFAULT 14 autoprewarm_main
187: 00000000000045ad 873 FUNC GLOBAL DEFAULT 14 autoprewarm_database_main

After commit (089480c077056) -
78: 0000000000002d79 609 FUNC LOCAL DEFAULT 14 autoprewarm_main
85: 00000000000035ad 873 FUNC LOCAL DEFAULT 14 autoprewarm_database_main

After applying the attached fix:
103: 0000000000003d79 609 FUNC GLOBAL DEFAULT 14 autoprewarm_main
84: 00000000000045ad 873 FUNC LOCAL DEFAULT 14 autoprewarm_database_main
129: 0000000000003d79 609 FUNC GLOBAL DEFAULT 14 autoprewarm_main

Please let me know your thoughts on this approach.

[1] https://www.postgresql.org/message-id/A737B7A37273E048B164557ADEF4A58B5393038C%40ntex2010a.host.magwien.gv.at

diff --git a/contrib/pg_prewarm/autoprewarm.c b/contrib/pg_prewarm/autoprewarm.c
index b2d6026093..ec619be9f2 100644
--- a/contrib/pg_prewarm/autoprewarm.c
+++ b/contrib/pg_prewarm/autoprewarm.c
@@ -82,7 +82,7 @@ typedef struct AutoPrewarmSharedState
int prewarmed_blocks;
} AutoPrewarmSharedState;

-void autoprewarm_main(Datum main_arg);
+PGDLLEXPORT void autoprewarm_main(Datum main_arg);
void autoprewarm_database_main(Datum main_arg);

PG_FUNCTION_INFO_V1(autoprewarm_start_worker);

-
Robins Tharakan
Amazon Web Services