LWLocks for dynamic shared memory, with an ad-hoc implementation for parallel foreign scans

Cargo.toml

@@ -40,6 +40,7 @@ exclude = [
     "pgrx-examples/numeric",
     "pgrx-examples/pgtrybuilder",
     "pgrx-examples/operators",
+    "pgrx-examples/parallel_scan_lwlock",
     "pgrx-examples/range",
     "pgrx-examples/schemas",
     "pgrx-examples/shmem",

pgrx-examples/parallel_scan_lwlock/.gitignore

@@ -0,0 +1,7 @@
+.DS_Store
+.idea/
+/target
+*.iml
+**/*.rs.bk
+Cargo.lock
+sql/shmem-1.0.sql

pgrx-examples/parallel_scan_lwlock/Cargo.toml

@@ -0,0 +1,38 @@
+[package]
+name = "parallel_scan_lwlock"
+version = "0.0.0"
+edition = "2021"
+publish = false
+
+[lib]
+crate-type = ["cdylib", "lib"]
+
+[[bin]]
+name = "pgrx_embed_parallel_scan_lwlock"
+path = "./src/bin/pgrx_embed.rs"
+
+[features]
+default = ["pg17"]
+pg13 = ["pgrx/pg13", "pgrx-tests/pg13"]
+pg14 = ["pgrx/pg14", "pgrx-tests/pg14"]
+pg15 = ["pgrx/pg15", "pgrx-tests/pg15"]
+pg16 = ["pgrx/pg16", "pgrx-tests/pg16"]
+pg17 = ["pgrx/pg17", "pgrx-tests/pg17"]
+pg18 = ["pgrx/pg18", "pgrx-tests/pg18"]
+pg_test = []
+
+[dependencies]
+pgrx = { path = "../../pgrx", default-features = false }
+
+[dev-dependencies]
+pgrx-tests = { path = "../../pgrx-tests" }
+
+# uncomment these if compiling outside of 'pgrx'
+# [profile.dev]
+# panic = "unwind"
+
+# [profile.release]
+# panic = "unwind"
+# opt-level = 3
+# lto = "fat"
+# codegen-units = 1

pgrx-examples/parallel_scan_lwlock/README.md

@@ -0,0 +1,10 @@
+## Postgres Dynamic Shared Memory in Parallel Foreign Scans and LWLock Support
+
+Important:
+> Extensions that use shared memory **must** be loaded via `postgresql.conf`'s 
+>`shared_preload_libraries` configuration setting.
+
+The example in [src/lib.rs](src/lib.rs) implements a parallel scan implementation for a "generator" foreign table type
+that yields numbers from an integer counter, guarded by a dynamically allocated LWLock. It is meant to illustrate how to
+set up a lock for foreign scans, keeping the shared memory handling as simple as possible. Check out the `shmem` example
+project for more insights on shared memory handling in `pgrx` extensions.

pgrx-examples/parallel_scan_lwlock/parallel_scan_lwlock.control

@@ -0,0 +1,5 @@
+comment = 'parallel_scan_lwlock:  Created by pgrx'
+default_version = '@CARGO_VERSION@'
+module_pathname = 'parallel_scan_lwlock'
+relocatable = false
+superuser = false

pgrx-examples/parallel_scan_lwlock/src/bin/pgrx_embed.rs

@@ -0,0 +1 @@
+::pgrx::pgrx_embed!();