Add a minimal loadable extension example

.github/workflows/main.yml

@@ -65,6 +65,9 @@ jobs:
       - run: cargo test --features 'bundled-full session buildtime_bindgen' --all-targets --workspace --verbose
       - run: cargo test --features 'bundled-full session buildtime_bindgen' --doc --workspace --verbose
 
+      - name: loadable extension
+        run: cargo build --example loadable_extension --features "loadable_extension modern_sqlite functions vtab trace"
+
       # TODO: move into own action for better caching
       - name: Static build
         # Do we expect this to work / should we test with gnu toolchain?

Cargo.toml

@@ -157,6 +157,11 @@ harness = false
 name = "exec"
 harness = false
 
+[[example]]
+name = "loadable_extension"
+crate-type = ["cdylib"]
+required-features = ["loadable_extension", "modern_sqlite", "functions", "vtab", "trace"]
+
 [package.metadata.docs.rs]
 features = ["modern-full"]
 all-features = false

examples/loadable_extension.rs

@@ -0,0 +1,48 @@
+//! Adaptation of https://sqlite.org/loadext.html#programming_loadable_extensions
+use std::os::raw::{c_char, c_int};
+
+use rusqlite::ffi;
+use rusqlite::functions::FunctionFlags;
+use rusqlite::types::{ToSqlOutput, Value};
+use rusqlite::{to_sqlite_error, Connection, Result};
+
+/// # build
+/// ```sh
+/// cargo build --example loadable_extension --features "loadable_extension modern_sqlite functions vtab trace"
+/// ```
+/// # test
+/// ```sh
+/// sqlite> .log on
+/// sqlite> .load target/debug/examples/libloadable_extension.so
+/// (28) Rusqlite extension initialized
+/// sqlite> SELECT rusqlite_test_function();
+/// Rusqlite extension loaded correctly!
+/// ```
+#[allow(clippy::not_unsafe_ptr_arg_deref)]
+#[no_mangle]
+pub extern "C" fn sqlite3_extension_init(
+    db: *mut ffi::sqlite3,
+    pz_err_msg: *mut *mut c_char,
+    p_api: *mut ffi::sqlite3_api_routines,
+) -> c_int {
+    if let Err(err) = extension_init(db, p_api) {
+        return unsafe { to_sqlite_error(&err, pz_err_msg) };
+    }
+    ffi::SQLITE_OK
+}
+
+fn extension_init(db: *mut ffi::sqlite3, p_api: *mut ffi::sqlite3_api_routines) -> Result<()> {
+    let db = unsafe { Connection::extension_init2(db, p_api)? };
+    db.create_scalar_function(
+        "rusqlite_test_function",
+        0,
+        FunctionFlags::SQLITE_DETERMINISTIC,
+        |_ctx| {
+            Ok(ToSqlOutput::Owned(Value::Text(
+                "Rusqlite extension loaded correctly!".to_string(),
+            )))
+        },
+    )?;
+    rusqlite::trace::log(ffi::SQLITE_WARNING, "Rusqlite extension initialized");
+    Ok(())
+}