Change how Aggregate works when called on no rows.

Before this commit, if the aggregate function was called on 0 rows, it
would always return NULL (and never call Aggregate::init() or
finalize()). Now, init() and finalize() are always called to get the
result of the function, even if step() is never called.

src/functions.rs

@@ -155,14 +155,6 @@ impl ToResult for Null {
     }
 }
 
-
-// sqlite3_result_error_code, c_int
-// sqlite3_result_error_nomem
-// sqlite3_result_error_toobig
-// sqlite3_result_error, *const c_char, c_int
-// sqlite3_result_zeroblob
-// sqlite3_result_value
-
 /// A trait for types that can be created from a SQLite function parameter value.
 pub trait FromValue: Sized {
     unsafe fn parameter_value(v: *mut sqlite3_value) -> Result<Self>;
@@ -337,15 +329,19 @@ impl<'a> Context<'a> {
 /// `A` is the type of the aggregation context and `T` is the type of the final result.
 /// Implementations should be stateless.
 pub trait Aggregate<A, T> where T: ToResult {
-    /// Initializes the aggregation context.
+    /// Initializes the aggregation context. Will be called exactly once for each
+    /// invocation of the function.
     fn init(&self) -> A;
-    /// "step" function called once for each row in an aggregate group.
+
+    /// "step" function called once for each row in an aggregate group. May be called
+    /// 0 times if there are no rows.
     fn step(&self, &mut Context, &mut A) -> Result<()>;
-    /// Computes and returns the final result.
+
+    /// Computes and returns the final result. Will be called exactly once for each
+    /// invocation of the function.
     fn finalize(&self, A) -> Result<T>;
 }
 
-
 impl Connection {
     /// Attach a user-defined scalar function to this database connection.
     ///
@@ -489,7 +485,24 @@ impl InnerConnection {
         where D: Aggregate<A, T>,
               T: ToResult
     {
-        unsafe extern "C" fn call_boxed_closure<A, D, T>(ctx: *mut sqlite3_context,
+        // Get our aggregation context from the sqlite3_context.
+        unsafe fn aggregate_context<A, D, T>(agg: &D, ctx: *mut sqlite3_context) -> Result<*mut A>
+            where D: Aggregate<A, T>,
+                  T: ToResult
+        {
+            let pac = ffi::sqlite3_aggregate_context(ctx, ::std::mem::size_of::<*mut A>() as c_int)
+                            as *mut *mut A;
+            if pac.is_null() {
+                return Err(Error::SqliteFailure(ffi::Error::new(ffi::SQLITE_NOMEM), None));
+            }
+            if (*pac).is_null() {
+                let a = agg.init();
+                *pac = Box::into_raw(Box::new(a));
+            }
+            Ok(*pac)
+        }
+
+        unsafe extern "C" fn call_boxed_step<A, D, T>(ctx: *mut sqlite3_context,
                                                       argc: c_int,
                                                       argv: *mut *mut sqlite3_value)
             where D: Aggregate<A, T>,
@@ -499,18 +512,12 @@ impl InnerConnection {
             assert!(!boxed_aggr.is_null(),
                     "Internal error - null aggregate pointer");
 
-            // TODO Validate: double indirection: `pac` allocated/freed by SQLite and `ac` allocated/freed by Rust.
-            let pac = ffi::sqlite3_aggregate_context(ctx, ::std::mem::size_of::<*mut A>() as c_int) as *mut *mut A;
-            if pac.is_null() {
+            let agg_ctx = match aggregate_context(&*boxed_aggr, ctx) {
+                Ok(agg_ctx) => agg_ctx,
+                Err(_) => {
                     ffi::sqlite3_result_error_nomem(ctx);
                     return;
                 }
-            let ac: *mut A = if (*pac).is_null() {
-                let a = (*boxed_aggr).init();
-                *pac = Box::into_raw(Box::new(a));
-                *pac
-            } else {
-                *pac
             };
 
             let mut ctx = Context {
@@ -518,7 +525,7 @@ impl InnerConnection {
                 args: slice::from_raw_parts(argv, argc as usize),
             };
 
-            match (*boxed_aggr).step(&mut ctx, &mut *ac) {
+            match (*boxed_aggr).step(&mut ctx, &mut *agg_ctx) {
                 Ok(_) => {}
                 Err(Error::SqliteFailure(err, s)) => {
                     ffi::sqlite3_result_error_code(ctx.ctx, err.extended_code);
@@ -534,6 +541,7 @@ impl InnerConnection {
                 }
             };
         }
+
         unsafe extern "C" fn call_boxed_final<A, D, T>(ctx: *mut sqlite3_context)
             where D: Aggregate<A, T>,
                   T: ToResult
@@ -542,12 +550,15 @@ impl InnerConnection {
             assert!(!boxed_aggr.is_null(),
                     "Internal error - null aggregate pointer");
 
-            let pac = ffi::sqlite3_aggregate_context(ctx, 0) as *mut *mut A;
-            if pac.is_null() || (*pac).is_null() {
+            let agg_ctx = match aggregate_context(&*boxed_aggr, ctx) {
+                Ok(agg_ctx) => agg_ctx,
+                Err(_) => {
+                    ffi::sqlite3_result_error_nomem(ctx);
                     return;
                 }
-            let ac: *mut A = *pac;
-            let a = Box::from_raw(mem::transmute(ac)); // to be freed
+            };
+
+            let a = Box::from_raw(agg_ctx); // to be freed
 
             match (*boxed_aggr).finalize(*a) {
                 Ok(r) => r.set_result(ctx),
@@ -579,7 +590,7 @@ impl InnerConnection {
                                             flags,
                                             mem::transmute(boxed_aggr),
                                             None,
-                                            Some(call_boxed_closure::<A, D, T>),
+                                            Some(call_boxed_step::<A, D, T>),
                                             Some(call_boxed_final::<A, D, T>),
                                             Some(mem::transmute(free_boxed_value::<D>)))
         };
@@ -772,14 +783,30 @@ mod test {
     }
 
     struct Sum;
+    struct Count;
 
-    impl Aggregate<i64, i64> for Sum {
+    impl Aggregate<Option<i64>, Option<i64>> for Sum {
+        fn init(&self) -> Option<i64> {
+            None
+        }
+
+        fn step(&self, ctx: &mut Context, sum: &mut Option<i64>) -> Result<()> {
+            *sum = Some(sum.unwrap_or(0) + try!(ctx.get::<i64>(0)));
+            Ok(())
+        }
+
+        fn finalize(&self, sum: Option<i64>) -> Result<Option<i64>> {
+            Ok(sum)
+        }
+    }
+
+    impl Aggregate<i64, i64> for Count {
         fn init(&self) -> i64 {
             0
         }
 
-        fn step(&self, ctx: &mut Context, sum: &mut i64) -> Result<()> {
-            *sum = *sum + try!(ctx.get::<i64>(0));
+        fn step(&self, _ctx: &mut Context, sum: &mut i64) -> Result<()> {
+            *sum += 1;
             Ok(())
         }
 
@@ -792,6 +819,8 @@ mod test {
     fn test_sum() {
         let db = Connection::open_in_memory().unwrap();
         db.create_aggregate_function("my_sum", 1, true, Sum).unwrap();
+
+        // sum should return NULL when given no columns (contrast with count below)
         let no_result = "SELECT my_sum(i) FROM (SELECT 2 AS i WHERE 1 <> 1)";
         let result: Option<i64> = db.query_row(no_result, &[], |r| r.get(0))
                                     .unwrap();
@@ -808,4 +837,20 @@ mod test {
                                    .unwrap();
         assert_eq!((4, 2), result);
     }
+
+    #[test]
+    fn test_count() {
+        let db = Connection::open_in_memory().unwrap();
+        db.create_aggregate_function("my_count", -1, true, Count).unwrap();
+
+        // count should return 0 when given no columns (contrast with sum above)
+        let no_result = "SELECT my_count(i) FROM (SELECT 2 AS i WHERE 1 <> 1)";
+        let result: i64 = db.query_row(no_result, &[], |r| r.get(0)).unwrap();
+        assert_eq!(result, 0);
+
+        let single_sum = "SELECT my_count(i) FROM (SELECT 2 AS i UNION ALL SELECT 2)";
+        let result: i64 = db.query_row(single_sum, &[], |r| r.get(0))
+                            .unwrap();
+        assert_eq!(2, result);
+    }
 }