Merge 'eliminate the need for another Once in Completion' from Pedro Muniz

I added the `Once` before so fix a bug, but it was a bit hackery. We can
`get_or_init` to achieve the same purpose, and the code becomes much
cleaner. `get_or_init` guarantees the init will happen only once as
well.

Reviewed-by: Preston Thorpe <preston@turso.tech>
Reviewed-by: bit-aloo (@Shourya742)

Closes #3578

core/io/mod.rs

@@ -3,7 +3,6 @@ use crate::storage::sqlite3_ondisk::WAL_FRAME_HEADER_SIZE;
 use crate::{BufferPool, CompletionError, Result};
 use bitflags::bitflags;
 use cfg_block::cfg_block;
-use parking_lot::Once;
 use std::cell::RefCell;
 use std::fmt;
 use std::ptr::NonNull;
@@ -143,8 +142,6 @@ struct CompletionInner {
     // Thread safe with OnceLock
     result: std::sync::OnceLock<Option<CompletionError>>,
     needs_link: bool,
-    /// before calling callback we check if done is true
-    done: Once,
     /// Optional parent group this completion belongs to
     parent: OnceLock<Arc<GroupCompletionInner>>,
 }
@@ -298,7 +295,6 @@ impl Completion {
                 completion_type,
                 result: OnceLock::new(),
                 needs_link: false,
-                done: Once::new(),
                 parent: OnceLock::new(),
             }),
         }
@@ -310,7 +306,6 @@ impl Completion {
                 completion_type,
                 result: OnceLock::new(),
                 needs_link: true,
-                done: Once::new(),
                 parent: OnceLock::new(),
             }),
         }
@@ -420,7 +415,7 @@ impl Completion {
     }
 
     fn callback(&self, result: Result<i32, CompletionError>) {
-        self.inner.done.call_once(|| {
+        self.inner.result.get_or_init(|| {
             match &self.inner.completion_type {
                 CompletionType::Read(r) => r.callback(result),
                 CompletionType::Write(w) => w.callback(result),
@@ -428,10 +423,6 @@ impl Completion {
                 CompletionType::Truncate(t) => t.callback(result),
                 CompletionType::Group(g) => g.callback(result),
             };
-            self.inner
-                .result
-                .set(result.err())
-                .expect("result must be set only once");
 
             if let Some(group) = self.inner.parent.get() {
                 // Capture first error in group
@@ -447,6 +438,8 @@ impl Completion {
                 }
                 // TODO: remove self from parent group
             }
+
+            result.err()
         });
     }
 
@@ -459,15 +452,6 @@ impl Completion {
         }
     }
 
-    /// only call this method if you are sure that the completion is
-    /// a WriteCompletion, panics otherwise
-    pub fn as_write(&self) -> &WriteCompletion {
-        match self.inner.completion_type {
-            CompletionType::Write(ref w) => w,
-            _ => unreachable!(),
-        }
-    }
-
     /// Link this completion to a group completion (internal use only)
     fn link_internal(&mut self, group: &Completion) {
         let group_inner = match &group.inner.completion_type {