Add functions to take/release just producer or consumer

See jamesmunns#40
See jamesmunns#67

core/src/bbbuffer.rs

@@ -12,13 +12,19 @@ use core::{
     result::Result as CoreResult,
     slice::from_raw_parts_mut,
     sync::atomic::{
-        AtomicBool, AtomicUsize,
+        AtomicBool, AtomicU8, AtomicUsize,
         Ordering::{AcqRel, Acquire, Release},
     },
 };
 pub use generic_array::typenum::consts;
 use generic_array::{ArrayLength, GenericArray};
 
+/// Bit to define producer taken.
+const BIT_PRODUCER: u8 = 1 << 0;
+
+/// Bit to define consumer taken.
+const BIT_CONSUMER: u8 = 1 << 1;
+
 /// A backing structure for a BBQueue. Can be used to create either
 /// a BBQueue or a split Producer/Consumer pair
 pub struct BBBuffer<N: ArrayLength<u8>>(
@@ -41,7 +47,7 @@ where
     /// is placed at `static` scope within the `.bss` region, the explicit initialization
     /// will be elided (as it is already performed as part of memory initialization)
     ///
-    /// NOTE:  If the `thumbv6` feature is selected, this function takes a short critical section
+    /// NOTE: If the `thumbv6` feature is selected, this function takes a short critical section
     /// while splitting.
     ///
     /// ```rust
@@ -64,7 +70,13 @@ where
     /// # }
     /// ```
     pub fn try_split(&'a self) -> Result<(Producer<'a, N>, Consumer<'a, N>)> {
-        if atomic::swap(&self.0.already_split, true, AcqRel) {
+        // Set producer/consumer taken bit, error and reset if one was already set
+        let prev = self
+            .0
+            .split_prod_cons
+            .fetch_or(BIT_PRODUCER | BIT_CONSUMER, AcqRel);
+        if prev > 0 {
+            self.0.split_prod_cons.store(prev, Release);
             return Err(Error::AlreadySplit);
         }
 
@@ -101,14 +113,82 @@ where
     /// is placed at `static` scope within the `.bss` region, the explicit initialization
     /// will be elided (as it is already performed as part of memory initialization)
     ///
-    /// NOTE:  If the `thumbv6` feature is selected, this function takes a short critical
+    /// NOTE: If the `thumbv6` feature is selected, this function takes a short critical
     /// section while splitting.
     pub fn try_split_framed(&'a self) -> Result<(FrameProducer<'a, N>, FrameConsumer<'a, N>)> {
         let (producer, consumer) = self.try_split()?;
         Ok((FrameProducer { producer }, FrameConsumer { consumer }))
     }
 
-    /// Attempt to release the Producer and Consumer
+    /// Attempt to take a `Producer` from the `BBBuffer` to gain access to the
+    /// buffer. If a producer has already been taken, an error will be returned.
+    ///
+    /// NOTE: When splitting, the underlying buffer will be explicitly initialized
+    /// to zero. This may take a measurable amount of time, depending on the size
+    /// of the buffer. This is necessary to prevent undefined behavior. If the buffer
+    /// is placed at `static` scope within the `.bss` region, the explicit initialization
+    /// will be elided (as it is already performed as part of memory initialization)
+    ///
+    /// NOTE: If the `thumbv6` feature is selected, this function takes a short critical section
+    /// while splitting.
+    pub fn try_take_producer(&'a self) -> Result<Producer<'a, N>> {
+        // Set producer taken bit, error if already set
+        if self.0.split_prod_cons.fetch_or(BIT_PRODUCER, AcqRel) & BIT_PRODUCER > 0 {
+            return Err(Error::AlreadySplit);
+        }
+
+        unsafe {
+            // TODO: do we need to zero buffer here, like try_split?
+            // // Explicitly zero the data to avoid undefined behavior.
+            // // This is required, because we hand out references to the buffers,
+            // // which mean that creating them as references is technically UB for now
+            // let mu_ptr = self.0.buf.get();
+            // (*mu_ptr).as_mut_ptr().write_bytes(0u8, 1);
+
+            let nn1 = NonNull::new_unchecked(self as *const _ as *mut _);
+
+            Ok(Producer {
+                bbq: nn1,
+                pd: PhantomData,
+            })
+        }
+    }
+
+    /// Attempt to take a `Consumer` from the `BBBuffer` to gain access to the
+    /// buffer. If a consumer has already been taken, an error will be returned.
+    ///
+    /// NOTE: When splitting, the underlying buffer will be explicitly initialized
+    /// to zero. This may take a measurable amount of time, depending on the size
+    /// of the buffer. This is necessary to prevent undefined behavior. If the buffer
+    /// is placed at `static` scope within the `.bss` region, the explicit initialization
+    /// will be elided (as it is already performed as part of memory initialization)
+    ///
+    /// NOTE: If the `thumbv6` feature is selected, this function takes a short critical section
+    /// while splitting.
+    pub fn try_take_consumer(&'a self) -> Result<Consumer<'a, N>> {
+        // Set producer taken bit, error if already set
+        if self.0.split_prod_cons.fetch_or(BIT_CONSUMER, AcqRel) & BIT_CONSUMER > 0 {
+            return Err(Error::AlreadySplit);
+        }
+
+        unsafe {
+            // TODO: do we need to zero buffer here, like try_split?
+            // // Explicitly zero the data to avoid undefined behavior.
+            // // This is required, because we hand out references to the buffers,
+            // // which mean that creating them as references is technically UB for now
+            // let mu_ptr = self.0.buf.get();
+            // (*mu_ptr).as_mut_ptr().write_bytes(0u8, 1);
+
+            let nn1 = NonNull::new_unchecked(self as *const _ as *mut _);
+
+            Ok(Consumer {
+                bbq: nn1,
+                pd: PhantomData,
+            })
+        }
+    }
+
+    /// Attempt to release the `Producer` and `Consumer`
     ///
     /// This re-initializes the buffer so it may be split in a different mode at a later
     /// time. There must be no read or write grants active, or an error will be returned.
@@ -178,7 +258,7 @@ where
         self.0.last.store(0, Release);
 
         // Mark the buffer as ready to be split
-        self.0.already_split.store(false, Release);
+        self.0.split_prod_cons.store(0, Release);
 
         Ok(())
     }
@@ -201,6 +281,148 @@ where
                 (FrameProducer { producer }, FrameConsumer { consumer })
             })
     }
+
+    /// Attempt to release the `Producer`.
+    ///
+    /// This re-initializes the buffer so it may be split in a different mode at a later
+    /// time. There must be no read or write grants active, or an error will be returned.
+    ///
+    /// The `Producer`  ust be from THIS `BBBuffer`, or an error will be returned.
+    ///
+    /// ```rust
+    /// # // bbqueue test shim!
+    /// # fn bbqtest() {
+    /// use bbqueue::{BBBuffer, consts::*};
+    ///
+    /// // Create and split a new buffer
+    /// let buffer: BBBuffer<U6> = BBBuffer::new();
+    /// let (prod, cons) = buffer.try_split().unwrap();
+    ///
+    /// // Not possible to split twice
+    /// assert!(buffer.try_split().is_err());
+    ///
+    /// // Release the producer and consumer
+    /// assert!(buffer.try_release(prod, cons).is_ok());
+    ///
+    /// // Split the buffer in framed mode
+    /// let (fprod, fcons) = buffer.try_split_framed().unwrap();
+    /// # // bbqueue test shim!
+    /// # }
+    /// #
+    /// # fn main() {
+    /// # #[cfg(not(feature = "thumbv6"))]
+    /// # bbqtest();
+    /// # }
+    /// ```
+    pub fn try_release_producer(
+        &'a self,
+        prod: Producer<'a, N>,
+    ) -> CoreResult<(), Producer<'a, N>> {
+        // Note: Re-entrancy is not possible because we require ownership
+        // of the producer, which are not cloneable. We also
+        // can assume the buffer has been split, because
+
+        // Is this our producer?
+        let our_prod = prod.bbq.as_ptr() as *const Self == self;
+
+        if !(our_prod) {
+            // Can't release, not our producer
+            return Err(prod);
+        }
+
+        let wr_in_progress = self.0.write_in_progress.load(Acquire);
+        let rd_in_progress = self.0.read_in_progress.load(Acquire);
+
+        if wr_in_progress || rd_in_progress {
+            // Can't release, active grant(s) in progress
+            return Err(prod);
+        }
+
+        // Drop the producer
+        drop(prod);
+
+        // Re-initialize the buffer (not totally needed, but nice to do)
+        self.0.write.store(0, Release);
+        self.0.read.store(0, Release);
+        self.0.reserve.store(0, Release);
+        self.0.last.store(0, Release);
+
+        // Mark the buffer as ready to retake producer
+        self.0.split_prod_cons.fetch_and(!BIT_PRODUCER, Release);
+
+        Ok(())
+    }
+
+    /// Attempt to release the `Consumer`.
+    ///
+    /// This re-initializes the buffer so it may be split in a different mode at a later
+    /// time. There must be no read or write grants active, or an error will be returned.
+    ///
+    /// The `Consumer` must be from THIS `BBBuffer`, or an error will be returned.
+    ///
+    /// ```rust
+    /// # // bbqueue test shim!
+    /// # fn bbqtest() {
+    /// use bbqueue::{BBBuffer, consts::*};
+    ///
+    /// // Create and split a new buffer
+    /// let buffer: BBBuffer<U6> = BBBuffer::new();
+    /// let (prod, cons) = buffer.try_split().unwrap();
+    ///
+    /// // Not possible to split twice
+    /// assert!(buffer.try_split().is_err());
+    ///
+    /// // Release the producer and consumer
+    /// assert!(buffer.try_release(prod, cons).is_ok());
+    ///
+    /// // Split the buffer in framed mode
+    /// let (fprod, fcons) = buffer.try_split_framed().unwrap();
+    /// # // bbqueue test shim!
+    /// # }
+    /// #
+    /// # fn main() {
+    /// # #[cfg(not(feature = "thumbv6"))]
+    /// # bbqtest();
+    /// # }
+    /// ```
+    pub fn try_release_consumer(
+        &'a self,
+        cons: Consumer<'a, N>,
+    ) -> CoreResult<(), Consumer<'a, N>> {
+        // Note: Re-entrancy is not possible because we require ownership
+        // of the consumer, which are not cloneable. We also
+        // can assume the buffer has been split, because
+
+        // Is this our consumer?
+        let our_cons = cons.bbq.as_ptr() as *const Self == self;
+
+        if !(our_cons) {
+            // Can't release, not our consumer
+            return Err(cons);
+        }
+
+        let wr_in_progress = self.0.write_in_progress.load(Acquire);
+        let rd_in_progress = self.0.read_in_progress.load(Acquire);
+
+        if wr_in_progress || rd_in_progress {
+            // Can't release, active grant(s) in progress
+            return Err(cons);
+        }
+
+        // Drop the consumer
+        drop(cons);
+
+        // Re-initialize the buffer (not totally needed, but nice to do)
+        self.0.write.store(0, Release);
+        self.0.read.store(0, Release);
+        self.0.reserve.store(0, Release);
+        self.0.last.store(0, Release);
+
+        // Mark the buffer as ready to retake consumer
+        self.0.split_prod_cons.fetch_and(!BIT_CONSUMER, Release);
+
+        Ok(())
+    }
 }
 
 /// `const-fn` version BBBuffer
@@ -237,8 +459,10 @@ pub struct ConstBBBuffer<A> {
     /// Is there an active write grant?
     write_in_progress: AtomicBool,
 
-    /// Have we already split?
-    already_split: AtomicBool,
+    /// Whether we have split the producer and/or consumer parts off.
+    ///
+    /// See the `BIT_PRODUCER` and `BIT_CONSUMER` bits which define what parts have been split off.
+    split_prod_cons: AtomicU8,
 }
 
 impl<A> ConstBBBuffer<A> {
@@ -292,7 +516,7 @@ impl<A> ConstBBBuffer<A> {
             write_in_progress: AtomicBool::new(false),
 
             /// We haven't split at the start
-            already_split: AtomicBool::new(false),
+            split_prod_cons: AtomicU8::new(0),
         }
     }
 }
@@ -820,7 +1044,7 @@ where
     /// If `used` is larger than the given grant, the maximum amount will
     /// be commited
     ///
-    /// NOTE:  If the `thumbv6` feature is selected, this function takes a short critical
+    /// NOTE: If the `thumbv6` feature is selected, this function takes a short critical
     /// section while committing.
     pub fn commit(mut self, used: usize) {
         self.commit_inner(used);
@@ -947,7 +1171,7 @@ where
     /// If `used` is larger than the given grant, the full grant will
     /// be released.
     ///
-    /// NOTE:  If the `thumbv6` feature is selected, this function takes a short critical
+    /// NOTE: If the `thumbv6` feature is selected, this function takes a short critical
     /// section while releasing.
     pub fn release(mut self, used: usize) {
         // Saturate the grant release
@@ -1065,7 +1289,7 @@ where
     /// If `used` is larger than the given grant, the full grant will
     /// be released.
     ///
-    /// NOTE:  If the `thumbv6` feature is selected, this function takes a short critical
+    /// NOTE: If the `thumbv6` feature is selected, this function takes a short critical
     /// section while releasing.
     pub fn release(mut self, used: usize) {
         // Saturate the grant release