Support empty IpcSharedMemory (#335)

* Add tests for empty shared mem

* Simple empty slice

* Bump version to 0.18.1

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "ipc-channel"
-version = "0.18.0"
+version = "0.18.1"
 description = "A multiprocess drop-in replacement for Rust channels"
 authors = ["The Servo Project Developers"]
 license = "MIT OR Apache-2.0"

src/ipc.rs

@@ -553,15 +553,20 @@ impl IpcReceiverSet {
     derive(PartialEq)
 )]
 pub struct IpcSharedMemory {
-    os_shared_memory: OsIpcSharedMemory,
+    /// None represents no data (empty slice)
+    os_shared_memory: Option<OsIpcSharedMemory>,
 }
 
 impl Deref for IpcSharedMemory {
     type Target = [u8];
 
     #[inline]
     fn deref(&self) -> &[u8] {
-        &self.os_shared_memory
+        if let Some(os_shared_memory) = &self.os_shared_memory {
+            os_shared_memory
+        } else {
+            &[]
+        }
     }
 }
 
@@ -571,16 +576,22 @@ impl<'de> Deserialize<'de> for IpcSharedMemory {
         D: Deserializer<'de>,
     {
         let index: usize = Deserialize::deserialize(deserializer)?;
-        let os_shared_memory = OS_IPC_SHARED_MEMORY_REGIONS_FOR_DESERIALIZATION.with(
-            |os_ipc_shared_memory_regions_for_deserialization| {
-                // FIXME(pcwalton): This could panic if the data was corrupt and the index was out
-                // of bounds. We should return an `Err` result instead.
-                os_ipc_shared_memory_regions_for_deserialization.borrow_mut()[index]
-                    .take()
-                    .unwrap()
-            },
-        );
-        Ok(IpcSharedMemory { os_shared_memory })
+        if index == usize::MAX {
+            Ok(IpcSharedMemory::empty())
+        } else {
+            let os_shared_memory = OS_IPC_SHARED_MEMORY_REGIONS_FOR_DESERIALIZATION.with(
+                |os_ipc_shared_memory_regions_for_deserialization| {
+                    // FIXME(pcwalton): This could panic if the data was corrupt and the index was out
+                    // of bounds. We should return an `Err` result instead.
+                    os_ipc_shared_memory_regions_for_deserialization.borrow_mut()[index]
+                        .take()
+                        .unwrap()
+                },
+            );
+            Ok(IpcSharedMemory {
+                os_shared_memory: Some(os_shared_memory),
+            })
+        }
     }
 }
 
@@ -589,32 +600,52 @@ impl Serialize for IpcSharedMemory {
     where
         S: Serializer,
     {
-        let index = OS_IPC_SHARED_MEMORY_REGIONS_FOR_SERIALIZATION.with(
-            |os_ipc_shared_memory_regions_for_serialization| {
-                let mut os_ipc_shared_memory_regions_for_serialization =
-                    os_ipc_shared_memory_regions_for_serialization.borrow_mut();
-                let index = os_ipc_shared_memory_regions_for_serialization.len();
-                os_ipc_shared_memory_regions_for_serialization.push(self.os_shared_memory.clone());
-                index
-            },
-        );
-        index.serialize(serializer)
+        if let Some(os_shared_memory) = &self.os_shared_memory {
+            let index = OS_IPC_SHARED_MEMORY_REGIONS_FOR_SERIALIZATION.with(
+                |os_ipc_shared_memory_regions_for_serialization| {
+                    let mut os_ipc_shared_memory_regions_for_serialization =
+                        os_ipc_shared_memory_regions_for_serialization.borrow_mut();
+                    let index = os_ipc_shared_memory_regions_for_serialization.len();
+                    os_ipc_shared_memory_regions_for_serialization.push(os_shared_memory.clone());
+                    index
+                },
+            );
+            debug_assert!(index < usize::MAX);
+            index
+        } else {
+            usize::MAX
+        }
+        .serialize(serializer)
     }
 }
 
 impl IpcSharedMemory {
+    const fn empty() -> Self {
+        Self {
+            os_shared_memory: None,
+        }
+    }
+
     /// Create shared memory initialized with the bytes provided.
     pub fn from_bytes(bytes: &[u8]) -> IpcSharedMemory {
-        IpcSharedMemory {
-            os_shared_memory: OsIpcSharedMemory::from_bytes(bytes),
+        if bytes.is_empty() {
+            IpcSharedMemory::empty()
+        } else {
+            IpcSharedMemory {
+                os_shared_memory: Some(OsIpcSharedMemory::from_bytes(bytes)),
+            }
         }
     }
 
     /// Create a chunk of shared memory that is filled with the byte
     /// provided.
     pub fn from_byte(byte: u8, length: usize) -> IpcSharedMemory {
-        IpcSharedMemory {
-            os_shared_memory: OsIpcSharedMemory::from_byte(byte, length),
+        if length == 0 {
+            IpcSharedMemory::empty()
+        } else {
+            IpcSharedMemory {
+                os_shared_memory: Some(OsIpcSharedMemory::from_byte(byte, length)),
+            }
         }
     }
 }

src/test.rs

@@ -465,6 +465,26 @@ fn shared_memory() {
         .all(|byte| *byte == 0xba));
 }
 
+#[test]
+fn shared_memory_slice() {
+    let (tx, rx) = ipc::channel().unwrap();
+    // test byte of size 0
+    let shared_memory = IpcSharedMemory::from_byte(42, 0);
+    tx.send(shared_memory.clone()).unwrap();
+    let received_shared_memory = rx.recv().unwrap();
+    assert_eq!(*received_shared_memory, *shared_memory);
+    // test empty slice
+    let shared_memory = IpcSharedMemory::from_bytes(&[]);
+    tx.send(shared_memory.clone()).unwrap();
+    let received_shared_memory = rx.recv().unwrap();
+    assert_eq!(*received_shared_memory, *shared_memory);
+    // test non-empty slice
+    let shared_memory = IpcSharedMemory::from_bytes(&[4, 2, 42]);
+    tx.send(shared_memory.clone()).unwrap();
+    let received_shared_memory = rx.recv().unwrap();
+    assert_eq!(*received_shared_memory, *shared_memory);
+}
+
 #[test]
 #[cfg(any(
     not(target_os = "windows"),