[Bugfix] Fix allocation & free logic of SingleWriterShmRingBuffer (#27117)

Signed-off-by: Kero Liang <kerorek@outlook.com>
Signed-off-by: Roger Wang <hey@rogerw.io>
Co-authored-by: donglu <donglu@cohere.com>
Co-authored-by: Roger Wang <hey@rogerw.io>

vllm/distributed/device_communicators/shm_object_storage.py

@@ -127,9 +127,7 @@ class SingleWriterShmRingBuffer:
 
         if create:
             # we are creating a buffer
-            self.metadata = {
-                self.monotonic_id_end: self.data_buffer_end
-            }  # monotonic_id -> start address
+            self.metadata: dict[int, int] = {}  # monotonic_id -> start address
             self.shared_memory = shared_memory.SharedMemory(
                 create=True, size=self.data_buffer_size, name=name
             )
@@ -288,7 +286,15 @@ class SingleWriterShmRingBuffer:
                     self.monotonic_id_start = (
                         self.monotonic_id_start + 1
                     ) % self.ID_MAX
-                    self.data_buffer_start = address
+                    if self.monotonic_id_start in self.metadata:
+                        # pointing to the start addr of next allocation
+                        self.data_buffer_start += (
+                            self.metadata[self.monotonic_id_start]
+                            - self.data_buffer_start
+                        ) % self.data_buffer_size
+                    else:
+                        # no remaining allocation, reset to zero
+                        self.data_buffer_start = self.data_buffer_end = 0
                     freed_bytes += metadata[1]
                 else:
                     # there are still readers, we cannot free the buffer