Issue-2734 - Clamp on readable bytes

Sources/NIOCore/ByteBuffer-core.swift

@@ -832,12 +832,13 @@ public struct ByteBuffer {
         return true
     }
 
-    /// Reset capacity of the buffer to the specified amount.
+    /// Clamps the buffers capacity to either the maximum of the `desiredCapacity` or the buffer's `readableBytes`.
     ///
-    /// - returns: `true` if one or more bytes have been discarded, `false` if there are no bytes to discard.
+    /// - returns: A boolean indicating if the capacity of the buffer has changed.
     @inlinable
-    @discardableResult public mutating func clampBufferCapacity(to maxRetainedCapacity: Int) -> Bool {
-        guard maxRetainedCapacity >= readableBytes && maxRetainedCapacity < capacity else {
+    @discardableResult public mutating func clampBufferCapacity(to desiredCapacity: Int) -> Bool {
+        let maxRetainedCapacity = max(self.readableBytes, desiredCapacity)
+        guard maxRetainedCapacity < capacity else {
             return false
         }
 

Tests/NIOCoreTests/ByteBufferTest.swift

@@ -1839,34 +1839,28 @@ class ByteBufferTest: XCTestCase {
     }
 
     func testRestoreBufferCapacity() {
-        let maxBufferCapacityToBeRetained = 1024
+        let desiredCapacity = 1024
         var buffer = self.allocator.buffer(capacity: 512)
-
-        // For an item equal to the maximum specified buffer capacity it should not clamp buffer capacity
+        
+        // If readable capacity and desired capacity are less than or equal to buffer capacity, should not clamp
         buffer.clear()
-        buffer.writeString(String(repeating: "x", count: maxBufferCapacityToBeRetained))
-        XCTAssertFalse(buffer.clampBufferCapacity(to: maxBufferCapacityToBeRetained))
-        XCTAssertEqual(buffer.capacity, 1024)
+        buffer.writeString(String(repeating: "x", count: desiredCapacity))
+        XCTAssertEqual(buffer.capacity, 1024) // Before
+        XCTAssertFalse(buffer.clampBufferCapacity(to: desiredCapacity))
+        XCTAssertEqual(buffer.capacity, 1024) // After
 
-        // For an item with readable bytes above the maximum specified buffer capacity it should not clamp
+        // If readable capacity is greater less than to buffer capacity, should clamp to readable bytes
         buffer.clear()
-        buffer.writeString(String(repeating: "x", count: maxBufferCapacityToBeRetained + 1))
-        XCTAssertFalse(buffer.clampBufferCapacity(to: maxBufferCapacityToBeRetained))
-        XCTAssertEqual(buffer.capacity, 2048)
+        buffer.writeString(String(repeating: "x", count: desiredCapacity + 1))
+        XCTAssertTrue(buffer.clampBufferCapacity(to: desiredCapacity))
+        XCTAssertEqual(buffer.capacity, 1025)
 
-        // For a subsequent item smaller than buffer capacity it should clamp if the maximum specified buffer capacity is equal to, or less than readable bytes
+        // If desiredCapacity capacity is less than buffer capacity and larger than readable bytes, should clamp to desiredCapacity bytes
         buffer.clear()
-        buffer.writeString(String(repeating: "x", count: maxBufferCapacityToBeRetained))
-        XCTAssertTrue(buffer.clampBufferCapacity(to: maxBufferCapacityToBeRetained))
+        buffer.writeString(String(repeating: "x", count: 512))
+        XCTAssertTrue(buffer.clampBufferCapacity(to: desiredCapacity))
         XCTAssertEqual(buffer.capacity, 1024)
 
-        // For a small item smaller than maxBufferCapacityToBeRetained, it should not clamp further.
-        // This is to avoid flip-flopping between clamping and expansion during buffer lifecycle
-        buffer.clear()
-        buffer.writeString("Small item")
-        XCTAssertFalse(buffer.clampBufferCapacity(to: maxBufferCapacityToBeRetained))
-        XCTAssertEqual(buffer.capacity, 1024)
-
         // For any item, it should not clamp buffer capacity to a value larger than the current buffer capacity
         buffer.clear()
         buffer.writeString("Any item")