Fix off-by-one when initializing Data with discontiguous, underestimated sequences

(cherry picked from commit 7ce6f84)
(cherry picked from commit 3a6685e)

Author: Itai Ferber

stdlib/public/Darwin/Foundation/Data.swift

@@ -2084,9 +2084,8 @@ public struct Data : ReferenceConvertible, Equatable, Hashable, RandomAccessColl
                 // ... and append the rest byte-wise, buffering through an InlineData.
                 var buffer = InlineData()
                 while let element = iter.next() {
-                    if buffer.count < buffer.capacity {
-                        buffer.append(byte: element)
-                    } else {
+                    buffer.append(byte: element)
+                    if buffer.count == buffer.capacity {
                         buffer.withUnsafeBytes { _representation.append(contentsOf: $0) }
                         buffer.count = 0
                     }
@@ -2378,9 +2377,8 @@ public struct Data : ReferenceConvertible, Equatable, Hashable, RandomAccessColl
             // ... and append the rest byte-wise, buffering through an InlineData.
             var buffer = InlineData()
             while let element = iter.next() {
-                if buffer.count < buffer.capacity {
-                    buffer.append(byte: element)
-                } else {
+                buffer.append(byte: element)
+                if buffer.count == buffer.capacity {
                     buffer.withUnsafeBytes { _representation.append(contentsOf: $0) }
                     buffer.count = 0
                 }

test/stdlib/TestData.swift

@@ -1198,10 +1198,13 @@ class TestData : TestDataSuper {
 
         // If we append a sequence of elements larger than a single InlineData, the internal append here should buffer.
         // We want to make sure that buffering in this way does not accidentally drop trailing elements on the floor.
-        d.append(contentsOf: 0x03...0x17)
+        d.append(contentsOf: 0x03...0x2F)
         expectEqual(Data([0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07,
                           0x08, 0x09, 0x0A, 0x0B, 0x0C, 0x0D, 0x0E, 0x0F,
-                          0x10, 0x11, 0x12, 0x13, 0x14, 0x15, 0x16, 0x17]), d)
+                          0x10, 0x11, 0x12, 0x13, 0x14, 0x15, 0x16, 0x17,
+                          0x18, 0x19, 0x1A, 0x1B, 0x1C, 0x1D, 0x1E, 0x1F,
+                          0x20, 0x21, 0x22, 0x23, 0x24, 0x25, 0x26, 0x27,
+                          0x28, 0x29, 0x2A, 0x2B, 0x2C, 0x2D, 0x2E, 0x2F]), d)
     }
 
     // This test is like test_appendingNonContiguousSequence_exactCount but uses a sequence which reports 0 for its `.underestimatedCount`.
@@ -1220,10 +1223,13 @@ class TestData : TestDataSuper {
 
         // If we append a sequence of elements larger than a single InlineData, the internal append here should buffer.
         // We want to make sure that buffering in this way does not accidentally drop trailing elements on the floor.
-        d.append(contentsOf: (0x03...0x17).makeIterator()) // `.makeIterator()` produces a sequence whose `.underestimatedCount` is 0.
+        d.append(contentsOf: (0x03...0x2F).makeIterator()) // `.makeIterator()` produces a sequence whose `.underestimatedCount` is 0.
         expectEqual(Data([0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07,
                           0x08, 0x09, 0x0A, 0x0B, 0x0C, 0x0D, 0x0E, 0x0F,
-                          0x10, 0x11, 0x12, 0x13, 0x14, 0x15, 0x16, 0x17]), d)
+                          0x10, 0x11, 0x12, 0x13, 0x14, 0x15, 0x16, 0x17,
+                          0x18, 0x19, 0x1A, 0x1B, 0x1C, 0x1D, 0x1E, 0x1F,
+                          0x20, 0x21, 0x22, 0x23, 0x24, 0x25, 0x26, 0x27,
+                          0x28, 0x29, 0x2A, 0x2B, 0x2C, 0x2D, 0x2E, 0x2F]), d)
     }
 
     func test_sequenceInitializers() {