Added a new public API bytesUntilLimit to GPBCodedInputStream.

This simply exposes preexisting logic. This read-only API is for convenience and doesn't reveal any information that was not already available. `[GPBCodedInputStream position]` is already a public API. The `limit` is known to the user because it's specified by them.

PiperOrigin-RevId: 635502694

objectivec/GPBCodedInputStream.h

@@ -217,6 +217,11 @@ __attribute__((objc_subclassing_restricted))
  */
 - (void)popLimit:(size_t)oldLimit;
 
+/**
+ * @return The number of bytes from the current position to the current limit.
+ */
+- (size_t)bytesUntilLimit;
+
 /**
  * Verifies that the last call to -readTag returned the given tag value. This
  * is used to verify that a nested group ended with the correct end tag.

objectivec/GPBCodedInputStream.m

@@ -384,6 +384,10 @@ - (void)popLimit:(size_t)oldLimit {
   GPBCodedInputStreamPopLimit(&state_, oldLimit);
 }
 
+- (size_t)bytesUntilLimit {
+  return GPBCodedInputStreamBytesUntilLimit(&state_);
+}
+
 - (double)readDouble {
   return GPBCodedInputStreamReadDouble(&state_);
 }

objectivec/Tests/GPBCodedInputStreamTests.m

@@ -283,6 +283,20 @@ - (void)testSkipWholeMessage {
   }
 }
 
+- (void)testLimit {
+  TestAllTypes* message = [self allSetRepeatedCount:kGPBDefaultRepeatCount];
+  NSData* rawBytes = message.data;
+  GPBCodedInputStream* input = [GPBCodedInputStream streamWithData:rawBytes];
+  XCTAssertEqual([input bytesUntilLimit], rawBytes.length);
+  [input pushLimit:8];
+  XCTAssertEqual([input bytesUntilLimit], 8u);
+  [input popLimit:3];
+  XCTAssertEqual([input bytesUntilLimit], 3u);
+  [input readTag];
+  XCTAssertEqual([input position], 1u);
+  XCTAssertEqual([input bytesUntilLimit], 2u);
+}
+
 - (void)testReadHugeBlob {
   // Allocate and initialize a 1MB blob.
   NSMutableData* blob = [NSMutableData dataWithLength:1 << 20];