Merge pull request #25308 from Azoy/demangle-bug-thing

[Runtime] swift_demangle: Update buffer size after copying

Author: compnerd

stdlib/public/runtime/Demangle.cpp

@@ -666,13 +666,14 @@ char *swift_demangle(const char *mangledName,
     return strdup(result.c_str());
   }
 
-  // Indicate a failure if the result does not fit and will be truncated
-  // and set the required outputBufferSize.
+  // Copy into the provided buffer.
+  _swift_strlcpy(outputBuffer, result.c_str(), *outputBufferSize);
+
+  // Indicate a failure if the result did not fit and was truncated
+  // by setting the required outputBufferSize.
   if (*outputBufferSize < result.length() + 1) {
     *outputBufferSize = result.length() + 1;
   }
 
-  // Copy into the provided buffer.
-  _swift_strlcpy(outputBuffer, result.c_str(), *outputBufferSize);
   return outputBuffer;
 }

test/stdlib/Runtime.swift.gyb

@@ -375,6 +375,37 @@ Runtime.test("demangleName") {
   expectEqual("Foobar", _stdlib_demangleName("$s13__lldb_expr_46FoobarCD"))
 }
 
+Runtime.test("demangleTruncate") {
+  // Swift.Int requires 10 bytes to be fully demangled.
+  let buffer = UnsafeMutableBufferPointer<Int8>.allocate(capacity: 10)
+
+  defer { buffer.deallocate() }
+
+  // Set last byte to a custom number, that way when we call swift_demangle
+  // the last byte should be unchanged rather than it being set to 0.
+  buffer[buffer.count - 1] = 16
+
+  // Only give 9 bytes though to exercise that swift_demangle doesn't write past
+  // the buffer.
+  var bufferSize = UInt(buffer.count - 1)
+
+  let mangled = "$sSi"
+
+  mangled.utf8CString.withUnsafeBufferPointer {
+    _ = _stdlib_demangleImpl(
+      mangledName: $0.baseAddress,
+      mangledNameLength: UInt($0.count - 1),
+      outputBuffer: buffer.baseAddress,
+      outputBufferSize: &bufferSize,
+      flags: 0
+    )
+  }
+
+  expectEqual(String(cString: buffer.baseAddress!), "Swift.In")
+  expectEqual(bufferSize, 10)
+  expectEqual(buffer[buffer.count - 1], 16)
+}
+
 % for optionality in ['', '?']:
 Runtime.test("_stdlib_atomicCompareExchangeStrongPtr") {
   typealias IntPtr = UnsafeMutablePointer<Int>