[Documentation] Update docs for TryCancel() on ServerContext (#32889)

Fix #32638

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include/grpcpp/server_context.h

@@ -206,7 +206,9 @@ class ServerContextBase {
   /// TryCancel() is called, the serverhandler must return Status::CANCELLED.
   /// The only exception is that if the serverhandler is already returning an
   /// error status code, it is ok to not return Status::CANCELLED even if
-  /// TryCancel() was called.
+  /// TryCancel() was called. Additionally, it is illegal to invoke TryCancel()
+  /// before the call has actually begun, i.e., before metadata has been
+  /// received from the client.
   ///
   /// For reasons such as the above, it is generally preferred to explicitly
   /// finish an RPC by returning Status::CANCELLED rather than using TryCancel.

src/core/lib/surface/call.cc

@@ -3521,6 +3521,9 @@ grpc_call* grpc_call_from_top_element(grpc_call_element* surface_element) {
 grpc_call_error grpc_call_cancel(grpc_call* call, void* reserved) {
   GRPC_API_TRACE("grpc_call_cancel(call=%p, reserved=%p)", 2, (call, reserved));
   GPR_ASSERT(reserved == nullptr);
+  if (call == nullptr) {
+    return GRPC_CALL_ERROR;
+  }
   grpc_core::ApplicationCallbackExecCtx callback_exec_ctx;
   grpc_core::ExecCtx exec_ctx;
   grpc_core::Call::FromC(call)->CancelWithError(absl::CancelledError());
@@ -3536,6 +3539,9 @@ grpc_call_error grpc_call_cancel_with_status(grpc_call* c,
       "c=%p, status=%d, description=%s, reserved=%p)",
       4, (c, (int)status, description, reserved));
   GPR_ASSERT(reserved == nullptr);
+  if (c == nullptr) {
+    return GRPC_CALL_ERROR;
+  }
   grpc_core::ApplicationCallbackExecCtx callback_exec_ctx;
   grpc_core::ExecCtx exec_ctx;
   grpc_core::Call::FromC(c)->CancelWithStatus(status, description);
@@ -3576,7 +3582,7 @@ grpc_call_error grpc_call_start_batch(grpc_call* call, const grpc_op* ops,
       "reserved=%p)",
       5, (call, ops, (unsigned long)nops, tag, reserved));
 
-  if (reserved != nullptr) {
+  if (reserved != nullptr || call == nullptr) {
     return GRPC_CALL_ERROR;
   } else {
     grpc_core::ApplicationCallbackExecCtx callback_exec_ctx;