[lldb] Fix crash when completing register names after program exit

[DavidSpickett]

Summary

Previously the following would crash:
(lldb) run
Process 2594053 launched: '/tmp/test.o' (aarch64)
Process 2594053 exited with status = 0 (0x00000000)
(lldb) register read <tab>

As the completer assumed that the execution context would always
have a register context. After a program has finished, it does not.

Split out the generic parts of the test from the x86 specific tests,
and added "register info" to both.

[DavidSpickett]

lldb/test/API/functionalities/completion/TestCompletion.py

782

I will add register info here later once I've got an x86 build going again.

[DavidSpickett]

Added register info to the x86 test cases.

[JDevlieghere]

LGTM

lldb/source/Commands/CommandCompletions.cpp

void CommandCompletions::Registers(CommandInterpreter &interpreter,
                                    CompletionRequest &request,
                                    SearchFilter *searcher) {
   std::string reg_prefix;
   if (request.GetCursorArgumentPrefix().startswith("$"))
     reg_prefix = "$";
 
   RegisterContext *reg_ctx =
       interpreter.GetExecutionContext().GetRegisterContext();
+  if (!reg_ctx)
+    return;
+
   const size_t reg_num = reg_ctx->GetRegisterCount();
   for (size_t reg_idx = 0; reg_idx < reg_num; ++reg_idx) {
     const RegisterInfo *reg_info = reg_ctx->GetRegisterInfoAtIndex(reg_idx);
     request.TryCompleteCurrentArg(reg_prefix + reg_info->name,
                                   reg_info->alt_name);
   }
 }
 

lldb/test/API/functionalities/completion/TestCompletion.py

def test_completion_type_formatter_delete(self):
         self.runCmd("type summary add -x -c Foo")
         self.complete_from_to("type summary delete ", ["Foo"])
 
         self.runCmd("type synthetic add Goo -l test")
         self.complete_from_to("type synthetic delete ", ["Goo"])
         self.runCmd("type synthetic add -x Hoo -l test")
         self.complete_from_to("type synthetic delete ", ["Hoo"])
 
-    @skipIf(archs=no_match(["x86_64"]))
-    def test_register_read_and_write_on_x86(self):
-        """Test the completion of the commands register read and write on x86"""
-
+    def test_register_no_complete(self):
         # The tab completion for "register read/write"  won't work without a running process.
         self.complete_from_to("register read ", "register read ")
         self.complete_from_to("register write ", "register write ")
+        self.complete_from_to("register info ", "register info ")
+
+        self.build()
+        self.runCmd("target create {}".format(self.getBuildArtifact("a.out")))
+        self.runCmd("run")
+
+        # Once a program has finished you have an execution context but no register
+        # context so completion cannot work.
+        self.complete_from_to("register read ", "register read ")
+        self.complete_from_to("register write ", "register write ")
+        self.complete_from_to("register info ", "register info ")
+
+    @skipIf(archs=no_match(["x86_64"]))
+    def test_register_read_and_write_on_x86(self):
+        """Test the completion of the commands register read and write on x86"""
 
         self.build()
         self.main_source_spec = lldb.SBFileSpec("main.cpp")
         lldbutil.run_to_source_breakpoint(self, "// Break here", self.main_source_spec)
 
         # test cases for register read
         self.complete_from_to("register read ", ["rax", "rbx", "rcx"])
         self.complete_from_to("register read r", ["rax", "rbx", "rcx"])
         self.complete_from_to("register read ra", "register read rax")
         # register read can take multiple register names as arguments
         self.complete_from_to("register read rax ", ["rax", "rbx", "rcx"])
         # complete with prefix '$'
         self.completions_match("register read $rb", ["$rbx", "$rbp"])
         self.completions_match("register read $ra", ["$rax"])
         self.complete_from_to("register read rax $", ["\$rax", "\$rbx", "\$rcx"])
         self.complete_from_to("register read $rax ", ["rax", "rbx", "rcx"])
 
         # test cases for register write
         self.complete_from_to("register write ", ["rax", "rbx", "rcx"])
         self.complete_from_to("register write r", ["rax", "rbx", "rcx"])
         self.complete_from_to("register write ra", "register write rax")
         self.complete_from_to("register write rb", ["rbx", "rbp"])
         # register write can only take exact one register name as argument
         self.complete_from_to("register write rbx ", [])
 
     def test_common_completion_target_stophook_ids(self):
         subcommands = ["delete", "enable", "disable"]
 
         for subcommand in subcommands:
             self.complete_from_to(
                 "target stop-hook " + subcommand + " ",
                 "target stop-hook " + subcommand + " ",
             )