Raise exception if ambiguous name is used in gdb.parameter

Currently gdb.parameter doesn't raise an exception if an
ambiguous name is used, it instead returns the value of the
last partly matching parameter:
```
(gdb) show print sym
Ambiguous show print command "sym": symbol, symbol-filename, symbol-loading.
(gdb) show print symbol-loading
Printing of symbol loading messages is "full".
(gdb) py print(gdb.parameter("print sym"))
full
```

It's because lookup_cmd_composition_1 tries to detect
ambigous names by checking the return value of find_cmd
for CMD_LIST_AMBIGUOUS, which never happens, since only
lookup_cmd_1 returns CMD_LIST_AMBIGUOUS.
Instead the nfound argument contains the number of found
matches.

By using it instead, and by setting *CMD to the special value
CMD_LIST_AMBIGUOUS in this case, gdbpy_parameter can now show
the appropriate error message:
```
(gdb) py print(gdb.parameter("print sym"))
Traceback (most recent call last):
  File "<string>", line 1, in <module>
RuntimeError: Parameter `print sym' is ambiguous.
Error while executing Python code.
(gdb) py print(gdb.parameter("print symbol"))
True
(gdb) py print(gdb.parameter("print symbol-"))
Traceback (most recent call last):
  File "<string>", line 1, in <module>
RuntimeError: Parameter `print symbol-' is ambiguous.
Error while executing Python code.
(gdb) py print(gdb.parameter("print symbol-load"))
full
```

Since the document command also uses lookup_cmd_composition, it needed
to check for CMD_LIST_AMBIGUOUS as well, so it now also shows an
"Ambiguous command" error message in this case.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=14639
Approved-By: Tom Tromey <tom@tromey.com>

gdb/cli/cli-decode.c

@@ -2549,7 +2549,8 @@ deprecated_cmd_warning (const char *text, struct cmd_list_element *list)
    If TEXT is a subcommand (i.e. one that is preceded by a prefix
    command) set *PREFIX_CMD.
 
-   Set *CMD to point to the command TEXT indicates.
+   Set *CMD to point to the command TEXT indicates, or to
+   CMD_LIST_AMBIGUOUS if there are multiple possible matches.
 
    If any of *ALIAS, *PREFIX_CMD, or *CMD cannot be determined or do not
    exist, they are NULL when we return.
@@ -2589,7 +2590,12 @@ lookup_cmd_composition_1 (const char *text,
       *cmd = find_cmd (command.c_str (), len, cur_list, 1, &nfound);
 
       /* We only handle the case where a single command was found.  */
-      if (*cmd == CMD_LIST_AMBIGUOUS || *cmd == nullptr)
+      if (nfound > 1)
+	{
+	  *cmd = CMD_LIST_AMBIGUOUS;
+	  return 0;
+	}
+      else if (*cmd == nullptr)
 	return 0;
       else
 	{
@@ -2623,7 +2629,8 @@ lookup_cmd_composition_1 (const char *text,
    If TEXT is a subcommand (i.e. one that is preceded by a prefix
    command) set *PREFIX_CMD.
 
-   Set *CMD to point to the command TEXT indicates.
+   Set *CMD to point to the command TEXT indicates, or to
+   CMD_LIST_AMBIGUOUS if there are multiple possible matches.
 
    If any of *ALIAS, *PREFIX_CMD, or *CMD cannot be determined or do not
    exist, they are NULL when we return.

gdb/cli/cli-script.c

@@ -1521,6 +1521,8 @@ do_document_command (const char *comname, int from_tty,
   lookup_cmd_composition (comfull, &alias, &prefix_cmd, &c);
   if (c == nullptr)
     error (_("Undefined command: \"%s\"."), comfull);
+  else if (c == CMD_LIST_AMBIGUOUS)
+    error (_("Ambiguous command: \"%s\"."), comfull);
 
   if (c->theclass != class_user
       && (alias == nullptr || alias->theclass != class_alias))

gdb/python/python.c

@@ -559,7 +559,10 @@ gdbpy_parameter (PyObject *self, PyObject *args)
       GDB_PY_HANDLE_EXCEPTION (ex);
     }
 
-  if (!found)
+  if (cmd == CMD_LIST_AMBIGUOUS)
+    return PyErr_Format (PyExc_RuntimeError,
+			 _("Parameter `%s' is ambiguous."), arg);
+  else if (!found)
     return PyErr_Format (PyExc_RuntimeError,
 			 _("Could not find parameter `%s'."), arg);
 

gdb/testsuite/gdb.base/document.exp

@@ -32,3 +32,13 @@ gdb_test "help do-document" "usage: do-document" "invoke help do-document"
 # Test that document of a non-existing command prints an error.  There
 # was a regression at one point causing this to crash.
 gdb_test "document nosuchcommand" "Undefined command: \"nosuchcommand\"\\."
+
+# Test that document of an ambiguous command prints an error.
+foreach_with_prefix cmd {1 2} {
+    gdb_test_multiple "define ambiguous_command_$cmd" "" {
+	-re "Type commands for definition of \"ambiguous_command_$cmd\".\r\nEnd with a line saying just \"end\".\r\n>$" {
+	    gdb_test "end" "" "define ambiguous_command_$cmd"
+	}
+    }
+}
+gdb_test "document ambiguous_command" "Ambiguous command: \"ambiguous_command\"."

gdb/testsuite/gdb.python/py-parameter.exp

@@ -588,6 +588,37 @@ proc_with_prefix test_language {} {
     gdb_test_no_output "set lang auto"
 }
 
+proc_with_prefix test_ambiguous_parameter {} {
+    gdb_test_multiline "create parameter" \
+	"python" "" \
+	"class TestAmbiguousParam (gdb.Parameter):" "" \
+	"   def __init__ (self, name, value):" "" \
+	"      super (TestAmbiguousParam, self).__init__ (name, gdb.COMMAND_DATA, gdb.PARAM_INTEGER)" "" \
+	"      self.value = value" "" \
+	"end"
+
+    # Create parameters.
+    gdb_test "python TestAmbiguousParam('test-ambiguous-value-1', 1)" ""
+    gdb_test "python TestAmbiguousParam('test-ambiguous-value-2-extra', 2)" ""
+    gdb_test "python TestAmbiguousParam('test-ambiguous', 3)" ""
+
+    # Test unambiguous matches.
+    gdb_test "python print(gdb.parameter('test-ambiguous-value-1'))" "1"
+    gdb_test "python print(gdb.parameter('test-ambiguous-value-2-extra'))" "2"
+    gdb_test "python print(gdb.parameter('test-ambiguous-value-2'))" "2"
+    gdb_test "python print(gdb.parameter('test-ambiguous'))" "3"
+
+    # Test ambiguous names.
+    gdb_test "python print(gdb.parameter('test-ambiguou'))" \
+	"Parameter .* is ambiguous.*Error while executing Python code."
+    gdb_test "python print(gdb.parameter('test-ambiguous-'))" \
+	"Parameter .* is ambiguous.*Error while executing Python code."
+    gdb_test "python print(gdb.parameter('test-ambiguous-v'))" \
+	"Parameter .* is ambiguous.*Error while executing Python code."
+    gdb_test "python print(gdb.parameter('test-ambiguous-value-1a'))" \
+	"Could not find parameter.*Error while executing Python code."
+}
+
 test_directories
 test_data_directory
 test_boolean_parameter
@@ -600,5 +631,6 @@ test_gdb_parameter
 test_integer_parameter
 test_throwing_parameter
 test_language
+test_ambiguous_parameter
 
 rename py_param_test_maybe_no_output ""