Fix problem that alias can be defined or not depending on the order.

When an alias name starts with the name of another alias,
GDB was accepting to define the aliases in one order (short first, long after),
but refused it the other way around.

So, fix the logic to recognise an already existing alias by using
lookup_cmd_composition.

Also, this revealed a bug in lookup_cmd_composition:
when the searched command is a prefix command, lookup_cmd_composition
was not returning the fact that a command was found even if the
TEXT to parse was fully consumed.

gdb/ChangeLog
YYYY-MM-DD  Philippe Waroquiers  <philippe.waroquiers@skynet.be>

	* cli/cli-cmds.c (alias_command): Check for an existing alias
	using lookup_cmd_composition, as valid_command_p is too strict
	and forbids aliases that are the prefix of an existing alias
	or command.
	* cli/cli-decode.c (lookup_cmd_composition): Ensure a prefix
	command is properly recognised as a valid command.

gdb/testsuite/ChangeLog
2020-05-15  Philippe Waroquiers  <philippe.waroquiers@skynet.be>

	* gdb.base/alias.exp: Test aliases starting with a prefix of
	another alias.

5 files changed, 51 insertions, 9 deletions

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index 30500f4..e5708da 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,5 +1,14 @@
 2020-05-15  Philippe Waroquiers  <philippe.waroquiers@skynet.be>
 
+	* cli/cli-cmds.c (alias_command): Check for an existing alias
+	using lookup_cmd_composition, as valid_command_p is too strict
+	and forbids aliases that are the prefix of an existing alias
+	or command.
+	* cli/cli-decode.c (lookup_cmd_composition): Ensure a prefix
+	command is properly recognised as a valid command.
+
+2020-05-15  Philippe Waroquiers  <philippe.waroquiers@skynet.be>
+
 	* unittests/help-doc-selftests.c: Rename to
 	unittests/command-def-selftests.c
 	* unittests/command-def-selftests.c (help_doc_tests): Update some
diff --git a/gdb/cli/cli-cmds.c b/gdb/cli/cli-cmds.c
index c17521b..8538fad 100644
--- a/gdb/cli/cli-cmds.c
+++ b/gdb/cli/cli-cmds.c
@@ -1694,8 +1694,29 @@ alias_command (const char *args, int from_tty)
   /* ALIAS must not exist.  */
   std::string alias_string (argv_to_string (alias_argv, alias_argc));
   alias = alias_string.c_str ();
-  if (valid_command_p (alias))
-    error (_("Alias already exists: %s"), alias);
+  {
+    cmd_list_element *alias_cmd, *prefix_cmd, *cmd;
+
+    if (lookup_cmd_composition (alias, &alias_cmd, &prefix_cmd, &cmd))
+      {
+	const char *alias_name = alias_argv[alias_argc-1];
+
+	/* If we found an existing ALIAS_CMD, check that the prefix differ or
+	   the name differ.  */
+
+	if (alias_cmd != nullptr
+	    && alias_cmd->prefix == prefix_cmd
+	    && strcmp (alias_name, alias_cmd->name) == 0)
+	  error (_("Alias already exists: %s"), alias);
+
+	/* Check ALIAS differs from the found CMD.  */
+
+	if (cmd->prefix == prefix_cmd
+	    && strcmp (alias_name, cmd->name) == 0)
+	  error (_("Alias %s is the name of an existing command"), alias);
+      }
+  }
+
 
   /* If ALIAS is one word, it is an alias for the entire COMMAND.
      Example: alias spe = set print elements
diff --git a/gdb/cli/cli-decode.c b/gdb/cli/cli-decode.c
index d951ead..78b8901 100644
--- a/gdb/cli/cli-decode.c
+++ b/gdb/cli/cli-decode.c
@@ -1843,6 +1843,8 @@ lookup_cmd_composition (const char *text,
 
   cur_list = cmdlist;
 
+  text = skip_spaces (text);
+
   while (1)
     {
       /* Go through as many command lists as we need to,
@@ -1850,9 +1852,6 @@ lookup_cmd_composition (const char *text,
 
       prev_cmd = *cmd;
 
-      while (*text == ' ' || *text == '\t')
-	(text)++;
-
       /* Identify the name of the command.  */
       len = find_command_name_length (text);
 
@@ -1861,7 +1860,7 @@ lookup_cmd_composition (const char *text,
 	return 0;
 
       /* TEXT is the start of the first command word to lookup (and
-	 it's length is len).  We copy this into a local temporary.  */
+	 it's length is LEN).  We copy this into a local temporary.  */
 
       command = (char *) alloca (len + 1);
       memcpy (command, text, len);
@@ -1890,12 +1889,14 @@ lookup_cmd_composition (const char *text,
 	    }
 	  *prefix_cmd = prev_cmd;
 	}
-      if ((*cmd)->prefixlist)
+
+      text += len;
+      text = skip_spaces (text);
+
+      if ((*cmd)->prefixlist && *text != '\0')
 	cur_list = *(*cmd)->prefixlist;
       else
 	return 1;
-
-      text += len;
     }
 }
 
diff --git a/gdb/testsuite/ChangeLog b/gdb/testsuite/ChangeLog
index dda5b68..820af44 100644
--- a/gdb/testsuite/ChangeLog
+++ b/gdb/testsuite/ChangeLog
@@ -1,3 +1,8 @@
+2020-05-15  Philippe Waroquiers  <philippe.waroquiers@skynet.be>
+
+	* gdb.base/alias.exp: Test aliases starting with a prefix of
+	another alias.
+
 2020-05-15  Gary Benson <gbenson@redhat.com>
 
 	* gdb.base/info-os.c (main): Add return statement.
diff --git a/gdb/testsuite/gdb.base/alias.exp b/gdb/testsuite/gdb.base/alias.exp
index be78d9e..9a95576 100644
--- a/gdb/testsuite/gdb.base/alias.exp
+++ b/gdb/testsuite/gdb.base/alias.exp
@@ -116,3 +116,9 @@ gdb_test "show print elements" "Limit .* is 56\[.\]" "verify 56"
 
 gdb_test_no_output "set print max-elements 57"
 gdb_test "show print elements" "Limit .* is 57\[.\]" "verify 57"
+
+# Test aliases having a common prefix.
+gdb_test_no_output "alias abcd  = backtrace"
+gdb_test_no_output "alias abcde = backtrace"
+gdb_test_no_output "alias fghij = backtrace"
+gdb_test_no_output "alias fghi  = backtrace"