Fix "thread find" with multiple inferiors/targets (PR gdb/26631)

"thread find" with multiple inferiors got broken with the multi-target
work:

 Thread 1 "gdb" hit Breakpoint 1, internal_error (...) at ../../src/gdbsupport/errors.cc:51
 51      {
 (top-gdb) bt
 #0  internal_error (file=0xffffd4d0 <error: Cannot access memory at address 0xffffd4d0>, line=0, fmt=0x555556330320 "en_US.UTF-8") at ../../src/gdbsupport/errors.cc:51
 #1  0x0000555555bca4c7 in target_thread_name (info=0x555556801290) at ../../src/gdb/target.c:2035
 #2  0x0000555555beb07a in thread_find_command (arg=0x7fffffffe08e "1", from_tty=0) at ../../src/gdb/thread.c:1959
 #3  0x000055555572ec49 in do_const_cfunc (c=0x555556786bc0, args=0x7fffffffe08e "1", from_tty=0) at ../../src/gdb/cli/cli-decode.c:95
 #4  0x0000555555732abd in cmd_func (cmd=0x555556786bc0, args=0x7fffffffe08e "1", from_tty=0) at ../../src/gdb/cli/cli-decode.c:2181
 #5  0x0000555555bf1245 in execute_command (p=0x7fffffffe08e "1", from_tty=0) at ../../src/gdb/top.c:664
 #6  0x00005555559cad10 in catch_command_errors (command=0x555555bf0c31 <execute_command(char const*, int)>, arg=0x7fffffffe082 "thread find 1", from_tty=0) at ../../src/gdb/main.c:457
 #7  0x00005555559cc33d in captured_main_1 (context=0x7fffffffdb60) at ../../src/gdb/main.c:1218
 #8  0x00005555559cc571 in captured_main (data=0x7fffffffdb60) at ../../src/gdb/main.c:1243
 #9  0x00005555559cc5e8 in gdb_main (args=0x7fffffffdb60) at ../../src/gdb/main.c:1268
 #10 0x0000555555623816 in main (argc=17, argv=0x7fffffffdc78) at ../../src/gdb/gdb.c:32

The problem is that we're not switching to the inferior/target before
calling target methods, which trips on an assertion put in place
exactly to catch this sort of problem.

gdb/testsuite/ChangeLog:

	PR gdb/26631
	* gdb.multi/multi-target-thread-find.exp: New file.

gdb/ChangeLog:

	PR gdb/26631
	* thread.c (thread_find_command): Switch inferior before calling
	target methods.

gdb/ChangeLog

@@ -1,3 +1,9 @@
+2020-09-18  Pedro Alves  <[email protected]>
+
+	PR gdb/26631
+	* thread.c (thread_find_command): Switch inferior before calling
+	target methods.
+
 2020-09-17  Tom Tromey  <[email protected]>
 
 	* tic6x-tdep.c (tic6x_gdbarch_init): Update.

gdb/testsuite/ChangeLog

@@ -1,3 +1,8 @@
+2020-09-18  Pedro Alves  <[email protected]>
+
+	PR gdb/26631
+	* gdb.multi/multi-target-thread-find.exp: New file.
+
 2020-09-18  Pedro Alves  <[email protected]>
 
 	* gdb.multi/multi-target-continue.exp: New file, factored out from

gdb/testsuite/gdb.multi/multi-target-thread-find.exp

@@ -0,0 +1,106 @@
+# Copyright 2020 Free Software Foundation, Inc.
+
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+# Test "thread find" with multiple inferiors/targets.  Regression test
+# for PR gdb/26631.
+
+source $srcdir/$subdir/multi-target.exp.tcl
+
+if {![multi_target_prepare]} {
+    return
+}
+
+proc test_thread_find {} {
+    global decimal gdb_prompt
+
+    set NUM_INFS 6
+
+    if {![setup "off"]} {
+	untested "setup failed"
+	return
+    }
+
+    # This makes sure we don't crash.  See PR gdb/26631.
+    gdb_test "thread find xxxyyyzzz" \
+	"No threads match 'xxxyyyzzz'"
+
+    # Create thread names.
+    for {set i 1} {$i <= $NUM_INFS} {incr i} {
+	gdb_test "thread apply $i.1 thread name threadname_$i" \
+	    "Thread $i.1 .*" \
+	    "name thread $i"
+    }
+
+    # Collect target ids.
+
+    for {set i 1} {$i <= $NUM_INFS} {incr i} {
+	set target_id($i) ""
+    }
+    set any "\[^\r\n\]*"
+    gdb_test_multiple "info threads" "collect thread id" {
+	-re ". ($decimal).$decimal  (Thread ${any}) \"threadname_\[0-9\]+\" $any" {
+	    set thr_num $expect_out(1,string)
+	    set target_id($thr_num) $expect_out(2,string)
+	    exp_continue
+	}
+	-re ".*$gdb_prompt $" {
+	    pass "collect target id"
+	}
+    }
+
+    # Find the threads by name.  Note we repeat the search with each
+    # inferior selected, so that we're sure that GDB doesn't get
+    # confused with which target stack to consult.
+
+    with_test_prefix "find by name" {
+	for {set sel_inf 1} {$sel_inf <= $NUM_INFS} {incr sel_inf} {
+	    with_test_prefix "inf $sel_inf" {
+
+		gdb_test "inferior $sel_inf" \
+		    "Switching to inferior $sel_inf .*"
+
+		for {set find_inf 1} {$find_inf <= $NUM_INFS} {incr find_inf} {
+		    gdb_test "thread find threadname_$find_inf" \
+			"Thread $find_inf.1 has name 'threadname_$find_inf'" \
+			"find thread name $find_inf"
+		}
+	    }
+	}
+    }
+
+    # Find the threads by target id.  Likewise we repeat the search
+    # with each inferior selected.
+
+    with_test_prefix "find by target id" {
+	for {set sel_inf 1} {$sel_inf <= $NUM_INFS} {incr sel_inf} {
+	    with_test_prefix "inf $sel_inf" {
+
+		gdb_test "inferior $sel_inf" \
+		    "Switching to inferior $sel_inf .*"
+
+		for {set find_inf 1} {$find_inf <= $NUM_INFS} {incr find_inf} {
+		    set target_id_re [string_to_regexp $target_id($find_inf)]
+		    gdb_test "thread find $target_id($find_inf)" \
+			"Thread $find_inf.1 has target id '$target_id_re'.*" \
+			"find thread target id $find_inf"
+		}
+	    }
+	}
+    }
+}
+
+test_thread_find
+
+multi_target_cleanup

gdb/thread.c

@@ -1946,9 +1946,15 @@ thread_find_command (const char *arg, int from_tty)
   if (tmp != 0)
     error (_("Invalid regexp (%s): %s"), tmp, arg);
 
+  /* We're going to be switching threads.  */
+  scoped_restore_current_thread restore_thread;
+
   update_thread_list ();
+
   for (thread_info *tp : all_threads ())
     {
+      switch_to_inferior_no_thread (tp->inf);
+
       if (tp->name != NULL && re_exec (tp->name))
 	{
 	  printf_filtered (_("Thread %s has name '%s'\n"),