[PATCH] gdb: Fix "info os <unknown>" command

[Paul Marechal]

Running `info os someUnknownOsType` is crashing when gdb is built with
-D_GLIBCXX_DEBUG:

	/usr/include/c++/5/debug/vector:439:error: attempt to
	access an element in
	    an empty container.

In target_read_stralloc from target.c, the call to
target_read_alloc_1 can return an empty vector, we then call vector::back on
this vector, which is invalid.

This commit adds a check for emptiness before trying to call
vector::back on it. It also adds test to check for `info os <unknown>`
to return the proper error message.

This is a regression in gdb 8.2 and this patch restores the behavior of
previous versions.

gdb/ChangeLog:

	PR gdb/23974
	* target.c (target_read_stralloc): Check for empty vector.

gdb/testsuite/ChangeLog:

	PR gdb/23974
	* gdb.base/info-os.exp: Check return for unknown "info os" type.
---
 gdb/target.c                       | 2 +-
 gdb/testsuite/gdb.base/info-os.exp | 3 +++
 2 files changed, 4 insertions(+), 1 deletion(-)

diff --git a/gdb/target.c b/gdb/target.c
index 80b8453176..6c63255d03 100644
--- a/gdb/target.c
+++ b/gdb/target.c
@@ -1793,7 +1793,7 @@ target_read_stralloc (struct target_ops *ops, enum target_object object,
   if (!buf)
     return {};
 
-  if (buf->back () != '\0')
+  if (buf->empty () || buf->back () != '\0')
     buf->push_back ('\0');
 
   /* Check for embedded NUL bytes; but allow trailing NULs.  */
diff --git a/gdb/testsuite/gdb.base/info-os.exp b/gdb/testsuite/gdb.base/info-os.exp
index 8d7eab9c53..c44e6b27c3 100644
--- a/gdb/testsuite/gdb.base/info-os.exp
+++ b/gdb/testsuite/gdb.base/info-os.exp
@@ -175,6 +175,9 @@ expect_multiline "info os semaphores" "$semkey +$semid +666 +1 .*" "get semaphor
 #                               key   msqid   perm  num used bytes  num messages  last msgsnd() command  last msgrcv() command  user  group  creator user  creator group  last msgsnd() time  last msgrcv() time  last msgctl() time
 expect_multiline "info os msg" "$msgkey +$msqid +666 .*" "get message queues"
 
+gdb_test "info os unknown_entry" [multi_line \
+	"warning: Empty data returned by target.  Wrong osdata type\\\?" \
+	"Can not fetch data now."]
 
 # The SysV IPC primitives linger on after the creating process is killed
 # unless they are destroyed explicitly, so allow the test program to tidy
-- 
2.20.1