[Crash-utility] [PATCH v4 06/16] Fix gdb_interface: restore gdb's output streams at end of gdb_interface

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From: Aditya Gupta <adityag@xxxxxxxxxxxxx>

Currently for most gdb_interface call, in which a non-null file pointer
is passed, GDB's output stream is replaced with the passed file pointer

Due to this, 'info threads', which is a gdb passthrough, doesn't print any
thread, after support was added to get registers from crash_target:

    crash> info threads
      Id   Target Id         Frame

This empty output of 'info threads' was due to a subtle bug in
gdb_interface.

After this gdb passthrough is run, 'datatype_info' is called, with file
pointer set to null_fp (pointing to /dev/null). And after
'datatype_info' returns, any output by gdb goes to /dev/null, and hence
the output is lost.

Fix this by restoring the original output streams, after gdb_interface
has handled the output

After this patch:

    crash> info threads
      Id   Target Id         Frame
    * 1    2131 bash         0xc000000000051e40 in crash_fadump (regs=0x0, str=0xc000000002c60510 <buf> "sysrq triggered crash") at arch/powerpc/kernel/fadump.c:735

Cc: Sourabh Jain <sourabhjain@xxxxxxxxxxxxx>
Cc: Hari Bathini <hbathini@xxxxxxxxxxxxx>
Cc: Mahesh J Salgaonkar <mahesh@xxxxxxxxxxxxx>
Cc: Naveen N. Rao <naveen.n.rao@xxxxxxxxxxxxxxxxxx>
Cc: Lianbo Jiang <lijiang@xxxxxxxxxx>
Cc: HAGIO KAZUHITO(萩尾 一仁) <k-hagio-ab@xxxxxxx>
Cc: Tao Liu <ltao@xxxxxxxxxx>
Cc: Alexey Makhalov <alexey.makhalov@xxxxxxxxxxxx>
Signed-off-by: Aditya Gupta <adityag@xxxxxxxxxxxxx>
Signed-off-by: Tao Liu <ltao@xxxxxxxxxx>
---
 gdb-10.2.patch | 53 ++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 53 insertions(+)

diff --git a/gdb-10.2.patch b/gdb-10.2.patch
index 3694b13..0bed96a 100644
--- a/gdb-10.2.patch
+++ b/gdb-10.2.patch
@@ -16118,3 +16118,56 @@ exit 0
        subclass (SYMBOL_NONE)
      {
        /* We can't use an initializer list for members of a base class, and
+--- gdb-10.2/gdb/ui-file.h.orig
++++ gdb-10.2/gdb/ui-file.h
+@@ -195,6 +195,7 @@ public:
+ 
+   bool can_emit_style_escape () override;
+ 
++  FILE *get_stream(void);
+   /* Sets the internal stream to FILE, and saves the FILE's file
+      descriptor in M_FD.  */
+   void set_stream (FILE *file);
+--- gdb-10.2/gdb/ui-file.c.orig
++++ gdb-10.2/gdb/ui-file.c
+@@ -161,6 +161,12 @@ stdio_file::~stdio_file ()
+     fclose (m_file);
+ }
+ 
++FILE*
++stdio_file::get_stream(void)
++{
++  return m_file;
++}
++
+ void
+ stdio_file::set_stream (FILE *file)
+ {
+--- gdb-10.2/gdb/symtab.c.orig
++++ gdb-10.2/gdb/symtab.c
+@@ -6964,8 +6964,12 @@ void
+ gdb_command_funnel_1(struct gnu_request *req)
+ {
+         struct symbol *sym;
++        FILE *original_stdout_stream = nullptr;
++        FILE *original_stderr_stream = nullptr;
+ 
+         if (req->command != GNU_VERSION && req->command != GNU_USER_PRINT_OPTION) {
++                original_stdout_stream = (dynamic_cast< stdio_file * >gdb_stdout)->get_stream();
++                original_stderr_stream = (dynamic_cast< stdio_file * >gdb_stderr)->get_stream();
+                 (dynamic_cast<stdio_file *>gdb_stdout)->set_stream(req->fp);
+                 (dynamic_cast<stdio_file *>gdb_stderr)->set_stream(req->fp);
+         }
+@@ -7068,6 +7072,12 @@ gdb_command_funnel_1(struct gnu_request *req)
+                 req->flags |= GNU_COMMAND_FAILED;
+                 break;
+         }
++
++        /* Restore the streams gdb output was using */
++        if (original_stdout_stream)
++            (dynamic_cast<stdio_file *>gdb_stdout)->set_stream(original_stdout_stream);
++        if (original_stderr_stream)
++            (dynamic_cast<stdio_file *>gdb_stderr)->set_stream(original_stderr_stream);
+ }
+ 
+ /*
-- 
2.40.1
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