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[binutils-gdb/gdb-7.10-branch] remote follow fork and spurious child stops in non-stop mode


https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=4ccc18f16da05e93aff0c7ac570017d5ef79ef6c

commit 4ccc18f16da05e93aff0c7ac570017d5ef79ef6c
Author: Pedro Alves <palves@redhat.com>
Date:   Thu Jul 30 18:55:37 2015 +0100

    remote follow fork and spurious child stops in non-stop mode
    
    Running gdb.threads/fork-plus-threads.exp against gdbserver in
    extended-remote mode, even though the test passes, we still see broken
    behavior:
    
     (gdb) PASS: gdb.threads/fork-plus-threads.exp: set detach-on-fork off
     continue &
     Continuing.
     (gdb) PASS: gdb.threads/fork-plus-threads.exp: continue &
     [New Thread 28092.28092]
    
     [Thread 28092.28092] #2 stopped.
     [New Thread 28094.28094]
     [Inferior 2 (process 28092) exited normally]
     [New Thread 28094.28105]
     [New Thread 28094.28109]
    
    ...
    
    [Thread 28174.28174] #18 stopped.
     [New Thread 28185.28185]
     [Inferior 10 (process 28174) exited normally]
     [New Thread 28185.28196]
    
     [Thread 28185.28185] #20 stopped.
     Cannot remove breakpoints because program is no longer writable.
     Further execution is probably impossible.
     [Inferior 11 (process 28185) exited normally]
     [Inferior 1 (process 28091) exited normally]
     PASS: gdb.threads/fork-plus-threads.exp: reached breakpoint
     info threads
     No threads.
     (gdb) PASS: gdb.threads/fork-plus-threads.exp: no threads left
     info inferiors
       Num  Description       Executable
     * 1    <null>            /home/pedro/gdb/mygit/build/gdb/testsuite/gdb.threads/fork-plus-threads
     (gdb) PASS: gdb.threads/fork-plus-threads.exp: only inferior 1 left
    
    All the "[Thread FOO] #NN stopped." above are bogus, as well as the
    "Cannot remove breakpoints because program is no longer writable.",
    which is a consequence.
    
    The problem is that when we intercept a fork event, we should report
    the event for the parent, only, and leave the child stopped, but not
    report its stop event.  GDB later decides whether to follow the parent
    or the child.  But because handle_extended_wait does not set the
    child's last_status.kind to TARGET_WAITKIND_STOPPED, a
    stop_all_threads/unstop_all_lwps sequence (e.g., from trying to access
    memory) by mistake ends up queueing a SIGSTOP on the child, resuming
    it, and then when that SIGSTOP is intercepted, because the LWP has
    last_resume_kind set to resume_stop, gdbserver reports the stop to
    GDB, as GDB_SIGNAL_0:
    
    ...
     >>>> entering unstop_all_lwps
     unstopping all lwps
     proceed_one_lwp: lwp 1600
        client wants LWP to remain 1600 stopped
     proceed_one_lwp: lwp 1828
     Client wants LWP 1828 to stop. Making sure it has a SIGSTOP pending
     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
     Sending sigstop to lwp 1828
     pc is 0x3615ebc7cc
     Resuming lwp 1828 (continue, signal 0, stop expected)
       continue from pc 0x3615ebc7cc
     unstop_all_lwps done
     sigchld_handler
     <<<< exiting unstop_all_lwps
     handling possible target event
     >>>> entering linux_wait_1
     linux_wait_1: [<all threads>]
     my_waitpid (-1, 0x40000001)
     my_waitpid (-1, 0x1): status(137f), 1828
     LWFE: waitpid(-1, ...) returned 1828, ERRNO-OK
     LLW: waitpid 1828 received Stopped (signal) (stopped)
     pc is 0x3615ebc7cc
     Expected stop.
     LLW: resume_stop SIGSTOP caught for LWP 1828.1828.
     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    ...
     linux_wait_1 ret = LWP 1828.1828, 1, 0
     <<<< exiting linux_wait_1
     Writing resume reply for LWP 1828.1828:1
     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    
    Tested on x86_64 Fedora 20, extended-remote.
    
    gdb/gdbserver/ChangeLog:
    2015-07-30  Pedro Alves  <palves@redhat.com>
    
    	* linux-low.c (handle_extended_wait): Set the child's last
    	reported status to TARGET_WAITKIND_STOPPED.

Diff:
---
 gdb/gdbserver/linux-low.c                       |  2 ++
 gdb/testsuite/ChangeLog                         |  5 ++++
 gdb/testsuite/gdb.threads/fork-plus-threads.exp | 32 +++++++++++++++++++++++++
 3 files changed, 39 insertions(+)

diff --git a/gdb/gdbserver/linux-low.c b/gdb/gdbserver/linux-low.c
index 2dafb03..8c90944 100644
--- a/gdb/gdbserver/linux-low.c
+++ b/gdb/gdbserver/linux-low.c
@@ -486,6 +486,8 @@ handle_extended_wait (struct lwp_info *event_lwp, int wstat)
 	  child_lwp->status_pending_p = 0;
 	  child_thr = get_lwp_thread (child_lwp);
 	  child_thr->last_resume_kind = resume_stop;
+	  child_thr->last_status.kind = TARGET_WAITKIND_STOPPED;
+
 	  parent_proc = get_thread_process (event_thr);
 	  child_proc->attached = parent_proc->attached;
 	  clone_all_breakpoints (&child_proc->breakpoints,
diff --git a/gdb/testsuite/ChangeLog b/gdb/testsuite/ChangeLog
index 479837f..5036e78 100644
--- a/gdb/testsuite/ChangeLog
+++ b/gdb/testsuite/ChangeLog
@@ -1,5 +1,10 @@
 2015-07-30  Pedro Alves  <palves@redhat.com>
 
+	* linux-low.c (handle_extended_wait): Set the child's last
+	reported status to TARGET_WAITKIND_STOPPED.
+
+2015-07-30  Pedro Alves  <palves@redhat.com>
+
 	PR threads/18600
 	* gdb.threads/fork-plus-threads.exp: Test that "info inferiors"
 	only shows inferior 1.
diff --git a/gdb/testsuite/gdb.threads/fork-plus-threads.exp b/gdb/testsuite/gdb.threads/fork-plus-threads.exp
index 8a503ec..2b34b6c 100644
--- a/gdb/testsuite/gdb.threads/fork-plus-threads.exp
+++ b/gdb/testsuite/gdb.threads/fork-plus-threads.exp
@@ -51,13 +51,45 @@ proc do_test { detach_on_fork } {
 	}
     }
 
+    # gdbserver had a bug that resulted in reporting the fork child's
+    # initial stop to gdb, which gdb does not expect, in turn
+    # resulting in a broken session, like:
+    #
+    #  [Thread 31536.31536] #16 stopped.                                <== BAD
+    #  [New Thread 31547.31547]
+    #  [Inferior 10 (process 31536) exited normally]
+    #  [New Thread 31547.31560]
+    #
+    #  [Thread 31547.31547] #18 stopped.                                <== BAD
+    #  Cannot remove breakpoints because program is no longer writable. <== BAD
+    #  Further execution is probably impossible.                        <== BAD
+    #  [Inferior 11 (process 31547) exited normally]
+    #  [Inferior 1 (process 31454) exited normally]
+    #
+    # These variables track whether we see such broken behavior.
+    set saw_cannot_remove_breakpoints 0
+    set saw_thread_stopped 0
+
     set test "inferior 1 exited"
     gdb_test_multiple "" $test {
+	-re "Cannot remove breakpoints" {
+	    set saw_cannot_remove_breakpoints 1
+	    exp_continue
+	}
+	-re "Thread \[^\r\n\]+ stopped\\." {
+	    set saw_thread_stopped 1
+	    exp_continue
+	}
 	-re "Inferior 1 \(\[^\r\n\]+\) exited normally" {
 	    pass $test
 	}
     }
 
+    gdb_assert !$saw_cannot_remove_breakpoints \
+	"no failure to remove breakpoints"
+    gdb_assert !$saw_thread_stopped \
+	"no spurious thread stop"
+
     gdb_test "info threads" "No threads\." \
 	"no threads left"


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