[binutils-gdb] Don't call sigtimedwait for scoped_ignore_sigttou

[Pedro Alves]

https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=336b30e58ae98fe66862ab8480d3f7bb885fef23

commit 336b30e58ae98fe66862ab8480d3f7bb885fef23
Author: Pedro Alves <pedro@palves.net>
Date:   Thu Jun 17 16:23:03 2021 +0100

    Don't call sigtimedwait for scoped_ignore_sigttou
    
    Because SIGTTOU is sent to the whole process instead of to a specific
    thread, consuming a pending SIGTTOU in the destructor of
    scoped_ignore_sigttou could consume a SIGTTOU signal raised due to
    actions done by some other thread.  Simply avoid sigtimedwait in
    scoped_ignore_sigttou, thus plugging the race.  This works because we
    know that when the thread writes to the terminal and the signal is
    blocked, the kernel does not raise the signal at all.
    
    Tested on GNU/Linux, Solaris 11 and FreeBSD.
    
    gdb/ChangeLog:
    yyyy-mm-dd  Pedro Alves  <pedro@palves.net>
    
            * scoped_ignore_signal.h (scoped_ignore_signal): Add
            ConsumePending template parameter.
            (scoped_ignore_signal::~scoped_ignore_signal): Skip calling
            sigtimedwait if ConsumePending is false.
            (scoped_ignore_sigpipe): Initialize with ConsumePending=true.
            * scoped_ignore_sigttou.h (scoped_ignore_sigttou)
            <m_ignore_signal>: Initialize with ConsumePending=false.
    
    Change-Id: I92f754dbc45c45819dce2ce68b8c067d8d5c61b1

Diff:
---
 gdb/ChangeLog                      | 10 ++++++++++
 gdbsupport/scoped_ignore_signal.h  | 18 +++++++++++++-----
 gdbsupport/scoped_ignore_sigttou.h |  2 +-
 3 files changed, 24 insertions(+), 6 deletions(-)

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index e4e58173ee6..c70f6ef5329 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,13 @@
+2021-06-17  Pedro Alves  <pedro@palves.net>
+
+	* scoped_ignore_signal.h (scoped_ignore_signal): Add
+	ConsumePending template parameter.
+	(scoped_ignore_signal::~scoped_ignore_signal): Skip calling
+	sigtimedwait if ConsumePending is false.
+	(scoped_ignore_sigpipe): Initialize with ConsumePending=true.
+	* scoped_ignore_sigttou.h (scoped_ignore_sigttou)
+	<m_ignore_signal>: Initialize with ConsumePending=false.
+
 2021-06-17  Pedro Alves  <pedro@palves.net>
 
 	* Makefile.in (SELFTESTS_SRCS): Add
diff --git a/gdbsupport/scoped_ignore_signal.h b/gdbsupport/scoped_ignore_signal.h
index 55a921cb332..a14c96779bf 100644
--- a/gdbsupport/scoped_ignore_signal.h
+++ b/gdbsupport/scoped_ignore_signal.h
@@ -25,9 +25,16 @@
 /* RAII class used to ignore a signal in a scope.  If sigprocmask is
    supported, then the signal is only ignored by the calling thread.
    Otherwise, the signal disposition is set to SIG_IGN, which affects
-   the whole process.  */
-
-template <int Sig>
+   the whole process.  If ConsumePending is true, the destructor
+   consumes a pending Sig.  SIGPIPE for example is queued on the
+   thread even if blocked at the time the pipe is written to.  SIGTTOU
+   OTOH is not raised at all if the thread writing to the terminal has
+   it blocked.  Because SIGTTOU is sent to the whole process instead
+   of to a specific thread, consuming a pending SIGTTOU in the
+   destructor could consume a signal raised due to actions done by
+   some other thread.  */
+
+template <int Sig, bool ConsumePending>
 class scoped_ignore_signal
 {
 public:
@@ -58,7 +65,8 @@ public:
 
 	/* If we got a pending Sig signal, consume it before
 	   unblocking.  */
-	sigtimedwait (&set, nullptr, &zero_timeout);
+	if (ConsumePending)
+	  sigtimedwait (&set, nullptr, &zero_timeout);
 
 	sigprocmask (SIG_UNBLOCK, &set, nullptr);
       }
@@ -89,7 +97,7 @@ struct scoped_ignore_signal_nop
 };
 
 #ifdef SIGPIPE
-using scoped_ignore_sigpipe = scoped_ignore_signal<SIGPIPE>;
+using scoped_ignore_sigpipe = scoped_ignore_signal<SIGPIPE, true>;
 #else
 using scoped_ignore_sigpipe = scoped_ignore_signal_nop;
 #endif
diff --git a/gdbsupport/scoped_ignore_sigttou.h b/gdbsupport/scoped_ignore_sigttou.h
index 1fc8f80d7fd..5695c5db905 100644
--- a/gdbsupport/scoped_ignore_sigttou.h
+++ b/gdbsupport/scoped_ignore_sigttou.h
@@ -75,7 +75,7 @@ public:
   DISABLE_COPY_AND_ASSIGN (scoped_ignore_sigttou);
 
 private:
-  lazy_init<scoped_ignore_signal<SIGTTOU>> m_ignore_signal;
+  lazy_init<scoped_ignore_signal<SIGTTOU, false>> m_ignore_signal;
 };
 
 #else