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  <[email protected]>

	* 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

Author: palves

gdb/ChangeLog

@@ -1,3 +1,13 @@
+2021-06-17  Pedro Alves  <[email protected]>
+
+	* 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  <[email protected]>
 
 	* Makefile.in (SELFTESTS_SRCS): Add

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 @@ class scoped_ignore_signal
 
 	/* 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

gdbsupport/scoped_ignore_sigttou.h

@@ -75,7 +75,7 @@ class scoped_ignore_sigttou
   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