[Ada] GNAT.Expect (Expect_Internal): Try to call 'poll' few times

'poll' returns -1 in case of any error (including interruption by a
signal), so call need to be repeated few times to avoid false failures.

2019-09-17  Vadim Godunko  <godunko@adacore.com>

gcc/ada/

	* libgnat/g-expect.adb (Expect_Internal): Try to call 'poll' few
	times.

From-SVN: r275782

gcc/ada/ChangeLog

 2019-09-17  Vadim Godunko  <godunko@adacore.com>
 
+	* libgnat/g-expect.adb (Expect_Internal): Try to call 'poll' few
+	times.
+
+2019-09-17  Vadim Godunko  <godunko@adacore.com>
+
 	* libgnat/g-expect.ads, libgnat/g-expect.adb (Close_Input): New
 	subprogram.
 	(Get_Command_Output): Call Close_Input to close input stream.

gcc/ada/libgnat/g-expect.adb

@@ -679,8 +679,17 @@ package body GNAT.Expect is
          --  Loop until we match or we have a timeout
 
          loop
-            Num_Descriptors :=
-              Poll (Fds'Address, Fds_Count, Timeout, D'Access, Is_Set'Address);
+            --  Poll may be interrupted on Linux by a signal and need to be
+            --  repeated. We don't want to check for errno = EINTER, so just
+            --  attempt to call Poll a few times.
+
+            for J in 1 .. 3 loop
+               Num_Descriptors :=
+                 Poll
+                   (Fds'Address, Fds_Count, Timeout, D'Access, Is_Set'Address);
+
+               exit when Num_Descriptors /= -1;
+            end loop;
 
             case Num_Descriptors is