Fix racy output matching in gdb.base/completion.exp

Testing with:
 $ make check-read1 TESTS="gdb.base/completion.exp"

Exposes a testcase bug that can result in racy fails:

  FAIL: gdb.base/completion.exp: command-name completion limiting using tab character
  ERROR: Undefined command "".
  FAIL: gdb.base/completion.exp: symbol-name completion limiting using tab character
  FAIL: gdb.base/completion.exp: symbol-name completion limiting using complete command

testsuite/gdb.log shows:

  (gdb) PASS: gdb.base/completion.exp: set max-completions 5
  p^G
  passcount     path          print         print-object  printf
  *** List may be truncated, max-completions reached. ***
  (gdb) FAIL: gdb.base/completion.exp: command-name completion limiting using tab character
  pcomplete p
  Undefined command: "pcomplete".  Try "help".
  (gdb) ERROR: Undefined command "".

The problem is that the expect buffer can get filled with partial
output that ends in the gdb prompt, and so the default FAIL inside
gdb_test_multiple matches.

Fix that by splitting the gdb_test_multiple in two stages.  Since that
is done in more than one place in the testcase, move the otherwise
duplicate code to helper procedures.

gdb/testsuite/ChangeLog:
2017-11-09  Pedro Alves  <palves@redhat.com>

	* gdb.base/completion.exp (ignore_and_resync, test_tab_complete):
	New procedures, factored out from ...
	(top level): ... here, and adjusted to avoid expecting beyond the
	prompt in one go.

2 files changed, 41 insertions, 52 deletions

diff --git a/gdb/testsuite/ChangeLog b/gdb/testsuite/ChangeLog
index 8566dc1..0886647 100644
--- a/gdb/testsuite/ChangeLog
+++ b/gdb/testsuite/ChangeLog
@@ -1,5 +1,12 @@
 2017-11-09  Pedro Alves  <palves@redhat.com>
 
+	* gdb.base/completion.exp (ignore_and_resync, test_tab_complete):
+	New procedures, factored out from ...
+	(top level): ... here, and adjusted to avoid expecting beyond the
+	prompt in one go.
+
+2017-11-09  Pedro Alves  <palves@redhat.com>
+
 	* gdb.asm/asm-source.exp ("kill" test): Match the whole query
 	output.  Fix '?' match.
 
diff --git a/gdb/testsuite/gdb.base/completion.exp b/gdb/testsuite/gdb.base/completion.exp
index f03bfc3..9c7c17a 100644
--- a/gdb/testsuite/gdb.base/completion.exp
+++ b/gdb/testsuite/gdb.base/completion.exp
@@ -863,36 +863,45 @@ gdb_test "complete break $srcfile:ma" "break\.c:main"
 
 gdb_test_no_output "set max-completions 5"
 
-set test "command-name completion limiting using tab character"
-send_gdb "p\t"
-gdb_test_multiple "" "$test" {
-    -re "^p\\\x07$" {
-	send_gdb "\t"
-	gdb_test_multiple "" "$test" {
-	    -re "List may be truncated, max-completions reached.*\r\n$gdb_prompt p$" {
-		# Complete the command and ignore the output to resync
-		# gdb for the next test.
-		send_gdb "\n"
-		gdb_test_multiple "" "$test" {
-		    -re "$gdb_prompt $" {
-			pass "$test"
-		    }
+proc ignore_and_resync {cmd result test} {
+    global gdb_prompt
+
+    gdb_test_multiple "" "$test" {
+	-re "^${cmd}$" {
+	    # Complete the command and ignore the output
+	    # to resync gdb for the next test.
+	    send_gdb "\n"
+	    gdb_test_multiple "" "$test" {
+		-re "$gdb_prompt $" {
+		    $result $test
 		}
 	    }
-	    -re "$gdb_prompt p$" {
-		# Complete the command and ignore the output to resync
-		# gdb for the next test.
-		send_gdb "\n"
-		gdb_test_multiple "" "$test" {
-		    -re "$gdb_prompt $" {
-			fail "$test"
-		    }
+	}
+    }
+}
+
+proc test_tab_complete {cmd test} {
+    global gdb_prompt
+
+    send_gdb "${cmd}\t"
+    gdb_test_multiple "" "$test" {
+	-re "^${cmd}\\\x07$" {
+	    send_gdb "\t"
+	    gdb_test_multiple "" "$test" {
+		-re "List may be truncated, max-completions reached.*\r\n$gdb_prompt " {
+		    ignore_and_resync $cmd pass $test
+		}
+		-re "$gdb_prompt " {
+		    ignore_and_resync $cmd fail $test
 		}
 	    }
-        }
+	}
     }
 }
 
+test_tab_complete "p" \
+    "command-name completion limiting using tab character"
+
 set test "command-name completion limiting using complete command"
 send_gdb "complete p\n"
 gdb_test_multiple "" "$test" {
@@ -903,35 +912,8 @@ gdb_test_multiple "" "$test" {
 
 gdb_test_no_output "set max-completions 3"
 
-set test "symbol-name completion limiting using tab character"
-send_gdb "p marker\t"
-gdb_test_multiple "" "$test" {
-    -re "^p marker\\\x07$" {
-	send_gdb "\t"
-	gdb_test_multiple "" "$test" {
-	    -re "List may be truncated, max-completions reached.*\r\n$gdb_prompt p marker$" {
-		# Complete the command and ignore the output to resync
-		# gdb for the next test.
-		send_gdb "\n"
-		gdb_test_multiple "" "$test" {
-		    -re "$gdb_prompt $" {
-			pass "$test"
-		    }
-		}
-	    }
-	    -re "$gdb_prompt p marker$" {
-		# Complete the command and ignore the output to resync
-		# gdb for the next test.
-		send_gdb "\n"
-		gdb_test_multiple "" "$test" {
-		    -re "$gdb_prompt $" {
-			fail "$test"
-		    }
-		}
-	    }
-        }
-    }
-}
+test_tab_complete "p marker" \
+    "symbol-name completion limiting using tab character"
 
 set test "symbol-name completion limiting using complete command"
 send_gdb "complete p mark\n"