Do not emit style escape sequences to log file

PR gdb/24502 requests that the "set logging" log file not contain
style escape sequences emitted by gdb.

This seemed like a reasonable request to me, so this patch implements
filtering for the log file.

This also updates a comment in ui-style.h that I noticed while writing
the patch.

Tested on x86-64 Fedora 29.

gdb/ChangeLog
2019-06-14  Tom Tromey  <tromey@adacore.com>

	PR gdb/24502:
	* ui-style.h (skip_ansi_escape): Update comment.
	* ui-file.h (class no_terminal_escape_file): New class.
	* ui-file.c (no_terminal_escape_file::write)
	(no_terminal_escape_file::puts): New methods.
	* cli/cli-logging.c (handle_redirections): Use
	no_terminal_escape_file.

gdb/testsuite/ChangeLog
2019-06-14  Tom Tromey  <tromey@adacore.com>

	PR gdb/24502:
	* gdb.base/style-logging.exp: New file.

diff --git a/gdb/cli/cli-logging.c b/gdb/cli/cli-logging.c
--- a/gdb/cli/cli-logging.c
+++ b/gdb/cli/cli-logging.c
@@ -117,7 +117,7 @@ handle_redirections (int from_tty)
       return;
     }
 
-  stdio_file_up log (new stdio_file ());
+  stdio_file_up log (new no_terminal_escape_file ());
   if (!log->open (logging_filename, logging_overwrite ? "w" : "a"))
     perror_with_name (_("set logging"));
 
diff --git a/gdb/testsuite/gdb.base/style-logging.exp b/gdb/testsuite/gdb.base/style-logging.exp
new file mode 100644
--- /dev/null
+++ b/gdb/testsuite/gdb.base/style-logging.exp
@@ -0,0 +1,64 @@
+# Copyright 2019 Free Software Foundation, Inc.
+
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+# Test that logging does not style.
+
+if {[is_remote host]} {
+    untested "does not work on remote host"
+    return 0
+}
+
+standard_testfile style.c
+
+save_vars { env(TERM) } {
+    # We need an ANSI-capable terminal to get the output.
+    setenv TERM ansi
+
+    if {[prepare_for_testing "failed to prepare" $testfile $srcfile debug]} {
+	return -1
+    }
+
+    if {![runto_main]} {
+	fail "style tests failed"
+	return
+    }
+
+    gdb_test_no_output "set style enabled on"
+
+    set log_name [standard_output_file log.txt]
+    gdb_test_no_output "set logging file $log_name"
+    gdb_test_no_output "set logging overwrite on"
+    gdb_test "set logging on" "Copying output to .*"
+
+    set main_expr [style main function]
+    set base_file_expr [style ".*style\\.c" file]
+    set file_expr "$base_file_expr:\[0-9\]"
+    set arg_expr [style "arg." variable]
+    gdb_test "frame" \
+	"$main_expr.*$arg_expr.*$arg_expr.*$file_expr.*"
+
+    gdb_test "set logging off" "Done logging to .*"
+
+    set fd [open $log_name]
+    set data [read -nonewline $fd]
+    close $fd
+
+    set testname "log is escape-free"
+    if {[regexp "\033" $data]} {
+	fail $testname
+    } else {
+	pass $testname
+    }
+}
diff --git a/gdb/ui-file.c b/gdb/ui-file.c
--- a/gdb/ui-file.c
+++ b/gdb/ui-file.c
@@ -332,3 +332,33 @@ tee_file::isatty ()
 {
   return m_one->isatty ();
 }
+
+void
+no_terminal_escape_file::write (const char *buf, long length_buf)
+{
+  std::string copy (buf, length_buf);
+  this->puts (copy.c_str ());
+}
+
+/* See ui-file.h.  */
+
+void
+no_terminal_escape_file::puts (const char *buf)
+{
+  while (*buf != '\0')
+    {
+      const char *esc = strchr (buf, '\033');
+      if (esc == nullptr)
+	break;
+
+      int n_read = 0;
+      if (!skip_ansi_escape (esc, &n_read))
+	++esc;
+
+      this->stdio_file::write (buf, esc - buf);
+      buf = esc + n_read;
+    }
+
+  if (*buf != '\0')
+    this->stdio_file::write (buf, strlen (buf));
+}
diff --git a/gdb/ui-file.h b/gdb/ui-file.h
--- a/gdb/ui-file.h
+++ b/gdb/ui-file.h
@@ -264,4 +264,20 @@ private:
   bool m_close_one, m_close_two;
 };
 
+/* A ui_file implementation that filters out terminal escape
+   sequences.  */
+
+class no_terminal_escape_file : public stdio_file
+{
+public:
+  no_terminal_escape_file ()
+  {
+  }
+
+  /* Like the stdio_file methods, but these filter out terminal escape
+     sequences.  */
+  void write (const char *buf, long length_buf) override;
+  void puts (const char *linebuffer) override;
+};
+
 #endif
diff --git a/gdb/ui-style.h b/gdb/ui-style.h
--- a/gdb/ui-style.h
+++ b/gdb/ui-style.h
@@ -233,8 +233,8 @@ private:
 
 /* Skip an ANSI escape sequence in BUF.  BUF must begin with an ESC
    character.  Return true if an escape sequence was successfully
-   skipped; false otherwise.  In either case, N_READ is updated to
-   reflect the number of chars read from BUF.  */
+   skipped; false otherwise.  If an escape sequence was skipped,
+   N_READ is updated to reflect the number of chars read from BUF.  */
 
 extern bool skip_ansi_escape (const char *buf, int *n_read);