I tested this change with the following script:

```
#!/usr/bin/perl

###############################################################################

use warnings;
use strict;

use Test::More;

BEGIN { use FindBin; chdir($FindBin::Bin); }

use lib 'lib';
use Test::Nginx;

###############################################################################

select STDERR; $| = 1;
select STDOUT; $| = 1;

my $t = Test::Nginx->new()
->has(qw/http limit_req/)
->plan(2)
->write_file_expand('nginx.conf', <<'EOF');

%%TEST_GLOBALS%%

daemon off;

events {}

http {
%%TEST_GLOBALS_HTTP%%

limit_req_zone $binary_remote_addr zone=one:1m rate=1r/m;
limit_req zone=one;

server {
listen 127.0.0.1:8080;
server_name localhost;

location / {
error_log stderr;
}
}
}

EOF

open OLDERR, '>&', \*STDERR;
open STDERR, '>', $t->testdir() . '/stderr' or die "Can't reopen STDERR:
$!";
open my $stderr, '<', $t->testdir() . '/stderr' or die "Can't open stderr
file: $!";

$t->run();

open STDERR, '>&', \*OLDERR;

###############################################################################

# charge limit_req
http_get('/');

http_get('/"foo bar"');
is(lines($t, 'request: "GET /\"foo bar\" HTTP/1.0"'), 1);

http_get_with_referrer('/',
'https://www.google.com.au/search?hl=en&q="BauGutachter"
+41 2016 .ch&num=100&start=0');
is(lines($t, 'referrer:
"https://www.google.com.au/search?hl=en&q=\"BauGutachter\"
+41 2016 .ch&num=100&start=0"'), 1);

###############################################################################

sub http_get_with_referrer {
my ($url, $referer) = @_;
return http(<<EOF);
GET $url HTTP/1.0
Host: localhost
Referer: $referer

EOF
}

sub lines {
my ($t, $pattern) = @_;
return map { $_ =~ /\Q$pattern\E/ } (<$stderr>);
}

###############################################################################
```

*Joshua Spence*

*mobile:* +61 422 990 263
*email:* josh@joshuaspence.com
*web:* http://www.joshuaspence.com

On 3 January 2017 at 17:50, Joshua Spence <josh@joshuaspence.com> wrote:

> # HG changeset patch
> # User Joshua Spence <josh@joshuaspence.com>
> # Date 1483420178 -39600
> # Tue Jan 03 16:09:38 2017 +1100
> # Branch error_log_escape
> # Node ID 30a9ce42aadfcf135f3ad95a296c95e7044d5f9d
> # Parent a42afc225e98afe1f7c3b428c81c8af7eba362c0
> Escape strings produced by the `%V` format specification
>
> Currently nginx error logs loosely adhere to the following pattern:
>
> ```
> %{TIMESTAMP} [%{SEVERITY}] %{PROCESS_ID}#%{THREAD_ID}:(
> *%{CONNECTION_ID})?%{MESSAGE}(, %{KEY}: "%{VALUE}")*
> ```
>
> In our environment we are using Filebeat to ship nginx error logs into
> Logstash. Our Logstash filter for parsing nginx error logs is as follows:
>
> ```
> filter {
> grok {
> match => {
> "message" => "(?<timestamp>%{YEAR}/%{MONTHNUM2}/%{MONTHDAY}
> %{TIME}) \[%{LOGLEVEL:severity}\] %{POSINT:process_id:int}#%{NONNEGINT:thread_id:int}:(?:
> \*%{NONNEGINT:connection_id:int})? (?<message>[^,]*)(?:,
> %{GREEDYDATA:nginx_keyvalue_data})?"
> }
> overwrite => ["message"]
> }
>
> date {
> match => ["timestamp", "yyyy/MM/dd HH:mm:ss"]
> remove_field => ["timestamp"]
> }
>
> kv {
> field_split => ", "
> remove_field => "nginx_keyvalue_data"
> source => "nginx_keyvalue_data"
> value_split => ": "
> }
> }
> ```
>
> The problem, however, is that it is difficult to unambigiously parse the
> key-value data from the nginx error log format because of a lack of data
> sanitization. Specifically, consider the following entry from our nginx
> error logs:
>
> ```
> 2016/11/15 03:45:51 [error] 23770#23770: *21535000 open()
> "/REDACTED/+"BauGutachter"++41+2016+.ch&tbo=1" failed (2: No such file or
> directory), client: REDACTED, server: REDACTED, request: "GET
> /job/+%22BauGutachter%22++41+2016+.ch&tbo=1 HTTP/1.1", host: "REDACTED",
> referrer: "https://www.google.com.au/search?hl=en&q="BauGutachter" +41
> 2016 .ch&num=100&start=0"
> ```
>
> Specifically, the problem in the above sample is the `referrer` field, the
> value of which contains double quotes. I think that it makes sense to
> escape `\` and `"` characters before they are written to the error log.
>
> diff -r a42afc225e98 -r 30a9ce42aadf src/core/ngx_string.c
> --- a/src/core/ngx_string.c Tue Dec 27 17:23:08 2016 +0300
> +++ b/src/core/ngx_string.c Tue Jan 03 16:09:38 2017 +1100
> @@ -15,7 +15,7 @@
> const u_char *basis, ngx_uint_t padding);
> static ngx_int_t ngx_decode_base64_internal(ngx_str_t *dst, ngx_str_t
> *src,
> const u_char *basis);
> -
> +static uintptr_t ngx_escape_string(u_char *dst, u_char *src, size_t size);
>
> void
> ngx_strlow(u_char *dst, u_char *src, size_t n)
> @@ -235,7 +235,7 @@
> v = va_arg(args, ngx_str_t *);
>
> len = ngx_min(((size_t) (last - buf)), v->len);
> - buf = ngx_cpymem(buf, v->data, len);
> + buf = (u_char *) ngx_escape_string(buf, v->data, len);
> fmt++;
>
> continue;
> @@ -1844,6 +1844,43 @@
> }
>
>
> +static uintptr_t
> +ngx_escape_string(u_char *dst, u_char *src, size_t size)
> +{
> + u_char ch;
> + ngx_uint_t len;
> +
> + if (dst == NULL) {
> + len = 0;
> +
> + while (size) {
> + ch = *src++;
> +
> + if (ch == '\\' || ch == '"') {
> + len++;
> + }
> +
> + size--;
> + }
> +
> + return (uintptr_t) len;
> + }
> +
> + while (size) {
> + ch = *src++;
> +
> + if (ch == '\\' || ch == '"') {
> + *dst++ = '\\';
> + }
> +
> + *dst++ = ch;
> + size--;
> + }
> +
> + return (uintptr_t) dst;
> +}
> +
> +
> void
> ngx_str_rbtree_insert_value(ngx_rbtree_node_t *temp,
> ngx_rbtree_node_t *node, ngx_rbtree_node_t *sentinel)
>
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