MINOR: sample: add param converter

Add a converter that extracts a parameter from string of delimited
key/value pairs.

Fixes: #1697

doc/configuration.txt

@@ -17702,6 +17702,32 @@ or(<value>)
   This prefix is followed by a name. The separator is a '.'. The name may only
   contain characters 'a-z', 'A-Z', '0-9', '.' and '_'.
 
+param(<name>,[<delim>])
+  This extracts the first occurrence of the parameter <name> in the input string
+  where parameters are delimited by <delim>, which defaults to "&", and the name
+  and value of the parameter are separated by a "=". If there is no "=" and
+  value before the end of the parameter segment, it is treated as equivalent to
+  a value of an empty string.
+
+  This can be useful for extracting parameters from a query string, or possibly
+  a x-www-form-urlencoded body. In particular, `query,param(<name>)` can be used
+  as an alternative to `urlp(<name>)` which only uses "&" as a delimiter,
+  whereas "urlp" also uses "?" and ";".
+
+  Note that this converter doesn't do anything special with url encoded
+  characters. If you want to decode the value, you can use the url_dec converter
+  on the output. If the name of the parameter in the input might contain encoded
+  characters, you'll probably want do normalize the input before calling
+  "param". This can be done using "http-request normalize-uri", in particular
+  the percent-decode-unreserved and percent-to-uppercase options.
+
+  Example :
+      str(a=b&c=d&a=r),param(a)   # b
+      str(a&b=c),param(a)         # ""
+      str(a=&b&c=a),param(b)      # ""
+      str(a=1;b=2;c=4),param(b,;) # 2
+      query,param(redirect_uri),urldec()
+
 port_only
   Converts a string which contains a Host header value into an integer by
   returning its port.

reg-tests/converter/param.vtc

@@ -0,0 +1,80 @@
+varnishtest "param converter Test"
+
+feature ignore_unknown_macro
+
+server s1 {
+	rxreq
+	txresp -hdr "Connection: close"
+} -repeat 10 -start
+
+haproxy h1 -conf {
+	defaults
+	mode http
+	timeout connect "${HAPROXY_TEST_TIMEOUT-5s}"
+	timeout client  "${HAPROXY_TEST_TIMEOUT-5s}"
+	timeout server  "${HAPROXY_TEST_TIMEOUT-5s}"
+
+	frontend fe
+	bind "fd@${fe}"
+
+	### requests
+	http-request set-var(txn.query) query
+	http-response set-header Found %[var(txn.query),param(test)] if { var(txn.query),param(test) -m found }
+
+	default_backend be
+
+	backend be
+	server s1 ${s1_addr}:${s1_port}
+} -start
+
+client c1 -connect ${h1_fe_sock} {
+	txreq -url "/foo/?test=1&b=4&d"
+	rxresp
+	expect resp.status == 200
+	expect resp.http.found == "1"
+
+	txreq -url "/?a=1&b=4&test=34"
+	rxresp
+	expect resp.status == 200
+	expect resp.http.found == "34"
+
+	txreq -url "/?test=bar"
+	rxresp
+	expect resp.status == 200
+	expect resp.http.found == "bar"
+
+	txreq -url "/?a=b&c=d"
+	rxresp
+	expect resp.status == 200
+	expect resp.http.found == "<undef>"
+
+	txreq -url "/?a=b&test=t&c=d"
+	rxresp
+	expect resp.status == 200
+	expect resp.http.found == "t"
+
+	txreq -url "/?a=b&test&c=d"
+	rxresp
+	expect resp.status == 200
+	expect resp.http.found == ""
+
+	txreq -url "/?test="
+	rxresp
+	expect resp.status == 200
+	expect resp.http.found == ""
+
+    txreq -url "/?a=b&test"
+    rxresp
+    expect resp.status == 200
+    expect resp.http.found == ""
+
+    txreq -url "/?testing=123"
+    rxresp
+    expect resp.status == 200
+    expect resp.http.found == "<undef>"
+
+    txreq -url "/?testing=123&test=4"
+    rxresp
+    expect resp.status == 200
+    expect resp.http.found == "4"
+} -run

src/sample.c

@@ -2607,6 +2607,69 @@ found:
 	return 1;
 }
 
+static int sample_conv_param_check(struct arg *arg, struct sample_conv *conv,
+                                   const char *file, int line, char **err)
+{
+	if (arg[1].type == ARGT_STR && arg[1].data.str.data != 1) {
+		memprintf(err, "Delimiter must be exactly 1 character.");
+		return 0;
+	}
+
+	return 1;
+}
+
+static int sample_conv_param(const struct arg *arg_p, struct sample *smp, void *private)
+{
+	char *pos, *end, *pend, *equal;
+	char delim = '&';
+	const char *name = arg_p[0].data.str.area;
+	size_t name_l = arg_p[0].data.str.data;
+
+	if (arg_p[1].type == ARGT_STR)
+		delim = *arg_p[1].data.str.area;
+
+	pos = smp->data.u.str.area;
+	end = pos + smp->data.u.str.data;
+	while (pos < end) {
+		equal = pos + name_l;
+		/* Parameter not found */
+		if (equal > end)
+			break;
+
+		if (equal == end || *equal == delim) {
+			if (memcmp(pos, name, name_l) == 0) {
+				/* input contains parameter, but no value is supplied */
+				smp->data.u.str.data = 0;
+				return 1;
+			}
+			pos = equal + 1;
+			continue;
+		}
+
+		if (*equal == '=' && memcmp(pos, name, name_l) == 0) {
+			pos = equal + 1;
+			pend = memchr(pos, delim, end - pos);
+			if (pend == NULL)
+				pend = end;
+
+			if (smp->data.u.str.size)
+				smp->data.u.str.size -= pos - smp->data.u.str.area;
+			smp->data.u.str.area = pos;
+			smp->data.u.str.data = pend - pos;
+			return 1;
+		}
+		/* find the next delimiter and set position to character after that */
+		pos = memchr(pos, delim, end - pos);
+		if (pos == NULL)
+			pos = end;
+		else
+			pos++;
+	}
+	/* Parameter not found */
+	smp->data.u.str.data = 0;
+	return 0;
+}
+
 static int sample_conv_regsub_check(struct arg *args, struct sample_conv *conv,
                                     const char *file, int line, char **err)
 {
@@ -4399,6 +4462,7 @@ static struct sample_conv_kw_list sample_conv_kws = {ILH, {
 	{ "bytes",   sample_conv_bytes,        ARG2(1,SINT,SINT),     NULL,                     SMP_T_BIN,  SMP_T_BIN  },
 	{ "field",   sample_conv_field,        ARG3(2,SINT,STR,SINT), sample_conv_field_check,  SMP_T_STR,  SMP_T_STR  },
 	{ "word",    sample_conv_word,         ARG3(2,SINT,STR,SINT), sample_conv_field_check,  SMP_T_STR,  SMP_T_STR  },
+	{ "param",   sample_conv_param,        ARG2(1,STR,STR),       sample_conv_param_check,  SMP_T_STR,  SMP_T_STR  },
 	{ "regsub",  sample_conv_regsub,       ARG3(2,REG,STR,STR),   sample_conv_regsub_check, SMP_T_STR,  SMP_T_STR  },
 	{ "sha1",    sample_conv_sha1,         0,                     NULL,                     SMP_T_BIN,  SMP_T_BIN  },
 	{ "strcmp",  sample_conv_strcmp,       ARG1(1,STR),           smp_check_strcmp,         SMP_T_STR,  SMP_T_SINT },