MINOR: sample: provide a generic var-to-sample conversion function

We're using variable-to-sample conversion at least 4 times in the code,
two of which are bogus. Let's introduce a generic conversion function
that performs the required checks.

include/haproxy/sample.h

@@ -40,6 +40,7 @@ struct sample *sample_process(struct proxy *px, struct session *sess,
 struct sample *sample_fetch_as_type(struct proxy *px, struct session *sess,
                                    struct stream *strm, unsigned int opt,
                                    struct sample_expr *expr, int smp_type);
+int sample_conv_var2smp(const struct var_desc *var, struct sample *smp, int type);
 int sample_conv_var2smp_sint(const struct arg *arg, struct sample *smp);
 int sample_conv_var2smp_str(const struct arg *arg, struct sample *smp);
 void release_sample_expr(struct sample_expr *expr);

src/sample.c

@@ -1630,6 +1630,24 @@ static int sample_conv_bin2base64url(const struct arg *arg_p, struct sample *smp
 	return 1;
 }
 
+/* This function returns a sample struct filled with the conversion of variable
+ * <var> to sample type <type> (SMP_T_*), via a cast to the target type. If the
+ * variable cannot be retrieved or casted, 0 is returned, otherwise 1.
+ *
+ * Keep in mind that the sample content may be written to a pre-allocated
+ * trash chunk as returned by get_trash_chunk().
+ */
+int sample_conv_var2smp(const struct var_desc *var, struct sample *smp, int type)
+{
+	if (!vars_get_by_desc(var, smp, NULL))
+		return 0;
+	if (!sample_casts[smp->data.type][type])
+		return 0;
+	if (!sample_casts[smp->data.type][type](smp))
+		return 0;
+	return 1;
+}
+
 static int sample_conv_sha1(const struct arg *arg_p, struct sample *smp, void *private)
 {
 	blk_SHA_CTX ctx;