trurl: support ?= for set, to only do it if not already set

Idea-by: @emanuele6 in #296

Due to missing support in the libcurl URL API, this is not really
working for the scheme component right now, as that will always be
considered set once a URL has been parsed. The scheme guessing the
parser does sets it internally and there is currently no way provided by
the API to figure that out.

When the URL API has been extended, we can expand this functionality for
working better with scheme.

Closes #298

tests.json

@@ -2484,5 +2484,33 @@
  "stderr": "",
  "returncode": 0
  }
+ },
+ {
+ "input": {
+ "arguments": [
+ "example.com:88",
+ "--set",
+ "port?=99"
+ ]
+ },
+ "expected": {
+ "stdout": "http://example.com:88/\n",
+ "stderr": "",
+ "returncode": 0
+ }
+ },
+ {
+ "input": {
+ "arguments": [
+ "example.com",
+ "--set",
+ "port?=99"
+ ]
+ },
+ "expected": {
+ "stdout": "http://example.com:99/\n",
+ "stderr": "",
+ "returncode": 0
+ }
  }
 ]

trurl.1

@@ -217,6 +217,11 @@ options, host, port, path, query, fragment and zoneid.
 If a simple "="-assignment is used, the data is URL encoded when applied. If
 ":=" is used, the data is assumed to already be URL encoded and stored as-is.
 
+If "?=" is used, the set is only performed if the component is not already
+set. It avoids overwriting any already set data.
+
+You can also combine : and ? into "?:=" if desired.
+
 If no URL or \fI--url-file\fP argument is provided, trurl tries to create
 a URL using the components provided by the \fI--set\fP options. If not enough
 components are specified, this fails.

trurl.c

@@ -899,20 +899,47 @@ static const struct var *setone(CURLU *uh, const char *setline,
  if(ptr && (ptr > setline)) {
  size_t vlen = ptr - setline;
  bool urlencode = true;
+ bool conditional = false;
  bool found = false;
- if(ptr[-1] == ':') {
- urlencode = false;
- vlen--;
+ if(vlen) {
+ int back = -1;
+ size_t reqlen = 1;
+ while(vlen > reqlen) {
+ if(ptr[back] == ':') {
+ urlencode = false;
+ vlen--;
+ }
+ else if(ptr[back] == '?') {
+ conditional = true;
+ vlen--;
+ }
+ else
+ break;
+ reqlen++;
+ back--;
+ }
  }
  v = comp2var(setline, vlen);
  if(v) {
- CURLUcode rc;
+ CURLUcode rc = CURLUE_OK;
+ bool skip = false;
  if((v->part == CURLUPART_HOST) && ('[' == ptr[1]))
  /* when setting an IPv6 numerical address, disable URL encoding */
  urlencode = false;
- rc = curl_url_set(uh, v->part, ptr[1] ? &ptr[1] : NULL,
- (o->curl ? 0 : CURLU_NON_SUPPORT_SCHEME)|
- (urlencode ? CURLU_URLENCODE : 0) );
+
+ if(conditional) {
+ char *piece;
+ rc = curl_url_get(uh, v->part, &piece, 0);
+ if(!rc) {
+ skip = true;
+ curl_free(piece);
+ }
+ }
+
+ if(!skip)
+ rc = curl_url_set(uh, v->part, ptr[1] ? &ptr[1] : NULL,
+ (o->curl ? 0 : CURLU_NON_SUPPORT_SCHEME)|
+ (urlencode ? CURLU_URLENCODE : 0) );
  if(rc)
  warnf("Error setting %s: %s", v->name, curl_url_strerror(rc));
  found = true;