libpkg: improve curl fetch error reporting

Currently all errors that occur in curl_do_fetch() are reported to the
user as "Failed to fetch X: No Error" which is both incorrect and very
unhelpful.

With this patch, the correct error message from libcurl is reported
to the user, for example "Failed to fetch X: Could not resolve hostname"

This patch does not fix all the error handling done by the libcurl
fetching code, but it does fix the case that seems to be most commonly
hit by users.

Closes:		#2498
Sponsored by:	The FreeBSD Foundation

Author: ifreund

libpkg/fetch_libcurl.c

@@ -141,8 +141,10 @@ int my_trace(CURL *handle, curl_infotype type,
   return 0;
 }
 
+/* Returns the HTTP response code on success.
+ * Returns -1 and sets the `error` out parameter on failure. */
 static long
-curl_do_fetch(struct curl_userdata *data, CURL *cl, struct curl_repodata *cr)
+curl_do_fetch(struct curl_userdata *data, CURL *cl, struct curl_repodata *cr, CURLcode *error)
 {
 	char *tmp;
 	int still_running = 1;
@@ -180,16 +182,21 @@ curl_do_fetch(struct curl_userdata *data, CURL *cl, struct curl_repodata *cr)
 
 	while ((msg = curl_multi_info_read(cr->cm, &msg_left))) {
 		if (msg->msg == CURLMSG_DONE) {
-			if (msg->data.result == CURLE_ABORTED_BY_CALLBACK)
-				return (-1); // FIXME: more clear return code?
 			if (msg->data.result == CURLE_COULDNT_CONNECT
 				|| msg->data.result == CURLE_COULDNT_RESOLVE_HOST
-				|| msg->data.result == CURLE_COULDNT_RESOLVE_PROXY)
+				|| msg->data.result == CURLE_COULDNT_RESOLVE_PROXY) {
 				pkg_emit_pkg_errno(EPKG_NONETWORK, "curl_do_fetch", NULL);
+			}
+			if (msg->data.result != CURLE_OK) {
+				if (error != NULL) {
+					*error = msg->data.result;
+				}
+				return (-1);
+			}
 			CURL *eh = msg->easy_handle;
-			long rc = 0;
-			curl_easy_getinfo(eh, CURLINFO_RESPONSE_CODE, &rc);
-			return (rc);
+			long response_code = 0;
+			curl_easy_getinfo(eh, CURLINFO_RESPONSE_CODE, &response_code);
+			return (response_code);
 		}
 	}
 	return (0);
@@ -233,7 +240,8 @@ http_getmirrors(struct pkg_repo *r, struct curl_repodata *cr)
 		curl_easy_setopt(cl, CURLOPT_IPRESOLVE, CURL_IPRESOLVE_V4);
 	if (ctx.ip == IPV6)
 		curl_easy_setopt(cl, CURLOPT_IPRESOLVE, CURL_IPRESOLVE_V6);
-	curl_do_fetch(&data, cl, cr);
+	// FIXME: handle fetch error?
+	curl_do_fetch(&data, cl, cr, NULL);
 	fclose(data.fh);
 	walk = buf;
 	while ((line = strsep(&walk, "\n\r")) != NULL) {
@@ -499,34 +507,42 @@ curl_fetch(struct pkg_repo *repo, int dest, struct fetch_item *fi)
 		curl_easy_setopt(cl, CURLOPT_LOW_SPEED_TIME, repo->fetcher->timeout);
 	}
 
-	long rc = curl_do_fetch(&data, cl, cr);
+	CURLcode fetch_error;
+	long response_code = curl_do_fetch(&data, cl, cr, &fetch_error);
 	curl_off_t t;
 	res = curl_easy_getinfo(cl, CURLINFO_FILETIME_T, &t);
 	curl_multi_remove_handle(cr->cm, cl);
 	curl_easy_cleanup(cl);
-	if (rc == 304) {
+	if (response_code == -1) {
+		if (fetch_error == CURLE_ABORTED_BY_CALLBACK) {
+			retcode = EPKG_CANCEL;
+		} else {
+			pkg_emit_error("Failed to fetch %s: %s",
+			    fi->url, curl_easy_strerror(fetch_error));
+			retcode = EPKG_FATAL;
+		}
+	} else if (response_code == 304) {
 		retcode = EPKG_UPTODATE;
-	} else if (rc == -1) {
-		retcode = EPKG_CANCEL;
-	} else if (!curl_response_is_ok(rc)) {
+	} else if (!curl_response_is_ok(response_code)) {
 		--retry;
 		if (retry <= 0) {
-			if (rc == 404) {
+			if (response_code == 404) {
 				pkg_emit_error("Failed to fetch %s: Not found",
 				    fi->url);
 				retcode = EPKG_ENOENT;
 			} else {
-				pkg_emit_error("Failed to fetch %s: %s",
-				    fi->url, curl_easy_strerror(rc));
+				pkg_emit_error("Failed to fetch %s: response code %ld",
+				    fi->url, response_code);
 				retcode = EPKG_FATAL;
 			}
-		} else
+		} else {
 			goto do_retry;
+		}
 	}
 
 	if (res == CURLE_OK && t >= 0) {
 		fi->mtime = t;
-	} else if (rc != 304 && retcode != EPKG_FATAL &&
+	} else if (response_code != 304 && retcode != EPKG_FATAL &&
 	    retcode != EPKG_CANCEL && retcode != EPKG_ENOENT) {
 		pkg_emit_error("Impossible to get the value from Last-Modified"
 		    " HTTP header");